The GNU database manager 1 Copying Conditions 2 Introduction to GNU 'dbm' 3 Opening the database 4 Closing the database 5 Number of Records 6 Inserting and replacing records in the database 7 Searching for records in the database 8 Removing records from the database 9 Sequential access to records 10 Database reorganization 11 Database Synchronization 12 Changing database format 13 Export and Import 14 Error handling 15 Database consistency 16 Recovering structural consistency 17 Crash Tolerance 18 Setting options 19 File Locking 20 Useful global variables 21 Additional functions 22 Error codes 23 Compatibility with standard 'dbm' and 'ndbm' 24 Examine and modify a GDBM database 25 The 'gdbm_dump' utility 26 The 'gdbm_load' utility 27 Exit codes 28 Problems and bugs 29 Additional resources Appendix A GNU Free Documentation License Index The GNU database manager 1 Copying Conditions 2 Introduction to GNU 'dbm' 3 Opening the database 4 Closing the database 5 Number of Records 6 Inserting and replacing records in the database 7 Searching for records in the database 8 Removing records from the database 9 Sequential access to records 10 Database reorganization 11 Database Synchronization 12 Changing database format 13 Export and Import 14 Error handling 15 Database consistency 16 Recovering structural consistency 17 Crash Tolerance 17.1 Using Proper Filesystem 17.2 Enabling crash tolerance 17.3 Synchronizing the Database 17.4 Crash recovery 17.5 Manual crash recovery 17.6 Performance Impact 17.7 Availability 17.8 Numsync Extension 17.9 Crash Tolerance API 18 Setting options 19 File Locking 20 Useful global variables 21 Additional functions 22 Error codes 23 Compatibility with standard 'dbm' and 'ndbm' 23.1 NDBM interface functions 23.2 DBM interface functions 24 Examine and modify a GDBM database 24.1 gdbmtool invocation 24.2 gdbmtool interactive mode 24.2.1 Shell Variables 24.2.2 Gdbmtool Commands 24.2.3 Data Definitions 24.2.4 Startup Files 25 The 'gdbm_dump' utility 26 The 'gdbm_load' utility 27 Exit codes 28 Problems and bugs 29 Additional resources Appendix A GNU Free Documentation License Index The GNU database manager ************************ GNU 'dbm' ('GDBM') is a library of functions implementing a hashed database on a disk file. This manual documents GNU 'dbm' Version 1.23. The software was originally written by Philip A. Nelson. This document was originally written by Pierre Gaumond from texts written by Phil. 1 Copying Conditions ******************** This library is "free"; this means that everyone is free to use it and free to redistribute it on a free basis. GNU 'dbm' ('GDBM') is not in the public domain; it is copyrighted and there are restrictions on its distribution, but these restrictions are designed to permit everything that a good cooperating citizen would want to do. What is not allowed is to try to prevent others from further sharing any version of 'GDBM' that they might get from you. Specifically, we want to make sure that you have the right to give away copies of 'GDBM', that you receive source code or else can get it if you want it, that you can change these functions or use pieces of them in new free programs, and that you know you can do these things. To make sure that everyone has such rights, we have to forbid you to deprive anyone else of these rights. For example, if you distribute copies of 'GDBM', you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must tell them their rights. Also, for our own protection, we must make certain that everyone finds out that there is no warranty for anything in the 'GDBM' distribution. If these functions are modified by someone else and passed on, we want their recipients to know that what they have is not what we distributed, so that any problems introduced by others will not reflect on our reputation. 'GDBM' is currently distributed under the terms of the GNU General Public License, Version 3. (_NOT_ under the GNU General Library Public License.) A copy the GNU General Public License is included with the distribution of 'GDBM'. 2 Introduction to GNU 'dbm' *************************** GNU 'dbm' ('GDBM') is a library of database functions that use extensible hashing and work similar to the standard UNIX 'dbm' functions. These routines are provided to a programmer needing to create and manipulate a hashed database. ('GDBM' is _NOT_ a complete database package for an end user.) The basic use of 'GDBM' is to store key/data pairs in a data file. Each key must be unique and each key is paired with only one data item. The keys can not be directly accessed in sorted order. The basic unit of data in 'GDBM' is the structure: typedef struct { char *dptr; int dsize; } datum; This structure allows for arbitrary sized keys and data items. In particular, zero-length keys or data ('dsize = 0') are allowed. However, the 'dptr' field is required to point to a valid memory location. In other words, 'dptr' cannot be NULL. Note also that its type is 'char *' for purely historic reasons. You can use any C data type (either scalar or aggregate) both as for key and for data. The key/data pairs are stored in a 'GDBM' disk file, called a "gdbm database". An application must open a 'GDBM' database to be able manipulate the keys and data contained in it. 'GDBM' allows an application to have multiple databases open at the same time. When an application opens a 'GDBM' database, it is designated as a 'reader' or a 'writer'. A 'GDBM' database can be opened by at most one writer at a time. However, many readers may open the database simultaneously. Readers and writers can not open the 'GDBM' database at the same time. Speaking about "application" we usually mean a separate process. However, it is entirely normal for a multi-thread program to operate as a 'GDBM' reader in one thread and writer in another, provided, of course, that the two threads don't operate on the same database simultaneously. To use the 'GDBM' functions, the programmer must first include the header file 'gdbm.h'. This file defines, among others, the 'GDBM_FILE' data type, an opaque pointer to the structure that represents the opened 'GDBM' database. To access the database, the programmer must first open it using the 'gdbm_open' function. The function takes several arguments, the name of the database file being one of them, and returns a 'GDBM_FILE' object on success. This object is then passed to other functions in order to manipulate the database. When the database is no longer needed, the programmer "closes" it using the 'gdbm_close' call. These and other functions are discussed in detail in chapters that follow. Here we show an example illustrating the use of 'GDBM' to look up a key in the database. #include #include #include int main (int argc, char **argv) { GDBM_FILE gdbf; /* Database file object pointer */ datum key, content; /* Key and content data */ int status = 0; /* Exit status of the program: 0 - OK, 1 - key not found, 2 - error. */ /* * Validate arguments. */ if (argc != 3) { fprintf (stderr, "usage: %s DBFILE KEY\n", argv[0]); return 2; } /* * Open the database. The GDBM_READER flag indicates that we only * intend to read from it. */ gdbf = gdbm_open (argv[1], 0, GDBM_READER, 0, NULL); if (gdbf == NULL) { fprintf (stderr, "can't open database: %s\n", gdbm_strerror (gdbm_errno)); } /* * Prepare the lookup key. Notice, that the terminating \0 character * is not counted in the dsize computation. */ key.dptr = argv[2]; key.dsize = strlen (argv[2]); /* * Look up the key in the database. */ content = gdbm_fetch (gdbf, key); /* * Analyze the return. */ if (content.dptr != NULL) { /* * The key is found. Print the content on the stdout and * indicate success. */ fwrite (content.dptr, content.dsize, 1, stdout); putchar ('\n'); status = 0; } else if (gdbm_errno == GDBM_ITEM_NOT_FOUND) { /* * There is no such key in the database. */ fprintf (stderr, "no such key\n"); status = 1; } else { /* * An error occurred. */ fprintf (stderr, "%s\n", gdbm_db_strerror (gdbf)); status = 2; } /* * Close the database and return. */ gdbm_close (gdbf); return status; } To compile this example, run cc -oexample example.c -lgdbm To run it, you will need an example database. The easiest way to create it is by using the 'gdbtool' program, which is part of the 'GDBM' package (*note gdbmtool::): $ gdbmtool test.gdbm store foo bar This creates database file 'test.gdbm' and stores a single record in it. The record's key is 'foo', and the value is 'bar'. Now you can run the example program to see how it works: $ ./example test.gdbm foo bar $ ./example test.gdbm baz no such key 3 Opening the database ********************** -- gdbm interface: GDBM_FILE gdbm_open (const char *NAME, int BLOCK_SIZE, int FLAGS, int MODE, void (*FATAL_FUNC)(const char *)) Opens or creates a 'GDBM' database file. The arguments are: NAME The name of the file (the complete name, 'GDBM' does not append any characters to this name). BLOCK_SIZE This parameter is used only when 'gdbm_open' has to create a new database file and represents the size of a single transfer from disk to memory. If its value is less than 512, the file system block size is used instead. The size is adjusted so that the block can hold exact number of directory entries, so that the effective block size can be slightly greater than requested. However, if the 'GDBM_BSEXACT' flag is set and the size needs to be adjusted, the function will return with error status, setting the 'gdbm_errno' variable to 'GDBM_BLOCK_SIZE_ERROR'. FLAGS If 'flags' is set to 'GDBM_READER', the user wants to just read the database and any call to 'gdbm_store' or 'gdbm_delete' will fail. Many readers can access the database at the same time. If 'flags' is set to 'GDBM_WRITER', the user wants both read and write access to the database and requires exclusive access. If 'flags' is set to 'GDBM_WRCREAT', the user wants both read and write access to the database and wants it created if it does not already exist. If 'flags' is set to 'GDBM_NEWDB', the user want a new database created, regardless of whether one existed, and wants read and write access to the new database. If an existing database file is opened with the 'GDBM_NEWDB' flag, the existing data are destroyed, and an empty database structure is created in its place. The following constants may also be logically or'd into the database flags: -- gdbm_open flag: GDBM_CLOEXEC Set the close-on-exec flag on the database file descriptor. The 'libc' must support the 'O_CLOEXEC' flag (*note (open(2))O_CLOEXEC::). -- gdbm_open flag: GDBM_NOLOCK Don't lock the database file. Use this flag if you intend to do locking separately. *Note Locking::. -- gdbm_open flag: GDBM_NOMMAP Disable memory mapping mechanism. Note, that this degrades performance. -- gdbm_open flag: GDBM_PREREAD When mapping 'GDBM' file to memory, read its contents immediately, instead of when needed ("prefault reading"). This can be advantageous if you open a _read-only_ database and are going to do a lot of look-ups on it. In this case entire database will be pre-read and look-ups will operate on an in-memory copy. In contrast, 'GDBM_PREREAD' should not be used if you open a database (even in read-only mode) only to do a couple of look-ups. Finally, never use 'GDBM_PREREAD' when opening a database for updates, especially for inserts: this will degrade performance. This flag has no effect if 'GDBM_NOMMAP' is given, or if the operating system does not support prefault reading. It is known to work on Linux and FreeBSD kernels. -- gdbm_open flag: GDBM_XVERIFY Enable additional consistency checks. With this flag, eventual corruptions of the database are discovered when opening it, instead of when a corrupted structure is read during normal operation. However, on large databases, it can slow down the opening process. *Note Additional functions::. The following additional flags are valid when the database is opened for writing (i.e. together with 'GDBM_WRITER', 'GDBM_WRCREAT', or 'GDBM_NEWDB'): -- gdbm_open flag: GDBM_SYNC Synchronize all database operations to disk immediately. Notice, that this option entails severe performance degradation and does not necessarily ensure that the resulting database state is consistent. In general, we discourage its use (*note Sync::). *Note Crash Tolerance::, for a discussion of how to ensure database consistency with minimal performance overhead. -- gdbm_open flag: GDBM_FAST A reverse of 'GDBM_SYNC'. Synchronize writes only when needed. This is the default. The flag is provided for compatibility with previous versions of 'GDBM'. The following flags can be used together with 'GDBM_NEWDB'. They also take effect when used with 'GDBM_WRCREAT', if the requested database file doesn't exist: -- gdbm_open flag: GDBM_BSEXACT If this flag is set and the requested BLOCK_SIZE cannot be used without adjustment, 'gdbm_open' will refuse to create the databases. In this case it will set the 'gdbm_errno' variable to 'GDBM_BLOCK_SIZE_ERROR' and return 'NULL'. -- gdbm_open flag: GDBM_NUMSYNC Useful only together with 'GDBM_NEWDB', this bit instructs 'gdbm_open' to create new database in "extended database format", a format best suitable for effective crash recovery. *Note Numsync::, for a detailed discussion of this format, and *note Crash Tolerance::, for a discussion of crash recovery. MODE File mode(1), which is used if the file is created. FATAL_FUNC This parameter is deprecated and must always be 'NULL'. Early versions of 'GDBM' (prior to 1.13) lacked proper error handling and would abort on any "fatal" error (such as out of memory condition, disk write error, or the like). In these versions, 'fatal_func' was provided as a hook, allowing the caller to do proper cleanup before such abnormal exit. As of version 1.23, this functionality is deprecated, although still supported for backward compatibility. The return value, is the pointer needed by all other functions to access that 'GDBM' file. If the return is the 'NULL' pointer, 'gdbm_open' was not successful. The errors can be found in 'gdbm_errno' variable (*note gdbm_errno: Variables.). Available error codes are discussed in *note Error codes::. In all of the following calls, the parameter DBF refers to the pointer returned from 'gdbm_open' (or 'gdbm_fd_open', described below). -- gdbm interface: GDBM_FILE gdbm_fd_open (int FD, const char *NAME, int BLOCK_SIZE, int FLAGS, int MODE, void (*FATAL_FUNC)(const char *)) Alternative function for opening a 'GDBM' database. The FD argument is the file descriptor of the database file obtained by a call to 'open'(2), 'creat'(2) or similar functions. The descriptor is not dup'ed, and will be closed when the returned 'GDBM_FILE' is closed. Use 'dup'(2) if that is not desirable. In case of error, the function behaves like 'gdbm_open' and _does not close_ FD. This can be altered by the following value passed in the FLAGS argument: -- gdbm_open flag: GDBM_CLOERROR Close FD before exiting on error. -- gdbm interface: int gdbm_copy_meta (GDBM_FILE DST, GDBM_FILE SRC) Copy file ownership and mode from SRC to DST. ---------- Footnotes ---------- (1) *Note (chmod(2))chmod::, and *Note open a file: (open(2))open. 4 Closing the database ********************** It is important that every file opened is also closed. This is needed to properly update its disk structure and maintain a consistent locking state on the file. -- gdbm interface: int gdbm_close (GDBM_FILE DBF) This function closes the 'GDBM' file and frees all memory associated with it. The parameter is: DBF The pointer returned by 'gdbm_open'. 'Gdbm_close' returns 0 on success. On error, it sets 'gdbm_errno' and system 'errno' variables to the codes describing the error and returns -1. 5 Number of Records ******************* -- gdbm interface: int gdbm_count (GDBM_FILE DBF, gdbm_count_t *PCOUNT) Counts the number of records in the database DBF. On success, stores it in the memory location pointed to by PCOUNT and returns 0. On error, sets 'gdbm_errno' (if relevant, also 'errno') and returns -1. -- gdbm interface: int gdbm_bucket_count (GDBM_FILE DBF, size_t *PCOUNT) Counts the number of buckets in the database DBF. On success, stores it in the memory location pointed to by PCOUNT and return 0. On error, sets 'gdbm_errno' (if relevant, also 'errno') and returns -1. 6 Inserting and replacing records in the database ************************************************* -- gdbm interface: int gdbm_store (GDBM_FILE DBF, datum KEY, datum CONTENT, int FLAG) The function 'gdbm_store' inserts or replaces records in the database. The parameters are: DBF The pointer returned by 'gdbm_open'. KEY The search key. CONTENT The data to be associated with the key. FLAG Defines the action to take when the key is already in the database. The value 'GDBM_REPLACE' asks that the old data be replaced by the new CONTENT. The value 'GDBM_INSERT' asks that an error be returned and no action taken if the KEY already exists. This function can return the following values: 0 Success. The value of CONTENT is keyed by KEY in the database. -1 An error occurred which prevented the item from being stored in the database. Examine the 'gdbm_errno' variable to determine the actual cause of the error. +1 The item was not stored because the argument FLAG was 'GDBM_INSERT' and the KEY was already in the database. The 'gdbm_errno' variable is set to 'GDBM_CANNOT_REPLACE'. If the function returns -1, 'gdbm_errno' can have the following values: 'GDBM_READER_CANT_STORE' Database was open in read-only mode, i.e. with the 'GDBM_READER' flag. *Note Open::. 'GDBM_MALFORMED_DATA' Either KEY or CONTENT had their 'dptr' field set to 'NULL'. It is OK to have a "zero-length" key or content, i.e. a datum with 'dsize' set to 0, but the 'dptr' field must always be a non-NULL value. 'GDBM_BAD_HASH_TABLE' Database hash table is malformed. This usually means that some error in the application or the library caused memory overrun. The database is marked as needing recovery. All further calls on this database will return with 'gdbm_error' set to 'GDBM_NEED_RECOVERY'. *Note Recovery::, for a discussion of database recovery process. 'GDBM_BAD_DIR_ENTRY' Database directory entry is corrupted. The database is marked as needing recovery. *Note Recovery::. 'GDBM_BAD_BUCKET' Database bucket is corrupted. The database is marked as needing recovery. *Note Recovery::. 'GDBM_BAD_AVAIL' Database available storage index is corrupted. The database is marked as needing recovery. *Note Recovery::. 'GDBM_FILE_SEEK_ERROR' A seek error occurred on the underlying disk file. Examine the system 'errno' variable for more detail. If you store data for a KEY that is already in the data base, 'GDBM' replaces the old data with the new data if called with 'GDBM_REPLACE'. You do not get two data items for the same 'key' and you do not get an error from 'gdbm_store'. The size of datum in 'GDBM' is restricted only by the maximum value for an object of type 'int' (type of the 'dsize' member of 'datum'). 7 Searching for records in the database *************************************** -- gdbm interface: datum gdbm_fetch (GDBM_FILE DBF, datum KEY) Looks up a given KEY and returns the information associated with it. The 'dptr' field in the structure that is returned points to a memory block allocated by 'malloc'. It is the caller's responsibility to free it when no longer needed. If the 'dptr' is 'NULL', inspect the value of the 'gdbm_errno' variable (*note gdbm_errno: Variables.). If it is 'GDBM_ITEM_NOT_FOUND', no data was found. Any other value means an error occurred. Use 'gdbm_strerror' function to convert 'gdbm_errno' to a human-readable string. The parameters are: DBF The pointer returned by 'gdbm_open'. KEY The search key. An example of using this function: content = gdbm_fetch (dbf, key); if (content.dptr == NULL) { if (gdbm_errno == GDBM_ITEM_NOT_FOUND) fprintf(stderr, "key not found\n"); else fprintf(stderr, "error: %s\n", gdbm_db_strerror (dbf)); } else { /* do something with content.dptr */ } You may also search for a particular key without retrieving it: -- gdbm interface: int gdbm_exists (GDBM_FILE DBF, datum KEY) Checks whether the KEY exists in the database DBF. If KEY is found, returns 'true' ('1'). If it is not found, returns 'false' ('0') and sets 'gdbm_errno' to 'GDBM_NO_ERROR' ('0'). On error, returns '0' and sets 'gdbm_errno' to a non-'0' error code. The parameters are: DBF The pointer returned by 'gdbm_open'. KEY The search key. 8 Removing records from the database ************************************ To remove some data from the database, use the 'gdbm_delete' function. -- gdbm interface: int gdbm_delete (GDBM_FILE DBF, datum KEY) Deletes the data associated with the given KEY, if it exists in the database DBF. The parameters are: DBF The pointer returned by 'gdbm_open'. DATUM KEY The search key. The function returns '-1' if the item is not present or if an error is encountered. Examine the 'gdbm_errno' variable or the return from 'gdbm_last_errno (DBF)' to know the reason. The return of '0' marks a successful delete. 9 Sequential access to records ****************************** The next two functions allow for accessing all items in the database. This access is not 'key' sequential, but it is guaranteed to visit every 'key' in the database once. The order has to do with the hash values. 'gdbm_firstkey' starts the visit of all keys in the database. 'gdbm_nextkey' finds and reads the next entry in the hash structure for 'dbf'. -- gdbm interface: datum gdbm_firstkey (GDBM_FILE DBF) Initiate sequential access to the database DBF. The returned value is the first key accessed in the database. If the 'dptr' field in the returned datum is 'NULL', inspect the 'gdbm_errno' variable (*note gdbm_errno: Variables.). The value of 'GDBM_ITEM_NOT_FOUND' means that the database contains no data. Other value means an error occurred. On success, 'dptr' points to a memory block obtained from 'malloc', which holds the key value. The caller is responsible for freeing this memory block when no longer needed. -- gdbm interface: datum gdbm_nextkey (GDBM_FILE DBF, datum PREV) This function continues iteration over the keys in DBF, initiated by 'gdbm_firstkey'. The parameter PREV holds the value returned from a previous call to 'gdbm_nextkey' or 'gdbm_firstkey'. The function returns next key from the database. If the 'dptr' field in the returned datum is 'NULL' inspect the 'gdbm_errno' variable (*note gdbm_errno: Variables.). The value of 'GDBM_ITEM_NOT_FOUND' means that all keys in the database has been visited. Any other value means an error occurred. Otherwise, 'dptr' points to a memory block obtained from 'malloc', which holds the key value. The caller is responsible for freeing this memory block when no longer needed. These functions are intended to visit the database in read-only algorithms, for instance, to validate the database or similar operations. The usual algorithm for sequential access is: key = gdbm_firstkey (dbf); while (key.dptr) { datum nextkey; /* do something with the key */ ... /* Obtain the next key */ nextkey = gdbm_nextkey (dbf, key); /* Reclaim the memory used by the key */ free (key.dptr); /* Use nextkey in the next iteration. */ key = nextkey; } Don't use 'gdbm_delete' or 'gdbm_store' in such a loop. File visiting is based on a "hash table". The 'gdbm_delete' function re-arranges the hash table to make sure that any collisions in the table do not leave some item "un-findable". The original key order is _not_ guaranteed to remain unchanged in all instances. So it is possible that some key will not be visited or will be visited twice, if a loop like the following is executed: key = gdbm_firstkey (dbf); while (key.dptr) { datum nextkey; if (some condition) { gdbm_delete (dbf, key); } nextkey = gdbm_nextkey (dbf, key); free (key.dptr); key = nextkey; } 10 Database reorganization ************************** The following function should be used very seldom. -- gdbm interface: int gdbm_reorganize (GDBM_FILE DBF) Reorganizes the database. The parameter is: DBF The pointer returned by 'gdbm_open'. If you have had a lot of deletions and would like to shrink the space used by the 'GDBM' file, this function will reorganize the database. This results, in particular, in shortening the length of a 'GDBM' file by removing the space occupied by deleted records. This reorganization requires creating a new file and inserting all the elements in the old file DBF into the new file. The new file is then renamed to the same name as the old file and DBF is updated to contain all the correct information about the new file. If an error is detected, the return value is negative. The value zero is returned after a successful reorganization. 11 Database Synchronization *************************** Normally, 'GDBM' functions don't flush changed data to the disk immediately after a change. This allows for faster writing of databases at the risk of having a corrupted database if the application terminates in an abnormal fashion. The following function allows the programmer to make sure the disk version of the database has been completely updated with all changes to the current time. -- gdbm interface: int gdbm_sync (GDBM_FILE DBF) Synchronizes the changes in DBF with its disk file. The parameter is a pointer returned by 'gdbm_open'. This function would usually be called after a complete set of changes have been made to the database and before some long waiting time. This set of changes should preserve application-level invariants. In other words, call 'gdbm_sync' only when the database is in a consistent state with regard to the application logic, a state from which you are willing and able to recover. You can think about all database operations between two consecutive 'gdbm_sync' calls as constituting a single "transaction". *Note Synchronizing the Database::, for a detailed discussion about how to properly select the synchronization points. The 'gdbm_close' function automatically calls the equivalent of 'gdbm_sync' so no call is needed if the database is to be closed immediately after the set of changes have been made. 'Gdbm_sync' returns 0 on success. On error, it sets 'gdbm_errno' and system 'errno' variables to the codes describing the error and returns -1. Opening the database with 'GDBM_SYNC' flag ensures that 'gdbm_sync' function will be called after each change, thereby flushing the changes to disk immediately. You are advised against using this flag, however, because it incurs a severe performance penalty, while giving only a moderate guarantee that the _structural_ consistency of the database will be preserved in case of failure, and that only unless the failure occurs while being in the 'fsync' call. For the ways to ensure proper _logical_ consistency of the database, see *note Crash Tolerance::. 12 Changing database format *************************** As of version 1.23, 'GDBM' supports databases in two formats: "standard" and "extended". The standard format is used most often. The "extended" database format is used to provide additional crash resistance (*note Crash Tolerance::). Depending on the value of the FLAGS parameter in a call to 'gdbm_open' (*note GDBM_NUMSYNC: Open.), a database can be created in either format. The format of an existing database can be changed using the 'gdbm_convert' function: -- gdbm interface: int gdbm_convert (GDBM_FILE DBF, int FLAG) Changes the format of the database file DBF. Allowed values for FLAG are: '0' Convert database to the standard format. 'GDBM_NUMSYNC' Convert database to the extended "numsync" format (*note Numsync::). On success, the function returns 0. In this case, it should be followed by a call to 'gdbm_sync' (*note Sync::) or 'gdbm_close' (*note Close::) to ensure the changes are written to the disk. On error, returns -1 and sets the 'gdbm_errno' variable (*note gdbm_errno: Variables.). If the database is already in the requested format, the function returns success (0) without doing anything. 13 Export and Import ******************** 'GDBM' databases can be converted into so-called "flat format" files. Such files cannot be used for searching, their sole purpose is to keep the data from the database for restoring it when the need arrives. There are two flat file formats, which differ in the way they represent the data and in the amount of meta-information stored. Both formats can be used, for example, to migrate between the different versions of 'GDBM' databases. Generally speaking, flat files are safe to send over the network, and can be used to recreate the database on another machine. The recreated database is guaranteed to have the same format and contain the same set of key/value pairs as the database from which the flat file was created. However, it will not constitute a byte-to-byte equivalent of the latter. Various internal structures in the database can differ. In particular, ordering of key/value pairs can be different and the table of available file space will most probably differ, too. For databases in extended format, the 'numsync' counter will be reset to 0 (*note Numsync::). These details are not visible to the application programmer, and are mentioned here only for completeness sake. The fact that the restored database contains the same set of key/value pairs does not necessarily mean, however, that it can be used in the same way as the original one. For example, if the original database contained non-ASCII data (e.g. C structures, integers etc.), the recreated database can be of any use only if the target machine has the same integer size and byte ordering as the source one and if its C compiler uses the same packing conventions as the one which generated C which populated the original database. In general, such binary databases are not portable between machines, unless you follow some stringent rules on what data is written to them and how it is interpreted. 'GDBM' version 1.23 supports two flat file formats. The "binary" flat file format was first implemented in version 1.9.1. This format stores only key/data pairs, it does not keep information about the database file itself. As its name implies, files in this format are binary files. This format is supported for backward compatibility. The "ascii" flat file format encodes all data in Base64 and stores not only key/data pairs, but also the original database file metadata, such as file name, mode and ownership. Files in this format can be sent without additional encapsulation over transmission channels that normally allow only ASCII data, such as, e.g. SMTP. Due to additional metadata they allow for restoring an exact copy of the database, including file ownership and privileges, which is especially important if the database in question contained some security-related data. We call a process of creating a flat file from a database "exporting" or "dumping" this database. The reverse process, creating the database from a flat file is called "importing" or "loading" the database. -- gdbm interface: int gdbm_dump (GDBM_FILE DBF, const char *FILENAME, int FORMAT, int OPEN_FLAGS, int MODE) Dumps the database file to the named file in requested format. Arguments are: DBF A pointer to the source database, returned by a prior call to 'gdbm_open'. FILENAME Name of the dump file. FORMAT Output file format. Allowed values are: 'GDBM_DUMP_FMT_BINARY' to create a binary dump and 'GDBM_DUMP_FMT_ASCII' to create an ASCII dump file. OPEN_FLAGS How to create the output file. If FLAG is 'GDBM_WRCREAT' the file will be created if it does not exist. If it does exist, the 'gdbm_dump' will fail. If FLAG is 'GDBM_NEWDB', the function will create a new output file, replacing it if it already exists. MODE The permissions to use when creating the output file (*note open a file: (open(2))open.). -- gdbm interface: int gdbm_load (GDBM_FILE *PDBF, const char *FILENAME, int FLAG, int META_MASK, unsigned long *ERRLINE) Loads data from the dump file FILENAME into the database pointed to by PDBF. The latter can point to 'NULL', in which case the function will try to create a new database. If it succeeds, the function will return, in the memory location pointed to by PDBF, a pointer to the newly created database. If the dump file carries no information about the original database file name, the function will set 'gdbm_errno' to 'GDBM_NO_DBNAME' and return '-1', indicating failure. The FLAG has the same meaning as the FLAG argument to the 'gdbm_store' function (*note Store::). The META_MASK argument can be used to disable restoring certain bits of file's meta-data from the information in the input dump file. It is a binary OR of zero or more of the following: GDBM_META_MASK_MODE Do not restore file mode. GDBM_META_MASK_OWNER Do not restore file owner. The function returns 0 upon successful completion or -1 on fatal errors and 1 on mild (non-fatal) errors. If a fatal error occurs, 'gdbm_errno' will be set to one of the following values: GDBM_FILE_OPEN_ERROR Input file (FILENAME) cannot be opened. The 'errno' variable can be used to get more detail about the failure. GDBM_MALLOC_ERROR Not enough memory to load data. GDBM_FILE_READ_ERROR Reading from FILENAME failed. The 'errno' variable can be used to get more detail about the failure. GDBM_MALFORMED_DATA GDBM_ILLEGAL_DATA Input contained malformed data, i.e. it is not a valid 'GDBM' dump file. This often means that the dump file got corrupted during the transfer. The 'GDBM_ILLEGAL_DATA' is an alias for this error code, maintained for backward compatibility. GDBM_ITEM_NOT_FOUND This error can occur only when the input file is in ASCII format. It indicates that the data part of the record about to be read lacked length specification. Application developers are advised to treat this error equally as 'GDBM_MALFORMED_DATA'. Mild errors mean that the function was able to successfully load and restore the data, but was unable to change the database file metadata afterwards. The table below lists possible values for 'gdbm_errno' in this case. To get more detail, inspect the system 'errno' variable. GDBM_ERR_FILE_OWNER The function was unable to restore database file owner. GDBM_ERR_FILE_MODE The function was unable to restore database file mode (permission bits). If an error occurs while loading data from an input file in ASCII format, the number of line in which the error occurred will be stored in the location pointed to by the ERRLINE parameter, unless it is 'NULL'. If the line information is not available or applicable, ERRLINE will be set to '0'. -- gdbm interface: int gdbm_dump_to_file (GDBM_FILE DBF, FILE *FP, int FORMAT) This is an alternative entry point to 'gdbm_dump' (which see). Arguments are: DBF A pointer to the source database, returned by a call to 'gdbm_open'. FP File to write the data to. FORMAT Format of the dump file. See the FORMAT argument to the 'gdbm_dump' function. -- gdbm interface: int gdbm_load_from_file (GDBM_FILE *PDBF, FILE *FP, int REPLACE, int META_MASK, unsigned long *LINE) This is an alternative entry point to 'gdbm_load'. It writes the output to FP which must be a file open for writing. The rest of arguments is the same as for 'gdbm_load' (excepting of course FLAG, which is not needed in this case). -- gdbm interface: int gdbm_export (GDBM_FILE DBF, const char *EXPORTFILE, int FLAG, int MODE) This function is retained for compatibility with GDBM 1.10 and earlier. It dumps the database to a file in binary dump format and is equivalent to gdbm_dump(DBF, EXPORTFILE, GDBM_DUMP_FMT_BINARY, FLAG, MODE) -- gdbm interface: int gdbm_export_to_file (GDBM_FILE DBF, FILE *FP) This is an alternative entry point to 'gdbm_export'. This function writes to file FP a binary dump of the database DBF. -- gdbm interface: int gdbm_import (GDBM_FILE DBF, const char *IMPORTFILE, int FLAG) This function is retained for compatibility with 'GDBM' 1.10 and earlier. It loads the file IMPORTFILE, which must be a binary flat file, into the database DBF and is equivalent to the following construct: DBF = gdbm_open (IMPORTFILE, 0, FLAG == GDBM_REPLACE ? GDBM_WRCREAT : GDBM_NEWDB, 0600, NULL); gdbm_load (&DBF, EXPORTFILE, 0, FLAG, NULL) -- gdbm interface: int gdbm_import_from_file (GDBM_FILE DBF, FILE *FP, int FLAG) An alternative entry point to 'gdbm_import'. Reads the binary dump from the file FP and stores the key/value pairs to DBF. *Note Store::, for a description of FLAG. This function is equivalent to: DBF = gdbm_open (IMPORTFILE, 0, FLAG == GDBM_REPLACE ? GDBM_WRCREAT : GDBM_NEWDB, 0600, NULL); gdbm_load_from_file (DBF, FP, FLAG, 0, NULL); 14 Error handling ***************** The global variable 'gdbm_errno' (*note gdbm_errno: Variables.) keeps the error code of the most recent error encountered by 'GDBM' functions. To convert this code to human-readable string, use the following function: -- gdbm interface: const char * gdbm_strerror (gdbm_error ERRNO) Converts ERRNO (an integer value) into a human-readable descriptive text. Returns a pointer to a static string. The caller must not free the returned pointer or alter the string it points to. Detailed information about the most recent error that occurred while operating on a 'GDBM' file is stored in the 'GDBM_FILE' object itself. To retrieve it, the following functions are provided: -- gdbm interface: gdbm_error gdbm_last_errno (GDBM_FILE DBF) Returns the code of the most recent error encountered when operating on DBF. When 'gdbm_last_errno' called immediately after the failed function, its return equals the value of the 'gdbm_errno' variable. However, 'gdbm_errno' can be changed if any 'GDBM' functions (operating on another databases) were called afterwards, and 'gdbm_last_errno' will always return the code of the last error that occurred while working with _that_ database. -- gdbm interface: int gdbm_last_syserr (GDBM_FILE DBF) Returns the value of the system 'errno' variable associated with the most recent error. Notice, that not all 'GDBM' errors have an associated system error code. The following are the ones that have: * GDBM_FILE_OPEN_ERROR * GDBM_FILE_WRITE_ERROR * GDBM_FILE_SEEK_ERROR * GDBM_FILE_READ_ERROR * GDBM_FILE_STAT_ERROR * GDBM_BACKUP_FAILED * GDBM_BACKUP_FAILED * GDBM_FILE_CLOSE_ERROR * GDBM_FILE_SYNC_ERROR * GDBM_FILE_TRUNCATE_ERROR For other errors, 'gdbm_last_syserr' will return 0. -- gdbm interface: int gdbm_check_syserr (gdbm_errno ERR) Returns '1', if the system 'errno' value should be inspected to get more info on the error described by 'GDBM' error code ERR. To get a human-readable description of the recent error for a particular database file, use the 'gdbm_db_strerror' function: -- gdbm interface: const char * gdbm_db_strerror (GDBM_FILE DBF) Returns textual description of the most recent error encountered when operating on the database DBF. The resulting string is often more informative than what would be returned by 'gdbm_strerror(gdbm_last_errno(DBF))'. In particular, if there is a system error associated with the recent failure, it will be described as well. -- gdbm interface: void gdbm_clear_error (GDBM_FILE DBF) Clears the error state for the database DBF. Normally, this function is called upon the entry to any 'GDBM' function. Certain errors (such as write error when saving stored key) can leave database file in inconsistent state (*note Database consistency::). When such a critical error occurs, the database file is marked as needing recovery. Subsequent calls to any 'GDBM' functions for that database file (except 'gdbm_recover'), will return immediately with 'GDBM' error code 'GDBM_NEED_RECOVERY'. Additionally, the following function can be used to check the state of the database file: -- gdbm interface: int gdbm_needs_recovery (GDBM_FILE DBF) Returns '1' if the database file DBF is in inconsistent state and needs recovery. To restore structural consistency of the database, use the 'gdbm_recover' function (*note Recovery::). Crash tolerance provides a better way of recovery, because it restores both structural and logical consistency. *Note Crash Tolerance::, for a detailed discussion, 15 Database consistency *********************** In the chapters that follow we will cover different aspects of "database consistency" and ways to maintain it. Speaking about consistency, it is important to distinguish between two different aspects of it: structural and logical consistency. "Structural consistency" means that all internal structures of the database are in good order, contain valid data and are coherent with one another. Structural consistency means that the database is in good shape "technically", but it does not imply that the data it contains are in any way meaningful. "Logical consistency" means that the data stored in the database are coherent with respect to the application logic. Usually this implies that structural consistency is observed as well. For as long as the program is free from memory management errors and each opened database is properly closed before the program terminates, structural consistency is maintained. Maintaining logical consistency is more complex task and its maintenance is entirely the responsibility of the application programmer. *Note Crash Tolerance::, for a detailed discussion. Both consistency aspects can suffer as a result of both application errors that cause the program to terminate prematurely without properly saving the database, and hardware errors, such as disk failures or power outages. When such situations occur, it becomes necessary to "recover the database". In the next chapter we will discuss how to recover structural consistency of a database. 16 Recovering structural consistency ************************************ Certain errors (such as write error when saving stored key) can leave database file in "structurally inconsistent state". When such a critical error occurs, the database file is marked as needing recovery. Subsequent calls to any GDBM functions for that database file (except 'gdbm_recover'), will return immediately with 'GDBM' error code 'GDBM_NEED_RECOVERY'. To escape from this state and bring the database back to operational state, use the following function: -- gdbm interface: int gdbm_recover (GDBM_FILE DBF, gdbm_recovery *RCVR, int FLAGS) Check the database file DBF and fix eventual errors. The RCVR argument points to a structure that has "input members", providing additional information to alter the behavior of 'gdbm_recover', and "output members", which are used to return additional statistics about the recovery process (RCVR can be 'NULL' if no such information is needed). Each input member has a corresponding flag bit, which must be set in FLAGS, in order to instruct the function to use it. The 'gdbm_recover' type is defined as: typedef struct gdbm_recovery_s { /* Input members. These are initialized before call to gdbm_recover. The flags argument specifies which of them are initialized. */ void (*errfun) (void *data, char const *fmt, ...); void *data; size_t max_failed_keys; size_t max_failed_buckets; size_t max_failures; /* Output members. The gdbm_recover function fills these before returning. */ size_t recovered_keys; size_t recovered_buckets; size_t failed_keys; size_t failed_buckets; char *backup_name; } gdbm_recovery; The "input members" modify the behavior of 'gdbm_recover': -- input member of gdbm_recovery: void (*errfun) (void *DATA, char const *FMT, ...) If the 'GDBM_RCVR_ERRFUN' flag bit is set, 'errfun' points to a function that will be called upon each recoverable or non-fatal error that occurred during the recovery. The 'data' field of 'gdbm_recovery' will be passed to it as its first argument. The FMT argument is a 'printf'-like (*note (printf(3))Format of the format string::), format string. The rest of arguments supply parameters for that format. -- input member of gdbm_recovery: void * data Supplies first argument for the 'errfun' invocations. -- input member of gdbm_recovery: size_t max_failed_keys If 'GDBM_RCVR_MAX_FAILED_KEYS' is set, this member sets the limit on the number of keys that cannot be retrieved. If the number of failed keys becomes equal to 'max_failed_keys', recovery is aborted and error is returned. -- input member of gdbm_recovery: size_t max_failed_buckets If 'GDBM_RCVR_MAX_FAILED_BUCKETS' is set, this member sets the limit on the number of buckets that cannot be retrieved or that contain bogus information. If the number of failed buckets becomes equal to 'max_failed_buckets', recovery is aborted and error is returned. -- output member of gdbm_recovery: size_t max_failures If 'GDBM_RCVR_MAX_FAILURES' is set, this member sets the limit of failures that are tolerated during recovery. If the number of errors becomes equal to 'max_failures', recovery is aborted and error is returned. The following members are filled on output, upon successful return from the function: -- output member of gdbm_recovery: size_t recovered_keys Number of recovered keys. -- output member of gdbm_recovery: size_t recovered_buckets Number of recovered buckets. -- output member of gdbm_recovery: size_t failed_keys Number of key/data pairs that could not be retrieved. -- output member of gdbm_recovery: size_t failed_buckets Number of buckets that could not be retrieved. -- output member of gdbm_recovery: char * backup_name Name of the file keeping the copy of the original database, in the state prior to recovery. It is filled if the GDBM_RCVR_BACKUP flag is set. The string is allocated using the 'malloc' call. The caller is responsible for freeing that memory when no longer needed. By default, 'gdbm_recovery' first checks the database for inconsistencies and attempts recovery only if some were found. The special flag bit 'GDBM_RCVR_FORCE' instructs 'gdbm_recovery' to omit this check and to perform database recovery unconditionally. 17 Crash Tolerance ****************** Crash tolerance is a new (as of release 1.21) feature that can be enabled at compile time, and used in environments with appropriate support from the OS and the filesystem. As of version 1.23, this means a Linux kernel 5.12.12 or later and a filesystem that supports reflink copying, such as XFS, BtrFS, or OCFS2. If these prerequisites are met, crash tolerance code will be enabled automatically by the 'configure' script when building the package. The crash-tolerance mechanism, when used correctly, guarantees that a logically consistent (*note Database consistency::) recent state of application data can be recovered following a crash. Specifically, it guarantees that the state of the database file corresponding to the most recent successful 'gdbm_sync' call can be recovered. If the new mechanism is used correctly, crashes such as power outages, OS kernel panics, and (some) application process crashes will be tolerated. Non-tolerated failures include physical destruction of storage devices and corruption due to bugs in application logic. For example, the new mechanism won't help if a pointer bug in your application corrupts 'GDBM''s private in-memory data which in turn corrupts the database file. In the following sections we will describe how to enable crash tolerance in your application and what to do if a crash occurs. The design rationale of the crash tolerance mechanism is described in detail in the article, 'Crashproofing the Original NoSQL Key-Value Store', by Terence Kelly, 'ACM Queue magazine', July/August 2021, available from the ACM Digital Library (https://queue.acm.org/DrillBits5/). If you have difficulty retrieving this paper, please contact the author at , , or . 17.1 Using Proper Filesystem ============================ Use a filesystem that supports reflink copying. Currently XFS, BtrFS, and OCFS2 support reflink. You can create such a filesystem if you don't have one already. (Note that reflink support may require that special options be specified at the time of filesystem creation; this is true of XFS.) The most conventional way to create a filesystem is on a dedicated storage device. However it is also possible to create a filesystem _within an ordinary file_ on some other filesystem. For example, the following commands, executed as root, will create a smallish XFS filesystem inside a file on another filesystem: mkdir XFS cd XFS truncate --size 512m XFSfile mkfs -t xfs -m crc=1 -m reflink=1 XFSfile mkdir XFSmountpoint mount -o loop XFSfile XFSmountpoint The XFS filesystem is now available in directory 'XFSmountpoint'. Now, create a directory where your unprivileged user account may create and delete files: cd XFSmountpoint mkdir test chown USER:GROUP test (where USER and GROUP are the user and group names of the unprivileged account the application uses). Reflink copying via 'ioctl(FICLONE)' should work for files in and below this directory. You can test reflink copying using the GNU 'cp' program: cp --reflink=always file1 file2 *Note reflink: (coreutils)cp invocation. Your GNU dbm database file and two "snapshot" files described below must all reside on the same reflink-capable filesystem. 17.2 Enabling crash tolerance ============================= Open a GNU dbm database with 'gdbm_open'. Whenever possible, use the extended 'GDBM' format (*note Numsync::). Generally speaking, this means using the 'GDBM_NUMSYNC' flag when creating the database. Unless you know what you are doing, do not specify the 'GDBM_SYNC' flag when opening the database. The reason is that you want your application to explicitly control when 'gdbm_sync' is called; you don't want an implicit sync on every database operation (*note Sync::). Request crash tolerance by invoking the following interface: int gdbm_failure_atomic (GDBM_FILE DBF, const char *EVEN, const char *ODD); The EVEN and ODD arguments are the pathnames of two files that will be created and filled with "snapshots" of the database file. These two files must not exist when 'gdbm_failure_atomic' is called and must reside on the same reflink-capable filesystem as the database file. After you call 'gdbm_failure_atomic', every call to 'gdbm_sync' will make an efficient reflink snapshot of the database file in either the EVEN or the ODD snapshot file; consecutive 'gdbm_sync' calls alternate between the two, hence the names. The permission bits and 'mtime' timestamps on the snapshot files determine which one contains the state of the database file corresponding to the most recent successful 'gdbm_sync'. *Note Crash recovery::, for discussion of crash recovery. 17.3 Synchronizing the Database =============================== When your application knows that the state of the database is consistent (i.e., all relevant application-level invariants hold), you may call 'gdbm_sync'. For example, if your application manages bank accounts, transferring money from one account to another should maintain the invariant that the sum of the two accounts is the same before and after the transfer: It is correct to decrement account 'A' by $7, increment account 'B' by $7, and then call 'gdbm_sync'. However it is _not_ correct to call 'gdbm_sync' _between_ the decrement of 'A' and the increment of 'B', because a crash immediately after that call would destroy money. The general rule is simple, sensible, and memorable: Call 'gdbm_sync' only when the database is in a state from which you are willing and able to recover following a crash. (If you think about it you'll realize that there's never any other moment when you'd really want to call 'gdbm_sync', regardless of whether crash-tolerance is enabled. Why on earth would you push the state of an inconsistent unrecoverable database down to durable media?). 17.4 Crash recovery =================== If a crash occurs, the snapshot file (EVEN or ODD) containing the database state reflecting the most recent successful 'gdbm_sync' call is the snapshot file whose permission bits are read-only and whose last-modification timestamp is greatest. If both snapshot files are readable, we choose the one with the most recent last-modification timestamp. Modern operating systems record timestamps in nanoseconds, which gives sufficient confidence that the timestamps of the two snapshots will differ. However, one can't rule out the possibility that the two snapshot files will both be readable and have identical timestamps(1). To cope with this, 'GDBM' version 1.21 introduced the new "extended database format", which stores in the database file header the number of synchronizations performed so far. This number can reliably be used to select the most recent snapshot, independently of its timestamp. We strongly suggest using this new format when writing crash-tolerant applications. *Note Numsync::, for a detailed discussion. The 'gdbm_latest_snapshot' function is provided, that selects the right snapshot among the two. Invoke it as: const char *recovery_file = NULL; result = gdbm_latest_snapshot (even, odd, &recovery_file); where EVEN and ODD are names of the snapshot files. On success, it stores the pointer to the most recent snapshot file name in RECOVERY_FILE and returns 'GDBM_SNAPSHOT_OK'. To finalize the recovery, rename this file to the name of your database file and re-open it using 'gdbm_open'. You should discard the remaining snapshot. If an error occurs, 'gdbm_latest_snapshot' returns one of the following error codes. -- gdbm_latest_snapshot: GDBM_SNAPSHOT_BAD Neither snapshot file is readable. This means that the crash has occurred before 'gdbm_failure_atomic' completed. In this case, it is best to fall back on a safe backup copy of the data file. -- gdbm_latest_snapshot: GDBM_SNAPSHOT_ERR System error occurred in 'gdbm_latest_snapshot'. Examine the system 'errno' variable for details. Its possible values are: 'EACCES' The file mode of one of the snapshot files was incorrect. Each snapshot file can be either readable (0400) or writable (0200), but not both. This probably means that someone touched one or both snapshot files after the crash and before your attempt to recover from it. This case needs additional investigation. If you're sure that the only change someone made to the files is altering their modes, and your database is in "numsync" format (*note Numsync::), you can reset the modes to 0400 and retry the recovery. This error can also be returned by underlying 'stat' call, meaning that search permission was denied for one of the directories in the path prefix of a snapshot file name. That again means that someone has messed with permissions after the crash. 'EINVAL' Some arguments passed to 'gdbm_latest_snapshot' were not valid. It is a programmer's error which means that your application needs to be fixed. 'ENOSYS' Function is not implemented. This means 'GDBM' was built without crash-tolerance support. 'Other value (EBADF, EFAULT, etc)' An error occurred when trying to 'stat' the snapshot file. *Note (stat(2))ERRORS::, for a discussion of possible 'errno' values. -- gdbm_latest_snapshot: GDBM_SNAPSHOT_SAME File modes and modification dates of both snapshot files are exactly the same. This can happen only if numsync is not available (*note Numsync::). -- gdbm_latest_snapshot: GDBM_SNAPSHOT_SUSPICIOUS For the database in extended "numsync" format (*note Numsync::): the 'numsync' values of the two snapshot differ by more than one. Check the arguments to the 'gdbm_latest_snapshot' function. The most probably reason of such an error is that the EVEN and ODD parameters point to snapshot files belonging to different database files. If you get any of these errors, we strongly suggest to undertake "manual recovery". ---------- Footnotes ---------- (1) This can happen, for example, if the storage is very fast and the system clock is low-resolution, or if the system administrator sets the system clock backwards. In the latter case one can end up with the most recent snapshot file having modification time earlier than that of the obsolete snapshot. 17.5 Manual crash recovery ========================== "Manual recovery" is usually performed with the help of the 'gdbmtool' utility. Start 'gdbmtool' in read-only mode (the '-r') option. Once in the command shell, issue the following command: snapshot A B where A and B are names of the two snapshot files you configured using the 'gdbm_failure_atomic' function. This command investigates both files and prints out detailed diagnostics. Its output begins with a line listing one of the error codes above, followed by a colon and a textual description of the error. The lines that follow show details for each snapshot file. Each snapshot description begins with the snapshot file name followed by a colon and four fields, in this order: 1. File permission bits in octal. 2. File permission bits in 'ls -l' notation. 3. Modification timestamp. 4. Numsync counter. For databases in standard 'GDBM' format, this field is 'N/A'. If the counter cannot be obtained because of error, this field is '?'. Any errors or inconsistencies discovered are reported in the lines that follow, one error per line. Here's an example of the 'snapshot' command output, describing the 'GDBM_SNAPSHOT_ERR' condition: gdbmtool> snapshot even.dbf odd.dbf GDBM_SNAPSHOT_ERR: Error selecting snapshot. even.dbf: 200 -w------- 1627820627.485681330 ? odd.dbf: 600 rw------- 1627820627.689503918 301 odd.dbf: ERROR: bad file mode Line 2 lists the meta-data of the snapshot 'even.dbf'. The 'numsync' field contains question mark because the file permissions (write-only) prevented 'gdbmtool' from opening it. The lines for 'odd.dbf' show the actual reason for the error: bad file mode (read-write). Apparently, the file mode has been changed manually after the crash. The timestamp of the file, which is more recent than that of 'even.dbf', suggests that it might be used for recovery. To confirm this guess, change the mode of the 'even.dbf' to read-only and repeat the 'snapshot' command: gdbmtool> ! chmod 400 even.dbf gdbmtool> snapshot even.dbf odd.dbf GDBM_SNAPSHOT_ERR: Error selecting snapshot. even.dbf: 400 r-------- 1627820627.485681330 300 odd.dbf: 600 rw------- 1627820627.689503918 301 odd.dbf: ERROR: bad file mode This shows the numsync value of the 'even.dbf' file, which is exactly one less than that of 'odd.dbf'. This means that the latter should be selected for recovery. For completeness sake, you can change the mode of 'odd.dbf' to read-only as well and repeat the 'snapshot' command. In this case you will see: gdbmtool> ! chmod 400 odd.dbf gdbmtool> snapshot even.dbf odd.dbf GDBM_SNAPSHOT_OK: Selected the most recent snapshot. odd.dbf: 400 r-------- 1627820627.689503918 301 17.6 Performance Impact ======================= The purpose of a parachute is not to hasten descent. Crash tolerance is a safety mechanism, not a performance accelerator. Reflink copying is designed to be as efficient as possible, but making snapshots of the GNU dbm database file on every 'gdbm_sync' call entails overheads. The performance impact of 'GDBM' crash tolerance will depend on many factors including the type and configuration of the underlying storage system, how often the application calls 'gdbm_sync', and the extent of changes to the database file between consecutive calls to 'gdbm_sync'. 17.7 Availability ================= To ensure that application data can survive the failure of one or more storage devices, replicated storage (e.g., RAID) may be used beneath the reflink-capable filesystem. Some cloud providers offer block storage services that mimic the interface of individual storage devices but that are implemented as high-availability fault-tolerant replicated distributed storage systems. Installing a reflink-capable filesystem atop a high-availability storage system is a good starting point for a high-availability crash-tolerant 'GDBM'. 17.8 Numsync Extension ====================== In *note Crash recovery::, we have shown that for database recovery, one should select the snapshot whose permission bits are read-only and whose last-modification timestamp is greatest. However, there may be cases when a crash occurs at such a time that both snapshot files remain readable. It may also happen, that their permissions had been reset to read-only and/or modification times inadvertently changed before recovery. To make it possible to select the right snapshot in such cases, a new "extended database format" was introduced in 'GDBM' version 1.21. This format adds to the database header the 'numsync' field, which holds the number of synchronizations the database underwent before being closed or abandoned due to a crash. A readable snapshot is a consistent copy of the database at a given point of time. Thus, if both snapshots of a database in extended format are readable, it will suffice to examine their 'numsync' counters and select the one whose 'numsync' is greater. That's what the 'gdbm_latest_snapshot' function does in this case. It is worth noticing, that the two counters should differ exactly by one. If the difference is greater than that, 'gdbm_latest_snapshot' will return a special status code, 'GDBM_SNAPSHOT_SUSPICIOUS'. If, during a recovery attempt, you get this status code, we recommend to proceed with the manual recovery (*note Manual crash recovery::). To create a database in extended format, call 'gdbm_open' with both 'GDBM_NEWDB' and 'GDBM_NUMSYNC' flags: dbf = gdbm_open(dbfile, 0, GDBM_NEWDB|GDBM_NUMSYNC, 0600, NULL); Notice, that this flag must always be used together with 'GDBM_NEWDB' (*note Open::). It is silently ignored when used together with another opening flag. A standard 'GDBM' database can be converted to the extended format and vice versa. To convert an existing database to the extended format, use the 'gdbm_convert' function (*note Database format::): rc = gdbm_convert(dbf, GDBM_NUMSYNC); You can do the same using the 'gdbmtool' utility (*note upgrade: commands.): gdbmtool DBNAME upgrade To convert a database from extended format back to the standard 'GDBM' format, do: rc = gdbm_convert(dbf, 0); To do the same from the command line, run: gdbmtool DBNAME downgrade 17.9 Crash Tolerance API ======================== -- gdbm interface: int gdbm_failure_atomic (GDBM_FILE DBF, const char *EVEN, const char *ODD) Enables crash tolerance for the database file DBF. The EVEN and ODD arguments are the pathnames of two files that will be created and filled with snapshots of the database file. These two files must not exist when 'gdbm_failure_atomic' is called and must reside on the same reflink-capable filesystem as the database file. Returns 0 on success. On failure, returns -1 and sets 'gdbm_errno' to one of the following values: 'GDBM_ERR_USAGE' Improper function usage. Either EVEN or ODD is 'NULL', or they point to the same string. 'GDBM_NEED_RECOVERY' The database needs recovery. *Note Recovery::. 'GDBM_ERR_SNAPSHOT_CLONE' Failed to clone the database file into a snapshot. Examine the system 'errno' variable for details. If one of the following error codes is returned, examine the system 'errno' variable for details: 'GDBM_ERR_REALPATH' Call to 'realpath' function failed. 'realpath' is used to determine actual path names of the snapshot files. 'GDBM_FILE_OPEN_ERROR' Unable to create snapshot file. 'GDBM_FILE_SYNC_ERROR' Failed to sync a snapshot file or one of directories in its pathname, during initial synchronization. 'GDBM_FILE_CLOSE_ERROR' Failed to close a snapshot file or one of directories in its pathname, during initial synchronization. 'GDBM_ERR_FILE_MODE' The 'fchmod' call on one of the snapshot files failed. Notes: * It is not an error to call 'gdbm_failure_atomic' several times. Each subsequent call closes the previously configured snapshot files and installs new ones instead. * Crash tolerance settings are cleared by functions 'gdbm_recover' (*note Recovery::) and 'gdbm_reorganize' (*note Reorganization::). In case of 'gdbm_recover', it should not be a problem, because if you enabled crash tolerance, the procedure described in *note Crash recovery:: is the preferred way of recovering the database. If, however, you decided to call either function even though you had enabled crash tolerance previously, be sure to call 'gdbm_failure_atomic' again with the same arguments as before (provided that the call returns successfully). -- gdbm interface: int gdbm_latest_snapshot (const char *EVEN, const char *ODD, const char **RETVAL) Selects between two snapshots, EVEN and ODD, the one to be used for crash recovery. On success, stores a pointer to the selected filename in the memory location pointed to by RETVAL and returns 'GDBM_SNAPSHOT_OK'. If neither snapshot file is usable, the function returns 'GDBM_SNAPSHOT_BAD'. If a system error occurs, it returns 'GDBM_SNAPSHOT_ERR' and sets 'errno' to the error code describing the problem. Finally, in the unlikely case that it cannot select between the two snapshots (this means they are both readable and have exactly the same 'mtime' timestamp), the function returns 'GDBM_SNAPSHOT_SAME'. If the 'numsync' extension is enabled (*note Numsync::), the function can also return the 'GDBM_SNAPSHOT_SUSPICIOUS' status code. This happens when the 'numsync' counters in the two snapshots differ by more than one. *Note Crash recovery::, for a detailed description of possible return codes and their interpretation. If any value other than 'GDBM_SNAPSHOT_OK' is returned, it is guaranteed that the function did not touch RETVAL. In this case it is recommended to switch to manual recovery procedure, letting the user examine the snapshots and take the appropriate action. *note Manual crash recovery::, for details. 18 Setting options ****************** 'GDBM' supports the ability to set certain options on an already open database. -- gdbm interface: int gdbm_setopt (GDBM_FILE DBF, int OPTION, void *VALUE, int SIZE) Sets an option on the database or returns the value of an option. The parameters are: DBF The pointer returned by 'gdbm_open'. OPTION The option to be set or retrieved. VALUE A pointer to the value to which OPTION will be set or where to place the option value (depending on the option). SIZE The length of the data pointed to by VALUE. The return value will be '-1' upon failure, or '0' upon success. The global variable 'gdbm_errno' will be set upon failure. The valid options are: -- Option: GDBM_SETCACHESIZE -- Option: GDBM_CACHESIZE Set the size of the internal bucket cache. The VALUE should point to a 'size_t' holding the desired cache size, or the constant 'GDBM_CACHE_AUTO', to adjust the cache size automatically. By default, a newly open database is configured to dynamically accommodate the cache size to the number of index buckets in the database file. This provides for the best performance. If another VALUE is set, it is adjusted to the nearest larger power of two. Use this option if you wish to limit the memory usage at the expense of performance. If you chose to do so, please bear in mind that cache becomes effective when its size is greater then 2/3 of the number of index bucket counts in the database. The best performance results are achieved when cache size equals the number of buckets. For example: size_t bn; gdbm_bucket_count (dbf, &bn); ret = gdbm_setopt (dbf, GDBM_SETCACHESIZE, &bn, sizeof (bn)); To request the automatically adjustable cache size, use the constant 'GDBM_CACHE_AUTO': size_t bn = GDBM_CACHE_AUTO; ret = gdbm_setopt (dbf, GDBM_SETCACHESIZE, &bn, sizeof (bn)); -- Option: GDBM_GETCACHESIZE Return the actual size of the internal bucket cache. The VALUE should point to a 'size_t' variable, where the size will be stored. -- Option: GDBM_SETCACHEAUTO Controls whether the cache size will be adjusted automatically as needed. The VALUE should point to an integer: 'TRUE' to enable automatic cache adjustment and 'FALSE' to disable it. The following two calls are equivalent: int t = TRUE; gdbm_setopt (dbf, GDBM_SETCACHEAUTO, &t, sizeof (t)); size_t n = GDBM_CACHE_AUTO; gdbm_setopt (dbf, GDBM_SETCACHESIZE, &n, sizeof (n)); -- Option: GDBM_GETCACHEAUTO Return the state of the automatic cache size adjustment. The VALUE should point to an integer which, upon successful return, will have the value 'TRUE' if the automatic cache size adjustment is enabled and 'FALSE' otherwise. -- Option: GDBM_GETFLAGS Return the flags describing the state of the database. The VALUE should point to an 'int' variable where to store the flags. On success, its value will be similar to the flags used when opening the database (*note gdbm_open: Open.), except that it will reflect the current state (which may have been altered by another calls to 'gdbm_setopt'). -- Option: GDBM_GETDBFORMAT Return the database format. The VALUE should point to an 'int' variable. Upon successful return, it will be set to '0' if the database is in standard format and 'GDBM_NUMSYNC' if it is in extended format. *Note Database format::. -- Option: GDBM_GETDIRDEPTH Returns the "directory depth": the number of initial (most significant) bits in hash value that are interpreted as index to the directory. The actual directory size can be computed as '1 << VALUE'. The VALUE argument should point to an 'int'. -- Option: GDBM_GETBUCKETSIZE Returns the "bucket capacity": maximum number of keys per bucket ('int'). -- Option: GDBM_FASTMODE Enable or disable the "fast writes mode", i.e. writes without subsequent synchronization. The VALUE should point to an integer: 'TRUE' to enable fast mode, and 'FALSE' to disable it. This option is retained for compatibility with previous versions of 'GDBM'. Its effect is the reverse of 'GDBM_SETSYNCMODE'. -- Option: GDBM_SETSYNCMODE -- Option: GDBM_SYNCMODE Turn on or off file system synchronization operations. This setting defaults to off. The VALUE should point to an integer: 'TRUE' to turn synchronization on, and 'FALSE' to turn it off. Note, that this option is a reverse of 'GDBM_FASTMODE', i.e. calling 'GDBM_SETSYNCMODE' with 'TRUE' has the same effect as calling 'GDBM_FASTMODE' with 'FALSE'. The 'GDBM_SYNCMODE' option is provided for compatibility with earlier versions. -- Option: GDBM_GETSYNCMODE Return the current synchronization status. The VALUE should point to an 'int' where the status will be stored. -- Option: GDBM_SETCENTFREE -- Option: GDBM_CENTFREE _NOTICE: This feature is still under study._ Set central free block pool to either on or off. The default is off, which is how previous versions of 'GDBM' handled free blocks. If set, this option causes all subsequent free blocks to be placed in the _global_ pool, allowing (in theory) more file space to be reused more quickly. The VALUE should point to an integer: 'TRUE' to turn central block pool on, and 'FALSE' to turn it off. The 'GDBM_CENTFREE' option is provided for compatibility with earlier versions. -- Option: GDBM_SETCOALESCEBLKS -- Option: GDBM_COALESCEBLKS _NOTICE: This feature is still under study._ Set free block merging to either on or off. The default is off, which is how previous versions of 'GDBM' handled free blocks. If set, this option causes adjacent free blocks to be merged. This can become a CPU expensive process with time, though, especially if used in conjunction with GDBM_CENTFREE. The VALUE should point to an integer: 'TRUE' to turn free block merging on, and 'FALSE' to turn it off. -- Option: GDBM_GETCOALESCEBLKS Return the current status of free block merging. The VALUE should point to an 'int' where the status will be stored. -- Option: GDBM_SETMAXMAPSIZE Sets maximum size of a memory mapped region. The VALUE should point to a value of type 'size_t', 'unsigned long' or 'unsigned'. The actual value is rounded to the nearest page boundary (the page size is obtained from 'sysconf(_SC_PAGESIZE)'). -- Option: GDBM_GETMAXMAPSIZE Return the maximum size of a memory mapped region. The VALUE should point to a value of type 'size_t' where to return the data. -- Option: GDBM_SETMMAP Enable or disable memory mapping mode. The VALUE should point to an integer: 'TRUE' to enable memory mapping or 'FALSE' to disable it. -- Option: GDBM_GETMMAP Check whether memory mapping is enabled. The VALUE should point to an integer where to return the status. -- Option: GDBM_GETDBNAME Return the name of the database disk file. The VALUE should point to a variable of type 'char**'. A pointer to the newly allocated copy of the file name will be placed there. The caller is responsible for freeing this memory when no longer needed. For example: char *name; if (gdbm_setopt (dbf, GDBM_GETDBNAME, &name, sizeof (name))) { fprintf (stderr, "gdbm_setopt failed: %s\n", gdbm_strerror (gdbm_errno)); } else { printf ("database name: %s\n", name); free (name); } -- Option: GDBM_GETBLOCKSIZE Return the block size in bytes. The VALUE should point to 'int'. 19 File Locking *************** With locking disabled (if 'gdbm_open' was called with 'GDBM_NOLOCK'), the user may want to perform their own file locking on the database file in order to prevent multiple writers operating on the same file simultaneously. In order to support this, the 'gdbm_fdesc' routine is provided. -- gdbm interface: int gdbm_fdesc (GDBM_FILE DBF) Returns the file descriptor of the database DBF. This value can be used as an argument to 'flock', 'lockf' or similar calls. 20 Useful global variables ************************** The following global variables and constants are available: -- Variable: gdbm_error gdbm_errno This variable contains error code from the last failed 'GDBM' call. *Note Error codes::, for a list of available error codes and their descriptions. Use 'gdbm_strerror' (*note Errors::) to convert it to a descriptive text. -- Variable: const char * gdbm_errlist[] This variable is an array of error descriptions, which is used by 'gdbm_strerror' to convert error codes to human-readable text (*note Errors::). You can access it directly, if you wish so. It contains '_GDBM_MAX_ERRNO + 1' elements and can be directly indexed by the error code to obtain a corresponding descriptive text. -- Variable: int const gdbm_syserr[] Array of boolean values indicating, for each 'GDBM' error code, whether the value of 'errno'(3) variable is meaningful for this error code. *Note gdbm_check_syserr::. -- Constant: _GDBM_MIN_ERRNO The minimum error code used by 'GDBM'. -- Constant: _GDBM_MAX_ERRNO The maximum error code used by 'GDBM'. -- Variable: const char * gdbm_version A string containing the version information. -- Variable: int const gdbm_version_number[3] This variable contains the 'GDBM' version numbers: Index Meaning ------------------------------------------------------------------- 0 Major number 1 Minor number 2 Patchlevel number Additionally, the following constants are defined in the 'gdbm.h' file: GDBM_VERSION_MAJOR Major number. GDBM_VERSION_MINOR Minor number. GDBM_VERSION_PATCH Patchlevel number. These can be used to verify whether the header file matches the library. To compare two split-out version numbers, use the following function: -- gdbm interface: int gdbm_version_cmp (int const A[3], int const B[3]) Compare two version numbers. Return '-1' if A is less than B, '1' if A is greater than B and '0' if they are equal. Comparison is done from left to right, so that: a = { 1, 8, 3 }; b = { 1, 8, 3 }; gdbm_version_cmp (a, b) => 0 a = { 1, 8, 3 }; b = { 1, 8, 2 }; gdbm_version_cmp (a, b) => 1 a = { 1, 8, 3 }; b = { 1, 9. 0 }; gdbm_version_cmp (a, b) => -1 21 Additional functions *********************** -- gdbm interface: int gdbm_avail_verify (GDBM_FILE DBF) Verify if the available block stack is in consistent state. On success, returns 0. If any errors are encountered, sets the 'gdbm_errno' to 'GDBM_BAD_AVAIL', marks the database as needing recovery (*note Recovery::) and return -1. 22 Error codes ************** This chapter summarizes error codes which can be set by the functions in 'GDBM' library. -- Error Code: GDBM_NO_ERROR No error occurred. -- Error Code: GDBM_MALLOC_ERROR Memory allocation failed. Not enough memory. -- Error Code: GDBM_BLOCK_SIZE_ERROR This error is set by the 'gdbm_open' function (*note Open::), if the value of its BLOCK_SIZE argument is incorrect and the 'GDBM_BSEXACT' flag is set. -- Error Code: GDBM_FILE_OPEN_ERROR The library was not able to open a disk file. This can be set by 'gdbm_open' (*note Open::), 'gdbm_dump' ('gdbm_export') and 'gdbm_load' ('gdbm_import') functions (*note Flat files::). Inspect the value of the system 'errno' variable to get more detailed diagnostics. -- Error Code: GDBM_FILE_WRITE_ERROR Writing to a disk file failed. This can be set by 'gdbm_open' (*note Open::), 'gdbm_dump' ('gdbm_export') and 'gdbm_load' ('gdbm_import') functions. Inspect the value of the system 'errno' variable to get more detailed diagnostics. -- Error Code: GDBM_FILE_SEEK_ERROR Positioning in a disk file failed. This can be set by 'gdbm_open' (*note Open::) function. Inspect the value of the system 'errno' variable to get a more detailed diagnostics. -- Error Code: GDBM_FILE_READ_ERROR Reading from a disk file failed. This can be set by 'gdbm_open' (*note Open::), 'gdbm_dump' ('gdbm_export') and 'gdbm_load' ('gdbm_import') functions. Inspect the value of the system 'errno' variable to get a more detailed diagnostics. -- Error Code: GDBM_BAD_MAGIC_NUMBER The file given as argument to 'gdbm_open' function is not a valid 'GDBM' file: it has a wrong magic number. -- Error Code: GDBM_EMPTY_DATABASE The file given as argument to 'gdbm_open' function is not a valid 'GDBM' file: it has zero length. -- Error Code: GDBM_CANT_BE_READER This error code is set by the 'gdbm_open' function if it is not able to lock file when called in 'GDBM_READER' mode (*note GDBM_READER: Open.). -- Error Code: GDBM_CANT_BE_WRITER This error code is set by the 'gdbm_open' function if it is not able to lock file when called in writer mode (*note Open::). -- Error Code: GDBM_READER_CANT_DELETE Set by the 'gdbm_delete' (*note Delete::) if it attempted to operate on a database that is open in read-only mode (*note GDBM_READER: Open.). -- Error Code: GDBM_READER_CANT_STORE Set by the 'gdbm_store' (*note Store::) if it attempted to operate on a database that is open in read-only mode (*note GDBM_READER: Open.). -- Error Code: GDBM_READER_CANT_REORGANIZE Set by the 'gdbm_reorganize' (*note Reorganization::) if it attempted to operate on a database that is open in read-only mode (*note GDBM_READER: Open.). -- Error Code: GDBM_ITEM_NOT_FOUND Requested item was not found. This error is set by 'gdbm_delete' (*note Delete::) and 'gdbm_fetch' (*note Fetch::) when the requested KEY value is not found in the database. -- Error Code: GDBM_REORGANIZE_FAILED The 'gdbm_reorganize' function is not able to create a temporary database. *Note Reorganization::. -- Error Code: GDBM_CANNOT_REPLACE Cannot replace existing item. This error is set by the 'gdbm_store' if the requested KEY value is found in the database and the FLAG parameter is not 'GDBM_REPLACE'. *Note Store::, for a detailed discussion. -- Error Code: GDBM_MALFORMED_DATA -- Error Code: GDBM_ILLEGAL_DATA Input data was malformed in some way. When returned by 'gdbm_load', this means that the input file was not a valid 'GDBM' dump file (*note gdbm_load function::). When returned by 'gdbm_store', this means that either KEY or CONTENT parameter had its 'dptr' field set to 'NULL' (*note Store::). The 'GDBM_ILLEGAL_DATA' is an alias for this error code, maintained for backward compatibility. Its use in modern applications is discouraged. -- Error Code: GDBM_OPT_ALREADY_SET Requested option can be set only once and was already set. As of version 1.23, this error code is no longer used. In prior versions it could have been returned by the 'gdbm_setopt' function when setting the 'GDBM_CACHESIZE' value. -- Error Code: GDBM_OPT_BADVAL -- Error Code: GDBM_OPT_ILLEGAL The OPTION argument is not valid or the VALUE argument points to an invalid value in a call to 'gdbm_setopt' function. *Note Options::. 'GDBM_OPT_ILLEGAL' is an alias for this error code, maintained for backward compatibility. Modern applications should not use it. -- Error Code: GDBM_BYTE_SWAPPED The 'gdbm_open' function (*note Open::) attempts to open a database which is created on a machine with different byte ordering. -- Error Code: GDBM_BAD_FILE_OFFSET The 'gdbm_open' function (*note Open::) sets this error code if the file it tries to open has a wrong magic number. -- Error Code: GDBM_BAD_OPEN_FLAGS Set by the 'gdbm_dump' ('gdbm_export') function if supplied an invalid FLAGS argument. *Note Flat files::. -- Error Code: GDBM_FILE_STAT_ERROR Getting information about a disk file failed. The system 'errno' will give more details about the error. This error can be set by the following functions: 'gdbm_open', 'gdbm_reorganize'. -- Error Code: GDBM_FILE_EOF End of file was encountered where more data was expected to be present. This error can occur when fetching data from the database and usually means that the database is truncated or otherwise corrupted. This error can be set by any 'GDBM' function that does I/O. Some of these functions are: 'gdbm_delete', 'gdbm_exists', 'gdbm_fetch', 'gdbm_dump', 'gdbm_load', 'gdbm_export', 'gdbm_import', 'gdbm_reorganize', 'gdbm_firstkey', 'gdbm_nextkey', 'gdbm_store'. -- Error Code: GDBM_NO_DBNAME Output database name is not specified. This error code is set by 'gdbm_load' (*note gdbm_load: gdbm_load function.) if the first argument points to 'NULL' and the input file does not specify the database name. -- Error Code: GDBM_ERR_FILE_OWNER This error code is set by 'gdbm_load' if it is unable to restore database file owner. It is a mild error condition, meaning that the data have been restored successfully, only changing the target file owner failed. Inspect the system 'errno' variable to get a more detailed diagnostics. -- Error Code: GDBM_ERR_FILE_MODE This error code is set by 'gdbm_load' if it is unable to restore database file mode. It is a mild error condition, meaning that the data have been restored successfully, only changing the target file owner failed. Inspect the system 'errno' variable to get a more detailed diagnostics. -- Error Code: GDBM_NEED_RECOVERY Database is in inconsistent state and needs recovery. Call 'gdbm_recover' if you get this error. *Note Recovery::, for a detailed description of recovery functions. -- Error Code: GDBM_BACKUP_FAILED The GDBM engine is unable to create backup copy of the file. -- Error Code: GDBM_DIR_OVERFLOW Bucket directory would overflow the size limit during an attempt to split hash bucket. This error can occur while storing a new key. -- Error Code: GDBM_BAD_BUCKET Invalid index bucket is encountered in the database. Database recovery is needed (*note Recovery::). -- Error Code: GDBM_BAD_HEADER This error is set by 'gdbm_open' and 'gdbm_fd_open', if the first block read from the database file does not contain a valid 'GDBM' header. -- Error Code: GDBM_BAD_AVAIL The available space stack is invalid. This error can be set by 'gdbm_open' and 'gdbm_fd_open', if the extended database verification was requested ('GDBM_XVERIFY'). It is also set by the 'gdbm_avail_verify' function (*note Additional functions::). Database recovery is needed (*note Recovery::). -- Error Code: GDBM_BAD_HASH_TABLE Hash table in a bucket is invalid. This error can be set by the following functions: 'gdbm_delete', 'gdbm_exists', 'gdbm_fetch', 'gdbm_firstkey', 'gdbm_nextkey', and 'gdbm_store'. Database recovery is needed (*note Recovery::). -- Error Code: GDBM_BAD_DIR_ENTRY Bad directory entry found in the bucket. The database recovery is needed (*note Recovery::). -- Error Code: GDBM_FILE_CLOSE_ERROR The 'gdbm_close' function was unable to close the database file descriptor. The system 'errno' variable contains the corresponding error code. -- Error Code: GDBM_FILE_SYNC_ERROR Cached content couldn't be synchronized to disk. Examine the 'errno' variable to get more info, Database recovery is needed (*note Recovery::). -- Error Code: GDBM_FILE_TRUNCATE_ERROR File cannot be truncated. Examine the 'errno' variable to get more info. This error is set by 'gdbm_open' and 'gdbm_fd_open' when called with the 'GDBM_NEWDB' flag. -- Error Code: GDBM_BUCKET_CACHE_CORRUPTED The bucket cache structure is corrupted. Database recovery is needed (*note Recovery::). -- Error Code: GDBM_BAD_HASH_ENTRY This error is set during sequential access (*note Sequential::), if the next hash table entry does not contain the expected key. This means that the bucket is malformed or corrupted and the database needs recovery (*note Recovery::). -- Error Code: GDBM_ERR_SNAPSHOT_CLONE Set by the 'gdbm_failure_atomic' function if it was unable to clone the database file into a snapshot. Inspect the system 'errno' variable for the underlying cause of the error. If 'errno' is 'EINVAL' or 'ENOSYS', crash tolerance settings will be removed from the database. *Note Crash Tolerance API::. -- Error Code: GDBM_ERR_REALPATH Set by the 'gdbm_failure_atomic' function if the call to 'realpath' function failed. 'realpath' is used to determine actual path names of the snapshot files. Examine the system 'errno' variable for details. *Note Crash Tolerance API::. -- Error Code: GDBM_ERR_USAGE Function usage error. That includes invalid argument values, and the like. 23 Compatibility with standard 'dbm' and 'ndbm' *********************************************** 'Gdbm' includes a compatibility layer, which provides traditional 'ndbm' and older 'dbm' functions. The layer is compiled and installed if the '--enable-libgdbm-compat' option is used when configuring the package. The compatibility layer consists of two header files: 'ndbm.h' and 'dbm.h' and the 'libgdbm_compat' library. Older programs using 'ndbm' or 'dbm' interfaces can use 'libgdbm_compat' without any changes. To link a program with the compatibility library, add the following two options to the 'cc' invocation: '-lgdbm -lgdbm_compat'. The '-L' option may also be required, depending on where 'GDBM' is installed, e.g.: cc ... -lgdbm -lgdbm_compat Databases created and manipulated by the compatibility interfaces consist of two different files: 'FILE.dir' and 'FILE.pag'. This is required by the POSIX specification and corresponds to the traditional usage. Note, however, that despite the similarity of the naming convention, actual data stored in these files has not the same format as in the databases created by other 'dbm' or 'ndbm' libraries. In other words, you cannot access a standard UNIX 'dbm' file with GNU 'dbm'! Compatibility interface includes only functions required by POSIX (*note ndbm::) or present in the traditional DBM implementation (*note dbm::). Advanced 'GDBM' features, such as crash tolerance, cannot be used with such databases. GNU 'dbm' files are not 'sparse'. You can copy them with the usual 'cp' command and they will not expand in the copying process. 23.1 NDBM interface functions ============================= The functions below implement the POSIX 'ndbm' interface: -- ndbm: DBM * dbm_open (char *FILE, int FLAGS, int MODE) Opens a database. The FILE argument is the full name of the database file to be opened. The function opens two files: 'FILE.pag' and 'FILE.dir'. The FLAGS and MODE arguments have the same meaning as the second and third arguments of 'open' (*note (open(2))open::), except that a database opened for write-only access opens the files for read and write access and the behavior of the 'O_APPEND' flag is unspecified. The function returns a pointer to the 'DBM' structure describing the database. This pointer is used to refer to this database in all operations described below. Any error detected will cause a return value of 'NULL' and an appropriate value will be stored in 'gdbm_errno' (*note Variables::). -- ndbm: void dbm_close (DBM *DBF) Closes the database. The DBF argument must be a pointer returned by an earlier call to 'dbm_open'. -- ndbm: datum dbm_fetch (DBM *DBF, datum KEY) Reads a record from the database with the matching key. The KEY argument supplies the key that is being looked for. If no matching record is found, the 'dptr' member of the returned datum is 'NULL'. Otherwise, the 'dptr' member of the returned datum points to the memory managed by the compatibility library. The application should never free it. -- ndbm: int dbm_store (DBM *DBF, datum KEY, datum CONTENT, int MODE) Writes a key/value pair to the database. The argument DBF is a pointer to the 'DBM' structure returned from a call to 'dbm_open'. The KEY and CONTENT provide the values for the record key and content. The MODE argument controls the behavior of 'dbm_store' in case a matching record already exists in the database. It can have one of the following two values: 'DBM_REPLACE' Replace existing record with the new one. 'DBM_INSERT' The existing record is left unchanged, and the function returns '1'. If no matching record exists in the database, new record will be inserted no matter what the value of the MODE is. -- ndbm: int dbm_delete (DBM *DBF, datum KEY) Deletes the record with the matching key from the database. If the function succeeds, '0' is returned. Otherwise, if no matching record is found or if an error occurs, '-1' is returned. -- ndbm: datum dbm_firstkey (DBM *DBF) Initializes iteration over the keys from the database and returns the first key. Note, that the word 'first' does not imply any specific ordering of the keys. If there are no records in the database, the 'dptr' member of the returned datum is 'NULL'. Otherwise, the 'dptr' member of the returned datum points to the memory managed by the compatibility library. The application should never free it. -- ndbm: datum dbm_nextkey (DBM *DBF) Continues the iteration started by 'dbm_firstkey'. Returns the next key in the database. If the iteration covered all keys in the database, the 'dptr' member of the returned datum is 'NULL'. Otherwise, the 'dptr' member of the returned datum points to the memory managed by the compatibility library. The application should never free it. The usual way of iterating over all the records in the database is: for (key = dbm_firstkey (dbf); key.ptr; key = dbm_nextkey (dbf)) { /* do something with the key */ } The loop above should not try to delete any records from the database, otherwise the iteration is not guaranteed to cover all the keys. *Note Sequential::, for a detailed discussion of this. -- ndbm: int dbm_error (DBM *DBF) Returns the error condition of the database: '0' if no errors occurred so far while manipulating the database, and a non-zero value otherwise. -- ndbm: void dbm_clearerr (DBM *DBF) Clears the error condition of the database. -- ndbm: int dbm_dirfno (DBM *DBF) Returns the file descriptor of the 'dir' file of the database. It is guaranteed to be different from the descriptor returned by the 'dbm_pagfno' function (see below). The application can lock this descriptor to serialize accesses to the database. -- ndbm: int dbm_pagfno (DBM *DBF) Returns the file descriptor of the 'pag' file of the database. See also 'dbm_dirfno'. -- ndbm: int dbm_rdonly (DBM *DBF) Returns '1' if the database DBF is open in a read-only mode and '0' otherwise. 23.2 DBM interface functions ============================ The functions below are provided for compatibility with the old UNIX 'DBM' interface. Only one database at a time can be manipulated using them. -- dbm: int dbminit (char *FILE) Opens a database. The FILE argument is the full name of the database file to be opened. The function opens two files: 'FILE.pag' and 'FILE.dir'. If any of them does not exist, the function fails. It never attempts to create the files. The database is opened in the read-write mode, if its disk permissions permit. The application must ensure that the functions described below in this section are called only after a successful call to 'dbminit'. -- dbm: int dbmclose (void) Closes the database opened by an earlier call to 'dbminit'. -- dbm: datum fetch (datum KEY) Reads a record from the database with the matching key. The KEY argument supplies the key that is being looked for. If no matching record is found, the 'dptr' member of the returned datum is 'NULL'. Otherwise, the 'dptr' member of the returned datum points to the memory managed by the compatibility library. The application should never free it. -- dbm: int store (datum KEY, datum CONTENT) Stores the key/value pair in the database. If a record with the matching key already exists, its content will be replaced with the new one. Returns '0' on success and '-1' on error. -- dbm: int delete (datum KEY) Deletes a record with the matching key. If the function succeeds, '0' is returned. Otherwise, if no matching record is found or if an error occurs, '-1' is returned. -- dbm: datum firstkey (void) Initializes iteration over the keys from the database and returns the first key. Note, that the word 'first' does not imply any specific ordering of the keys. If there are no records in the database, the 'dptr' member of the returned datum is 'NULL'. Otherwise, the 'dptr' member of the returned datum points to the memory managed by the compatibility library. The application should never free it. -- dbm: datum nextkey (datum KEY) Continues the iteration started by a call to 'firstkey'. Returns the next key in the database. If the iteration covered all keys in the database, the 'dptr' member of the returned datum is 'NULL'. Otherwise, the 'dptr' member of the returned datum points to the memory managed by the compatibility library. The application should never free it. 24 Examine and modify a GDBM database ************************************* The 'gdbmtool' utility allows you to view and modify an existing 'GDBM' database or to create a new one. When invoked without arguments, it tries to open a database file called 'junk.gdbm', located in the current working directory. You can change this default by supplying the name of the database as argument to the program, e.g.: $ gdbmtool file.db The database will be opened in read-write mode, unless the '-r' ('--read-only') option is specified, in which case it will be opened only for reading. If the database does not exist, 'gdbmtool' will create it. There is a special option '-n' ('--newdb'), which instructs the utility to create a new database. If it is used and if the database already exists, it will be deleted, so use it sparingly. 24.1 gdbmtool invocation ======================== When started without additional arguments, 'gdbmtool' operates on the default database 'junk.gdbm'. Otherwise, the first argument supplies the name of the database to operate upon. If neither any additional arguments nor the '-f' ('--file') option are given, 'gdbmtool' opens starts interactive shell and receives commands directly from the human operator. If more than one argument is given, all arguments past the database name are parsed as 'gdbmtool' commands (*note shell::, for a description of available commands) and executed in turn. All commands, except the last one, should be terminated with semicolons. Semicolon after the last command is optional. Note, that semicolons should be escaped in order to prevent them from being interpreted by the shell. Finally, if the '-f' ('--file') option is supplied, its argument specifies the name of the disk file with 'gdbmtool' script. The program will open that file and read commands from it. The following table summarizes all 'gdbmtool' command line options: '-b SIZE' '--block-size=SIZE' Set block size. '-c SIZE' '--cache-size=SIZE' Set cache size. '-d FD' '--db-descriptor=FD' Use the database referred to by the file descriptor FD. This must be a valid open file descriptor, obtained by a call to 'open' (*note open a file: (open(2))open.), 'creat' or a similar function. The database will be opened using 'gdbm_fd_open' (*note gdbm_fd_open::). This option is intended for use by automatic test suites. '-f FILE' '--file FILE' Read commands from FILE, instead of the standard input. '-h' '--help' Print a concise help summary. '-N' '--norc' Don't read startup files (*note startup files::). '-n' '--newdb' Create the database. '-l' '--no-lock' Disable file locking. '-m' '--no-mmap' Disable memory mapping. '-T' '--timing' Print time spent in each command. This is equivalent to setting the 'timing' variable. *Note timing: variables. '-t' '--trace' Enable command tracing. This is equivalent to setting the 'trace' variable. *Note trace: variables. '-q' '--quiet' Don't print the usual welcome banner at startup. This is the same as setting the variable 'quiet' in the startup file. *Note quiet::. '-r' '--read-only' Open the database in read-only mode. '-s' '--synchronize' Synchronize to the disk after each write. '-V' '--version' Print program version and licensing information and exit. '--usage' Print a terse invocation syntax summary along with a list of available command line options. '-x' '--extended' '--numsync' Create new database in extended (numsync) format (*note Numsync::). This option sets the 'format' variable to 'numsync'. *Note format variable::. 24.2 gdbmtool interactive mode ============================== After successful startup, 'gdbmtool' starts a loop, in which it reads commands from the standard input, executes them and prints results on the standard output. If the standard input is attached to a console, 'gdbmtool' runs in interactive mode, which is indicated by its "prompt": gdbmtool> _ The utility finishes when it reads the 'quit' command (see below) or detects end-of-file on its standard input, whichever occurs first. A 'gdbmtool' command consists of a "command verb", optionally followed by "arguments", separated by any amount of white space and terminated with a newline or semicolon. A command verb can be entered either in full or in an abbreviated form, as long as that abbreviation does not match any other verb. For example, 'co' can be used instead of 'count' and 'ca' instead of 'cache'. Any sequence of non-whitespace characters appearing after the command verb forms an argument. If the argument contains whitespace or unprintable characters it must be enclosed in double quotes. Within double quotes the usual "escape sequences" are understood, as shown in the table below: Sequence Replaced with \a Audible bell character (ASCII 7) \b Backspace character (ASCII 8) \f Form-feed character (ASCII 12) \n Newline character (ASCII 10) \r Carriage return character (ASCII 13) \t Horizontal tabulation character (ASCII 9) \v Vertical tabulation character (ASCII 11) \\ Single slash \" Double quote Table 24.1: Backslash escapes In addition, a backslash immediately followed by the end-of-line character effectively removes that character, allowing to split long arguments over several input lines. Command parameters may be optional or mandatory. If the number of actual arguments is less than the number of mandatory parameters, 'gdbmtool' will prompt you to supply missing arguments. For example, the 'store' command takes two mandatory parameters, so if you invoked it with no arguments, you would be prompted twice to supply the necessary data, as shown in example below: gdbmtool> store key? three data? 3 However, such prompting is possible only in interactive mode. In non-interactive mode (e.g. when running a script), all arguments must be supplied with each command, otherwise 'gdbmtool' will report an error and exit immediately. If the package is compiled with GNU Readline, the input line can be edited (*note Command Line Editing: (readline)Command Line Editing.). 24.2.1 Shell Variables ---------------------- A number of 'gdbmtool' parameters is kept in its internal variables. To examine or modify variables, use the 'set' command (*note set::). -- gdbmtool variable: bool confirm Whether to ask for confirmation before certain destructive operations, such as truncating the existing database. Default is 'true'. -- gdbmtool variable: string delim1 A string used to delimit fields of a structured datum on output (*note definitions::). Default is ',' (a comma). This variable cannot be unset. -- gdbmtool variable: string delim2 A string used to delimit array items when printing a structured datum (*note definitions::). Default is ',' (a comma). This variable cannot be unset. -- gdbmtool variable: string errorexit -- gdbmtool variable: bool errorexit Comma-delimited list of 'GDBM' error codes which cause program termination. Error codes are specified via their canonical names (*note Error codes::). The 'GDBM_' prefix can be omitted. Code name comparison is case-insensitive. Each error code can optionally be prefixed with minus sign, to indicate that it should be removed from the resulting list, or with plus sign (which is allowed for symmetry). A special code 'all' stands for all available error codes. In boolean context, the 'true' value is equivalent to 'all', and 'false' (i.e. variable unset) is equivalent to '-all'. -- gdbmtool variable: string errormask -- gdbmtool variable: bool errormask Comma-delimited list of 'GDBM' error codes which are masked, i.e. which won't trigger a diagnostic message if they occur. The syntax is the same as described for 'errorexit'. -- gdbmtool variable: string pager The name and command line of the pager program to pipe output to. This program is used in interactive mode when the estimated number of output lines is greater then the number of lines on your screen. The default value is inherited from the environment variable 'PAGER'. Unsetting this variable disables paging. -- gdbmtool variable: string ps1 Primary prompt string. Its value can contain "conversion specifiers", consisting of the '%' character followed by another character. These specifiers are expanded in the resulting prompt as follows: Sequence Expansion ------------------------------------------------------------------- %f name of the current database file %p program invocation name %P package name ('GDBM') %v program version %_ single space character %% % The default value is '%p>%_', i.e. the program name, followed by a "greater than" sign, followed by a single space. -- gdbmtool variable: string ps2 Secondary prompt. See 'ps1' for a description of its value. This prompt is displayed before reading the second and subsequent lines of a multi-line command. The default value is '%_>%_'. -- gdbmtool variable: bool timing When each command terminates, print an additional line listing times spent in that command. The line is formatted as follows: [reorganize r=0.070481 u=0.000200 s=0.000033] Here, 'reorganize' is the name of the command that finished, the number after 'r=' is real time spent executing the command, the number after 'u=' is the user CPU time used and the number after 's=' is the system CPU time used. -- gdbmtool variable: bool trace Enable command tracing. This is similar to the shell '-t' option: before executing each command, 'gdbmtool' will print on standard error a line starting with a plus sign and followed by the command name and its arguments. -- gdbmtool variable: bool quiet Whether to display a welcome banner at startup. To affect 'gdbmtool', this variable should be set in a startup script file (*note startup files::). *Note -q option::. The following variables control how the database is opened: -- gdbmtool variable: numeric blocksize Sets the block size. *Note block_size: Open. Unset by default. -- gdbmtool variable: numeric cachesize Sets the cache size. *Note GDBM_SETCACHESIZE: Options. This variable affects the currently opened database immediately. It is also used by 'open' command. To enable automatic cache size selection, unset this variable. This is the default. -- gdbmtool variable: string filename Name of the database file. If the 'open' command is called without argument (e.g. called implicitly), this variable names the database file to open. If 'open' is called with file name argument, upon successful opening of the database the 'filename' variable is initialized with its file name. This variable cannot be unset. -- gdbmtool variable: number fd File descriptor of the database file to open. If this variable is set, its value must be an open file descriptor referring to a 'GDBM' database file. The 'open' command will use 'gdbm_fd_open' function to use this file (*note gdbm_fd_open::). When this database is closed, the descriptor will be closed as well and the 'fd' variable will be unset. See also the '-d' ('--db-descriptor') command line option in *note invocation::. -- gdbmtool variable: string format Defines the format in which new databases will be created. Allowed values are: 'standard' Databases will be created in standard format. This is the format used by all 'GDBM' versions prior to 1.21. This value is the default. 'numsync' Extended format, best for crash-tolerant applications. *Note Numsync::, for a discussion of this format. -- gdbmtool variable: string open Open mode. The following values are allowed: newdb Truncate the database if it exists or create a new one. Open it in read-write mode. Technically, this sets the 'GDBM_NEWDB' flag in call to 'gdbm_open'. *Note GDBM_NEWDB: Open. wrcreat rw Open the database in read-write mode. Create it if it does not exist. This is the default. Technically speaking, it sets the 'GDBM_WRCREAT' flag in call to 'gdbm_open'. *Note GDBM_WRCREAT: Open. reader readonly Open the database in read-only mode. Signal an error if it does not exist. This sets the 'GDBM_READER' flag (*note GDBM_READER: Open.). Attempting to set any other value or to unset this variable results in error. -- gdbmtool variable: number filemode File mode (in octal) for creating new database files and database dumps. -- gdbmtool variable: bool lock Lock the database. This is the default. Setting this variable to false or unsetting it results in passing 'GDBM_NOLOCK' flag to 'gdbm_open' (*note GDBM_NOLOCK: Open.). -- gdbmtool variable: bool mmap Use memory mapping. This is the default. Setting this variable to false or unsetting it results in passing 'GDBM_NOMMAP' flag to 'gdbm_open' (*note GDBM_NOMMAP: Open.). -- gdbmtool variable: bool sync Flush all database writes on disk immediately. Default is false. *Note GDBM_SYNC: Open. -- gdbmtool variable: bool coalesce Enables the _coalesce_ mode, i.e. merging of the freed blocks of 'GDBM' files with entries in available block lists. This provides for effective memory management at the cost of slight increase in execution time when calling 'gdbm_delete'. *Note GDBM_SETCOALESCEBLKS: Options. This variable affects the currently opened database immediately and will be used by 'open' command, when it is invoked. -- gdbmtool variable: bool centfree Set to 'true', enables the use of central free block pool in newly opened databases. *Note GDBM_SETCENTFREE: Options. This variable affects the currently opened database immediately and will be used by 'open' command, when it is invoked. The following commands are used to list or modify the variables: -- command verb: set [ASSIGNMENTS] When used without arguments, lists all variables and their values. Unset variables are shown after a comment sign ('#'). For string and numeric variables, values are shown after an equals sign. For boolean variables, only the variable name is displayed if the variable is 'true'. If it is 'false', its name is prefixed with 'no'. For example: # blocksize is unset # cachesize is unset nocentfree nocoalesce confirm delim1="," delim2="," # fd is unset filemode=644 filename="junk.gdbm" format="standard" lock mmap open="wrcreat" pager="less" ps1="%p>%_" ps2="%_>%_" # quiet is unset nosync If used with arguments, the 'set' command alters the specified variables. In this case, arguments are variable assignments in the form 'NAME=VALUE'. For boolean variables, the VALUE is interpreted as follows: if it is numeric, '0' stands for 'false', any non-zero value stands for 'true'. Otherwise, the values 'on', 'true', and 'yes' denote 'true', and 'off', 'false', 'no' stand for 'false'. Alternatively, only the name of a boolean variable can be supplied to set it to 'true', and its name prefixed with 'no' can be used to set it to false. For example, the following command sets the 'delim2' variable to ';' and the 'confirm' variable to 'false': set delim2=";" noconfirm -- command verb: unset VARIABLES Unsets the listed variables. The effect of unsetting depends on the variable. Unless explicitly described in the discussion of the variables above, unsetting a boolean variable is equivalent to setting it to 'false'. Unsetting a string variable is equivalent to assigning it an empty string. 24.2.2 Gdbmtool Commands ------------------------ -- command verb: avail Print the "avail list". -- command verb: bucket NUM Print the bucket number NUM and set it as the current one. -- command verb: cache Print the bucket cache. -- command verb: close Close the currently open database. -- command verb: count Print the number of entries in the database. -- command verb: current Print the current bucket. -- command verb: debug [[+-]TOKEN...] If 'GDBM' is configured with additional debugging, this statement queries or sets 'GDBM' internal debugging level. This is intended for debugging and testing purposes and requires good knowledge of 'GDBM' internals. The use of this command is not recommended. -- command verb: delete KEY Delete record with the given KEY -- command verb: dir Print hash directory. -- command verb: downgrade Downgrade the database from extended to the standard database format. *Note Numsync::. -- command verb: export FILE-NAME [truncate] [binary|ascii] Export the database to the flat file FILE-NAME. *Note Flat files::, for a description of the flat file format and its purposes. This command will not overwrite an existing file, unless the 'truncate' parameter is also given. Another optional argument determines the type of the dump (*note Flat files::). By default, ASCII dump is created. The global variable 'filemode' specifies the permissions to use for the created output file. -- command verb: fetch KEY Fetch and display the record with the given KEY. -- command verb: first Fetch and display the first record in the database. Subsequent records can be fetched using the 'next' command (see below). *Note Sequential::, for more information on sequential access. -- command verb: hash KEY Compute and display the hash value for the given KEY. -- command verb: header Print file header. -- command verb: help -- command verb: ? Print a concise command summary, showing each command verb with its parameters and a short description of what it does. Optional arguments are enclosed in square brackets. -- command verb: import FILE-NAME [replace] [nometa] Import data from a flat dump file FILE-NAME (*note Flat files::). If the word 'replace' is given as an argument, any records with the same keys as the already existing ones will replace them. The word 'nometa' turns off restoring meta-information from the dump file. -- command verb: history -- command verb: history COUNT -- command verb: history N COUNT Shows the command history list with line numbers. When used without arguments, shows entire history. When used with one argument, displays COUNT last commands from the history. With two arguments, displays COUNT commands starting from Nth command. Command numbering starts with 1. This command is available only if 'GDBM' was compiled with GNU Readline. The history is saved in file '.gdbmtool_history' in the user's home directory. If this file exists upon startup, it is read to populate the history. Thus, command history is preserved between 'gdbmtool' invocations. -- command verb: list List the contents of the database. -- command verb: next [KEY] Sequential access: fetch and display the next record. If the KEY is given, the record following the one with this key will be fetched. Issuing several 'next' commands in row is rather common. A shortcut is provided to facilitate such use: if the last entered command was 'next', hitting the 'Enter' key repeats it without arguments. See also 'first', above. *Note Sequential::, for more information on sequential access. -- command verb: open FILENAME -- command verb: open Open the database file FILENAME. If used without arguments, the database name is taken from the variable 'filename'. If successful, any previously open database is closed and the 'filename' variable is updated. Otherwise, if the operation fails, the currently opened database remains unchanged. This command takes additional information from the following variables: 'filename' Name of the database to open, if no argument is given. 'fd' File descriptor to use. If set, this must be an open file descriptor referring to a valid database file. The database will be opened using 'gdbm_fd_open' (*note gdbm_fd_open::). The file descriptor will be closed and the variable unset upon closing the database. 'filemode' Specifies the permissions to use in case a new file is created. 'open' The database access mode. *Note The OPEN variable: openvar, for a list of its values. 'lock' Whether or not to lock the database. Default is 'on'. 'mmap' Use the memory mapping. Default is 'on'. 'sync' Synchronize after each write. Default is 'off'. *Note open parameters::, for a detailed description of these variables. -- command verb: perror [CODE] Describe the given 'GDBM' error code. The description occupies one or two lines. The second line is present if the system error number should be checked when handling this code. In this case, the second line states 'Examine errno'. If CODE is omitted, the latest error that occurred in the current database is described. Second line of the output (if present), contains description of the latest system error. Example: gdbmtool> perror 3 GDBM error code 3: "File open error" Examine errno. -- command verb: quit Close the database and quit the utility. -- command verb: recover [OPTIONS] Recover the database from structural inconsistencies. *Note Database consistency::. The following OPTIONS are understood: 'backup' Create a backup copy of the original database. 'max-failed-buckets=N' Abort recovery process if N buckets could not be recovered. 'max-failed-keys=N' Abort recovery process if N keys could not be recovered. 'max-failures=N' Abort recovery process after N failures. A "failure" in this context is either a key or a bucket that failed to be recovered. 'summary' Print the recovery statistics at the end of the run. The statistics includes number of successfully recovered, failed and duplicate keys and the number of recovered and failed buckets. 'verbose' Verbosely list each error encountered. -- command verb: reorganize Reorganize the database (*note Reorganization::). -- command verb: shell COMMAND -- command verb: ! COMMAND Execute COMMAND via current shell. If COMMAND is empty, shell is started without additional arguments. Otherwise, it is run as '$SHELL -c COMMAND'. For convenience, COMMAND is not parsed as 'gdbmtool' command line. It is passed to the shell verbatim. It can include newline characters if these are preceded by a backslash or appear within singly or doubly quoted strings. When using '!' form, be sure to separate it from COMMAND by whitespace, otherwise it will be treated as readline "event specifier". -- command verb: snapshot FILENAME FILENAME Analyze two snapshot files and select the most recent of them. In case of error, display a detailed diagnostics and meta-information of both snapshots. *Note Manual crash recovery::, for a detailed discussion. -- command verb: source FILENAME Read 'gdbmtool' commands from the file FILENAME. -- command verb: status Print current program status. The following example shows the information displayed: Database file: junk.gdbm Database is open define key string define content string The two 'define' strings show the defined formats for key and content data. *Note definitions::, for a detailed discussion of their meaning. -- command verb: store KEY DATA Store the DATA with KEY in the database. If KEY already exists, its data will be replaced. -- command verb: sync Synchronize the database with the disk storage (*note Sync::). -- command verb: upgrade Upgrade the database from standard to extended database format. *Note Numsync::. -- command verb: version Print the version of 'gdbm'. 24.2.3 Data Definitions ----------------------- 'GDBM' databases are able to keep data of any type, both in the key and in the content part of a record. Quite often these data are structured, i.e. they consist of several fields of various types. 'Gdbmtool' provides a mechanism for handling such kind of records. The 'define' command defines a record structure. The general syntax is: define WHAT DEFINITION where WHAT is 'key' to defining the structure of key data and 'content' to define the structure of the content records. The DEFINITION can be of two distinct formats. In the simplest case it is a single data type. For example, define content int defines content records consisting of a single integer field. Supported data types are: char Single byte (signed). short Signed short integer. ushort Unsigned short integer. int Signed integer. unsigned uint Unsigned integer. long Signed long integer. ulong Unsigned long integer. llong Signed long long integer. ullong Unsigned long long integer. float A floating point number. double Double-precision floating point number. string Array of bytes. stringz Null-terminated string, trailing null being part of the string. All numeric data types (integer as well as floating point) have the same respective widths as in C language on the host where the database file resides. The 'string' and 'stringz' are special. Both define a string of bytes, similar to 'char x[]' in C. The former defines an array of bytes, the latter - a null-terminated string. This makes a difference, in particular, when the string is the only part of datum. Consider the following two definitions: 1. 'define key string' 2. 'define key stringz' Now, suppose we want to store the string "ab" in the key. Using the definition (1), the 'dptr' member of 'GDBM' 'datum' will contain two bytes: 'a', and 'b'. Consequently, the 'dsize' member will have the value 2. Using the definition (2), the 'dptr' member will contain three bytes: 'a', 'b', and ASCII 0. The 'dsize' member will have the value 3. The definition (1) is the default for both key and content. The second form of the 'define' statement is similar to the C 'struct' statement and allows for defining structural data. In this form, the DEFINITION part is a comma-separated list of data types and variables enclosed in curly braces. In contrast to the rest of 'gdbm' commands, this command is inherently multiline and is terminated with the closing curly brace. For example: define content { int status, pad 8, char id[3], string name } This defines a structure consisting of three members: an integer 'status', an array of 3 bytes 'id', and an array of bytes 'name'. Notice the 'pad' statement: it allows to introduce padding between structure members. Another useful statement is 'offset': it specifies that the member following it begins at the given offset in the structure. Assuming the size of 'int' is 8 bytes, the above definition can also be written as define content { int status, offset 16, char id[3], string name } _NOTE_: The 'string' type can reasonably be used only if it is the last or the only member of the data structure. That's because it provides no information about the number of elements in the array, so it is interpreted to contain all bytes up to the end of the datum. When displaying the structured data, 'gdbmtool' precedes each value with the corresponding field name and delimits parts of the structure with the string defined in the 'delim1' variable (*note variables::). Array elements are delimited using the string from 'delim2'. For example: gdbmtool> fetch foo status=2,id={ a, u, x },name="quux" To supply a structured datum as an argument to a 'gdbmtool' command, use the same notation, e.g.: gdbmtool> store newkey { status=2, id={a,u,x}, name="quux" } The order in which the fields are listed is not significant. The above command can as well be written as: gdbmtool> store newkey { id={a,u,x}, status=2, name="quux" } You are not required to supply all defined fields. Any number of them can be omitted, provided that at least one remains. The omitted fields are filled with 0: gdbmtool> store newkey { name="bar" } gdbmtool> fetch newkey status=0,id={ ,, },name=bar Yet another way to supply structured data to a command is by listing the value for each field in the order they are defined, without field names: gdbmtool> store newkey { 2, {a,u,x}, "quux" } 24.2.4 Startup Files -------------------- Upon startup 'gdbmtool' looks for a file named '.gdbmtoolrc' first in the current working directory and, if not found, in the home directory of the user who started the command. If found, this file is read and interpreted as a list of 'gdbmtool' commands. This allows you to customize the program behavior. Following is an example startup file which disables the welcome banner, sets command line prompt to contain the name of the database file in parentheses and defines the structure of the database content records: set quiet set ps1="(%f) " define key stringz define content { int time, pad 4, int status } 25 The 'gdbm_dump' utility ************************** The 'gdbm_dump' utility creates a flat file dump of a 'GDBM' database (*note Flat files::). It takes one mandatory argument: the name of the source database file. The second argument, if given, specifies the name of the output file. If not given, 'gdbm_dump' will produce the dump on the standard output. For example, the following invocation creates a dump of the database 'file.db' in the file 'file.dump': $ gdbm_dump file.db file.dump By default the utility creates dumps in ASCII format (*note ASCII: Flat files.). Another format can be requested using the '--format' ('-H') option. The 'gdbm_dump' utility understands the following command line options: '-H FMT' '--format=FMT' Select output format. Valid values for FMT are: 'binary' or '0' to select binary dump format, and 'ascii' or '1' to select ASCII format. '-h' '--help' Print a concise help summary. '-V' '--version' Print program version and licensing information and exit. '--usage' Print a terse invocation syntax summary along with a list of available command line options. 26 The 'gdbm_load' utility ************************** The 'gdbm_load' utility restores a 'GDBM' database from a flat file. The utility requires at least one argument: the name of the input flat file. If it is '-', the standard input will be read. The format of the input file is detected automatically. By default the utility attempts to restore the database under its original name, as stored in the input file. It will fail to do so if the input is in binary format. In that case, the name of the database must be given as the second argument. In general, if two arguments are given, the second one is treated as the name of the database to create, overriding the file name specified in the flat file. The utility understands the following command line arguments: '-b NUM' '--block-size=NUM' Sets block size. *Note block_size: Open. '-c NUM' '--cache-size=NUM' Sets cache size. *Note GDBM_SETCACHESIZE: Options. '-M' '--mmap' Use memory mapping. '-m MODE' '--mode=MODE' Sets the file mode. The argument is the desired file mode in octal. '-n' '--no-meta' Do not restore file meta-data (ownership and mode) from the flat file. '-r' '--replace' Replace existing keys. '-u USER[:GROUP]' '--user=USER[:GROUP]' Set file owner. The USER can be either a valid user name or UID. Similarly, the GROUP is either a valid group name or GID. If GROUP is not given, the main group of USER is used. User and group parts can be separated by a dot, instead of the colon. '-h' '--help' Print a concise help summary. '-V' '--version' Print program version and licensing information and exit. '--usage' Print a terse invocation syntax summary along with a list of available command line options. 27 Exit codes ************* All 'GDBM' utilities return uniform exit codes. These are summarized in the table below: Code Meaning -------------------------------------------------------------------------- 0 Successful termination. 1 A fatal error occurred. 2 Program was unable to restore file ownership or mode. 3 Command line usage error. 28 Problems and bugs ******************** If you have problems with GNU 'dbm' or think you've found a bug, please report it. Before reporting a bug, make sure you've actually found a real bug. Carefully reread the documentation and see if it really says you can do what you're trying to do. If it's not clear whether you should be able to do something or not, report that too; it's a bug in the documentation! Before reporting a bug or trying to fix it yourself, try to isolate it to the smallest possible input file that reproduces the problem. Then send us the input file and the exact results 'GDBM' gave you. Also say what you expected to occur; this will help us decide whether the problem was really in the documentation. Once you've got a precise problem, send e-mail to . Please include the version number of GNU 'dbm' you are using. You can get this information by printing the variable 'gdbm_version' (*note Variables::). Non-bug suggestions are always welcome as well. If you have questions about things that are unclear in the documentation or are just obscure features, please report them too. You may contact the authors and maintainers by e-mail: Philip Nelson , Jason Downs , Sergey Poznyakoff or . Crash tolerance support written by Terence Kelly , , or . 29 Additional resources *********************** For the latest updates and pointers to additional resources, visit . In particular, a copy of 'GDBM' documentation in various formats is available online at . Latest versions of 'GDBM' can be downloaded from anonymous FTP: , or via HTTP from , or via HTTPS from , or from any GNU mirror worldwide. See , for a list of mirrors. To track 'GDBM' development, visit . Appendix A GNU Free Documentation License ***************************************** Version 1.3, 3 November 2008 Copyright (C) 2000-2002, 2007-2008, 2011, 2017-2022 Free Software Foundation, Inc. Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed. 0. PREAMBLE The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others. This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software. We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference. 1. 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ADDENDUM: How to use this License for your documents ==================================================== To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: Copyright (C) YEAR YOUR NAME. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled ``GNU Free Documentation License''. If you have Invariant Sections, Front-Cover Texts and Back-Cover Texts, replace the "with...Texts." line with this: with the Invariant Sections being LIST THEIR TITLES, with the Front-Cover Texts being LIST, and with the Back-Cover Texts being LIST. If you have Invariant Sections without Cover Texts, or some other combination of the three, merge those two alternatives to suit the situation. If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software. Index ***** * Menu: * !: commands. (line 3169) * (*errfun) of gdbm_recovery: Recovery. (line 1219) * --newdb, gdbmtool option: gdbmtool. (line 2533) * --read-only, gdbmtool option: gdbmtool. (line 2529) * -n, gdbmtool option: gdbmtool. (line 2533) * -r, gdbmtool option: gdbmtool. (line 2529) * .gdbmtoolrc: startup files. (line 3363) * ?: commands. (line 3031) * _GDBM_MAX_ERRNO: Variables. (line 1988) * _GDBM_MIN_ERRNO: Variables. (line 1985) * avail: commands. (line 2971) * backup_name of gdbm_recovery: Recovery. (line 1266) * blocksize: variables. (line 2808) * bucket: commands. (line 2974) * cache: commands. (line 2977) * cachesize: variables. (line 2811) * centfree: variables. (line 2909) * close: commands. (line 2980) * close-on-exec: Open. (line 327) * closing database: Close. (line 453) * coalesce: variables. (line 2899) * command line options, gdbmtool: invocation. (line 2541) * compatibility layer: Compatibility. (line 2312) * confirm: variables. (line 2713) * consistency, database: Database consistency. (line 1139) * count: commands. (line 2983) * creating a database, gdbmtool: gdbmtool. (line 2533) * current: commands. (line 2986) * data of gdbm_recovery: Recovery. (line 1229) * database options: Options. (line 1746) * database reorganization: Reorganization. (line 727) * database synchronization: Sync. (line 752) * database, closing: Close. (line 453) * database, opening or creating: Open. (line 284) * datum: Intro. (line 132) * DBM functions: dbm. (line 2457) * dbm.h: Compatibility. (line 2316) * dbmclose: dbm. (line 2473) * dbminit: dbm. (line 2461) * dbm_clearerr: ndbm. (line 2435) * dbm_close: ndbm. (line 2365) * dbm_delete: ndbm. (line 2396) * dbm_dirfno: ndbm. (line 2438) * dbm_error: ndbm. (line 2430) * dbm_fetch: ndbm. (line 2369) * dbm_firstkey: ndbm. (line 2401) * DBM_INSERT: ndbm. (line 2389) * dbm_nextkey: ndbm. (line 2411) * dbm_open: ndbm. (line 2348) * dbm_pagfno: ndbm. (line 2446) * dbm_rdonly: ndbm. (line 2450) * DBM_REPLACE: ndbm. (line 2386) * dbm_store: ndbm. (line 2378) * debug: commands. (line 2989) * default database, gdbmtool: gdbmtool. (line 2522) * delete: dbm. (line 2492) * delete <1>: commands. (line 2995) * deleting records: Delete. (line 627) * deletion in iteration loops: Sequential. (line 703) * delim1: variables. (line 2719) * delim2: variables. (line 2725) * dir: commands. (line 2998) * dir file: Compatibility. (line 2327) * downgrade: commands. (line 3001) * error code, most recent: Errors. (line 1067) * error codes: Error codes. (line 2051) * error strings: Errors. (line 1052) * errorexit: variables. (line 2731) * errorexit <1>: variables. (line 2732) * errormask: variables. (line 2745) * errormask <1>: variables. (line 2746) * exit code: Exit codes. (line 3492) * export: Flat files. (line 830) * export <1>: commands. (line 3005) * failed_buckets of gdbm_recovery: Recovery. (line 1263) * failed_keys of gdbm_recovery: Recovery. (line 1260) * fd: variables. (line 2829) * fetch: dbm. (line 2476) * fetch <1>: commands. (line 3016) * fetching records: Fetch. (line 572) * filemode: variables. (line 2879) * filename: variables. (line 2820) * first: commands. (line 3019) * firstkey: dbm. (line 2498) * Flat file format: Flat files. (line 830) * format: variables. (line 2840) * gdbm.h: Intro. (line 165) * gdbmtool: gdbmtool. (line 2519) * gdbm_avail_verify: Additional functions. (line 2042) * GDBM_BACKUP_FAILED: Error codes. (line 2225) * GDBM_BAD_AVAIL: Error codes. (line 2241) * GDBM_BAD_BUCKET: Error codes. (line 2232) * GDBM_BAD_DIR_ENTRY: Error codes. (line 2256) * GDBM_BAD_FILE_OFFSET: Error codes. (line 2174) * GDBM_BAD_HASH_ENTRY: Error codes. (line 2282) * GDBM_BAD_HASH_TABLE: Error codes. (line 2249) * GDBM_BAD_HEADER: Error codes. (line 2236) * GDBM_BAD_MAGIC_NUMBER: Error codes. (line 2096) * GDBM_BAD_OPEN_FLAGS: Error codes. (line 2178) * GDBM_BLOCK_SIZE_ERROR: Error codes. (line 2060) * GDBM_BSEXACT: Open. (line 387) * GDBM_BSEXACT <1>: Error codes. (line 2061) * GDBM_BUCKET_CACHE_CORRUPTED: Error codes. (line 2278) * gdbm_bucket_count: Count. (line 477) * GDBM_BYTE_SWAPPED: Error codes. (line 2170) * GDBM_CACHESIZE: Options. (line 1771) * GDBM_CANNOT_REPLACE: Error codes. (line 2137) * GDBM_CANT_BE_READER: Error codes. (line 2104) * GDBM_CANT_BE_WRITER: Error codes. (line 2109) * GDBM_CENTFREE: Options. (line 1875) * gdbm_check_syserr: Errors. (line 1098) * gdbm_clear_error: Errors. (line 1113) * GDBM_CLOERROR: Open. (line 440) * GDBM_CLOEXEC: Open. (line 326) * gdbm_close: Close. (line 457) * GDBM_COALESCEBLKS: Options. (line 1889) * gdbm_convert: Database format. (line 806) * gdbm_copy_meta: Open. (line 443) * gdbm_count: Count. (line 471) * gdbm_db_strerror: Errors. (line 1105) * gdbm_delete: Delete. (line 629) * gdbm_delete and sequential access: Sequential. (line 703) * GDBM_DIR_OVERFLOW: Error codes. (line 2228) * gdbm_dump: gdbm_dump. (line 3387) * gdbm_dump <1>: Flat files. (line 880) * gdbm_dump_to_file: Flat files. (line 987) * GDBM_EMPTY_DATABASE: Error codes. (line 2100) * gdbm_errlist[]: Variables. (line 1973) * gdbm_errno: Errors. (line 1052) * gdbm_errno <1>: Variables. (line 1965) * GDBM_ERR_FILE_MODE: Flat files. (line 975) * GDBM_ERR_FILE_MODE <1>: Error codes. (line 2213) * GDBM_ERR_FILE_OWNER: Flat files. (line 972) * GDBM_ERR_FILE_OWNER <1>: Error codes. (line 2206) * GDBM_ERR_REALPATH: Error codes. (line 2297) * GDBM_ERR_SNAPSHOT_CLONE: Error codes. (line 2288) * GDBM_ERR_USAGE: Error codes. (line 2305) * gdbm_exists: Fetch. (line 608) * gdbm_export: Flat files. (line 1010) * gdbm_export_to_file: Flat files. (line 1018) * gdbm_failure_atomic: Crash Tolerance API. (line 1656) * GDBM_FAST: Open. (line 378) * GDBM_FASTMODE: Options. (line 1849) * gdbm_fdesc: Locking. (line 1956) * gdbm_fd_open: Open. (line 426) * gdbm_fetch: Fetch. (line 572) * GDBM_FILE: Intro. (line 168) * GDBM_FILE_CLOSE_ERROR: Error codes. (line 2260) * GDBM_FILE_EOF: Error codes. (line 2189) * GDBM_FILE_OPEN_ERROR: Error codes. (line 2065) * GDBM_FILE_READ_ERROR: Error codes. (line 2088) * GDBM_FILE_SEEK_ERROR: Error codes. (line 2081) * GDBM_FILE_STAT_ERROR: Error codes. (line 2182) * GDBM_FILE_SYNC_ERROR: Error codes. (line 2265) * GDBM_FILE_TRUNCATE_ERROR: Error codes. (line 2271) * GDBM_FILE_WRITE_ERROR: Error codes. (line 2073) * gdbm_firstkey: Sequential. (line 656) * GDBM_GETBLOCKSIZE: Options. (line 1943) * GDBM_GETBUCKETSIZE: Options. (line 1845) * GDBM_GETCACHEAUTO: Options. (line 1817) * GDBM_GETCACHESIZE: Options. (line 1800) * GDBM_GETCOALESCEBLKS: Options. (line 1900) * GDBM_GETDBFORMAT: Options. (line 1831) * GDBM_GETDBNAME: Options. (line 1923) * GDBM_GETDIRDEPTH: Options. (line 1837) * GDBM_GETFLAGS: Options. (line 1823) * GDBM_GETMAXMAPSIZE: Options. (line 1910) * GDBM_GETMMAP: Options. (line 1919) * GDBM_GETSYNCMODE: Options. (line 1870) * GDBM_ILLEGAL_DATA: Error codes. (line 2144) * gdbm_import: Flat files. (line 1022) * gdbm_import_from_file: Flat files. (line 1035) * GDBM_INSERT: Store. (line 501) * GDBM_ITEM_NOT_FOUND: Error codes. (line 2128) * gdbm_last_errno: Errors. (line 1067) * gdbm_last_syserr: Errors. (line 1078) * gdbm_latest_snapshot: Crash Tolerance API. (line 1716) * gdbm_load: gdbm_load. (line 3426) * gdbm_load <1>: Flat files. (line 909) * gdbm_load_from_file: Flat files. (line 1003) * GDBM_MALFORMED_DATA: Error codes. (line 2143) * GDBM_MALLOC_ERROR: Error codes. (line 2057) * gdbm_needs_recovery: Errors. (line 1125) * GDBM_NEED_RECOVERY: Error codes. (line 2220) * GDBM_NEWDB: Open. (line 308) * gdbm_nextkey: Sequential. (line 668) * GDBM_NOLOCK: Open. (line 331) * GDBM_NOLOCK <1>: Locking. (line 1949) * GDBM_NOMMAP: Open. (line 335) * GDBM_NO_DBNAME: Error codes. (line 2200) * GDBM_NO_ERROR: Error codes. (line 2054) * GDBM_NUMSYNC: Database format. (line 813) * GDBM_NUMSYNC <1>: Open. (line 394) * gdbm_open: Open. (line 284) * GDBM_OPT_ALREADY_SET: Error codes. (line 2155) * GDBM_OPT_BADVAL: Error codes. (line 2161) * GDBM_OPT_ILLEGAL: Error codes. (line 2162) * GDBM_PREREAD: Open. (line 339) * GDBM_RCVR_BACKUP: Recovery. (line 1267) * GDBM_RCVR_ERRFUN: Recovery. (line 1221) * GDBM_RCVR_FORCE: Recovery. (line 1273) * GDBM_RCVR_MAX_FAILED_BUCKETS: Recovery. (line 1239) * GDBM_RCVR_MAX_FAILED_KEYS: Recovery. (line 1233) * GDBM_RCVR_MAX_FAILURES: Recovery. (line 1246) * GDBM_READER: Open. (line 308) * GDBM_READER_CANT_DELETE: Error codes. (line 2113) * GDBM_READER_CANT_REORGANIZE: Error codes. (line 2123) * GDBM_READER_CANT_STORE: Error codes. (line 2118) * gdbm_recover: Recovery. (line 1183) * gdbm_reorganize: Reorganization. (line 729) * GDBM_REORGANIZE_FAILED: Error codes. (line 2133) * GDBM_REPLACE: Store. (line 501) * GDBM_SETCACHEAUTO: Options. (line 1804) * GDBM_SETCACHESIZE: Options. (line 1770) * GDBM_SETCENTFREE: Options. (line 1874) * GDBM_SETCOALESCEBLKS: Options. (line 1888) * GDBM_SETMAXMAPSIZE: Options. (line 1904) * GDBM_SETMMAP: Options. (line 1914) * gdbm_setopt: Options. (line 1749) * GDBM_SETSYNCMODE: Options. (line 1857) * GDBM_SNAPSHOT_BAD: Crash Tolerance API. (line 1718) * GDBM_SNAPSHOT_BAD <1>: Crash recovery. (line 1441) * GDBM_SNAPSHOT_ERR: Crash Tolerance API. (line 1718) * GDBM_SNAPSHOT_ERR <1>: Crash recovery. (line 1446) * GDBM_SNAPSHOT_OK: Crash Tolerance API. (line 1718) * GDBM_SNAPSHOT_SAME: Crash Tolerance API. (line 1718) * GDBM_SNAPSHOT_SAME <1>: Crash recovery. (line 1481) * GDBM_SNAPSHOT_SUSPICIOUS: Crash Tolerance API. (line 1729) * GDBM_SNAPSHOT_SUSPICIOUS <1>: Crash recovery. (line 1486) * gdbm_store: Store. (line 487) * gdbm_strerror: Errors. (line 1058) * GDBM_SYNC: Open. (line 369) * GDBM_SYNC <1>: Sync. (line 782) * gdbm_sync: Sync. (line 759) * GDBM_SYNCMODE: Options. (line 1858) * gdbm_syserr[]: Variables. (line 1980) * gdbm_version: Variables. (line 1991) * gdbm_version_cmp: Variables. (line 2020) * GDBM_VERSION_MAJOR: Variables. (line 2006) * GDBM_VERSION_MINOR: Variables. (line 2009) * gdbm_version_number[3]: Variables. (line 1994) * GDBM_VERSION_PATCH: Variables. (line 2012) * GDBM_WRCREAT: Open. (line 308) * GDBM_WRITER: Open. (line 308) * GDBM_XVERIFY: Open. (line 356) * global error state: Errors. (line 1052) * GNU Readline: shell. (line 2704) * hash: commands. (line 3024) * header: commands. (line 3027) * help: commands. (line 3030) * history: commands. (line 3042) * history <1>: commands. (line 3043) * history <2>: commands. (line 3044) * import: Flat files. (line 830) * import <1>: commands. (line 3036) * init file, gdbmtool: startup files. (line 3363) * interactive mode, gdbmtool: shell. (line 2645) * iterating over records: Sequential. (line 649) * iteration and gdbm_delete: Sequential. (line 703) * iteration loop: Sequential. (line 683) * iteration loop, using NDBM: ndbm. (line 2419) * junk.gdbm: gdbmtool. (line 2522) * libgdbm_compat: Compatibility. (line 2316) * list: commands. (line 3057) * lock: variables. (line 2883) * locking: Locking. (line 1949) * logical consistency: Database consistency. (line 1150) * looking up records: Fetch. (line 572) * max_failed_buckets of gdbm_recovery: Recovery. (line 1238) * max_failed_keys of gdbm_recovery: Recovery. (line 1232) * max_failures of gdbm_recovery: Recovery. (line 1245) * mmap: variables. (line 2889) * most recent error code: Errors. (line 1067) * NDBM functions: ndbm. (line 2346) * ndbm.h: Compatibility. (line 2316) * next: commands. (line 3060) * nextkey: dbm. (line 2508) * number of records: Count. (line 471) * open: commands. (line 3074) * open <1>: commands. (line 3075) * open <2>: variables. (line 2853) * opening the database: Open. (line 284) * options, database: Options. (line 1746) * pag file: Compatibility. (line 2327) * pager: variables. (line 2751) * perror: commands. (line 3116) * ps1: variables. (line 2759) * ps2: variables. (line 2777) * quiet: variables. (line 2801) * quit: commands. (line 3133) * read-only mode, gdbmtool: gdbmtool. (line 2529) * readline: shell. (line 2704) * record, deleting: Delete. (line 627) * record, fetching: Fetch. (line 572) * records, iterating over: Sequential. (line 649) * records, storing: Store. (line 487) * records, testing existence: Fetch. (line 606) * recover: commands. (line 3136) * recovered_buckets of gdbm_recovery: Recovery. (line 1257) * recovered_keys of gdbm_recovery: Recovery. (line 1254) * reorganization, database: Reorganization. (line 727) * reorganize: commands. (line 3165) * sequential access: Sequential. (line 649) * sequential access, using NDBM: ndbm. (line 2419) * set: variables. (line 2918) * shell: commands. (line 3168) * snapshot: commands. (line 3183) * source: commands. (line 3190) * startup file, gdbmtool: startup files. (line 3363) * status: commands. (line 3193) * store: dbm. (line 2485) * store <1>: commands. (line 3206) * storing records: Store. (line 487) * structural consistency: Database consistency. (line 1144) * sync: commands. (line 3210) * sync <1>: variables. (line 2895) * synchronization, database: Sync. (line 752) * timing: variables. (line 2784) * trace: variables. (line 2795) * unset: variables. (line 2961) * upgrade: commands. (line 3213) * variables, gdbmtool: variables. (line 2710) * version: commands. (line 3217) * version number: Variables. (line 1991)