dbm_clearerr(3p) — Linux manual page

PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

DBM_CLEARERR(3P)        POSIX Programmer's Manual       DBM_CLEARERR(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch,
       dbm_firstkey, dbm_nextkey, dbm_open, dbm_store — database
       functions

SYNOPSIS         top

       #include <ndbm.h>

       int dbm_clearerr(DBM *db);
       void dbm_close(DBM *db);
       int dbm_delete(DBM *db, datum key);
       int dbm_error(DBM *db);
       datum dbm_fetch(DBM *db, datum key);
       datum dbm_firstkey(DBM *db);
       datum dbm_nextkey(DBM *db);
       DBM *dbm_open(const char *file, int open_flags, mode_t file_mode);
       int dbm_store(DBM *db, datum key, datum content, int store_mode);

DESCRIPTION         top

       These functions create, access, and modify a database.

       A datum consists of at least two members, dptr and dsize.  The
       dptr member points to an object that is dsize bytes in length.
       Arbitrary binary data, as well as character strings, may be
       stored in the object pointed to by dptr.

       A database shall be stored in one or two files. When one file is
       used, the name of the database file shall be formed by appending
       the suffix .db to the file argument given to dbm_open().  When
       two files are used, the names of the database files shall be
       formed by appending the suffixes .dir and .pag respectively to
       the file argument.

       The dbm_open() function shall open a database. The file argument
       to the function is the pathname of the database. The open_flags
       argument has the same meaning as the flags argument of 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 file_mode argument has the same meaning
       as the third argument of open().

       The dbm_open() function need not accept pathnames longer than
       {PATH_MAX}-4 bytes (including the terminating null), or pathnames
       with a last component longer than {NAME_MAX}-4 bytes (excluding
       the terminating null).

       The dbm_close() function shall close a database. The application
       shall ensure that argument db is a pointer to a dbm structure
       that has been returned from a call to dbm_open().

       These database functions shall support an internal block size
       large enough to support key/content pairs of at least 1023 bytes.

       The dbm_fetch() function shall read a record from a database. The
       argument db is a pointer to a database structure that has been
       returned from a call to dbm_open().  The argument key is a datum
       that has been initialized by the application to the value of the
       key that matches the key of the record the program is fetching.

       The dbm_store() function shall write a record to a database. The
       argument db is a pointer to a database structure that has been
       returned from a call to dbm_open().  The argument key is a datum
       that has been initialized by the application to the value of the
       key that identifies (for subsequent reading, writing, or
       deleting) the record the application is writing. The argument
       content is a datum that has been initialized by the application
       to the value of the record the program is writing. The argument
       store_mode controls whether dbm_store() replaces any pre-existing
       record that has the same key that is specified by the key
       argument. The application shall set store_mode to either
       DBM_INSERT or DBM_REPLACE. If the database contains a record that
       matches the key argument and store_mode is DBM_REPLACE, the
       existing record shall be replaced with the new record. If the
       database contains a record that matches the key argument and
       store_mode is DBM_INSERT, the existing record shall be left
       unchanged and the new record ignored. If the database does not
       contain a record that matches the key argument and store_mode is
       either DBM_INSERT or DBM_REPLACE, the new record shall be
       inserted in the database.

       If the sum of a key/content pair exceeds the internal block size,
       the result is unspecified. Moreover, the application shall ensure
       that all key/content pairs that hash together fit on a single
       block. The dbm_store() function shall return an error in the
       event that a disk block fills with inseparable data.

       The dbm_delete() function shall delete a record and its key from
       the database. The argument db is a pointer to a database
       structure that has been returned from a call to dbm_open().  The
       argument key is a datum that has been initialized by the
       application to the value of the key that identifies the record
       the program is deleting.

       The dbm_firstkey() function shall return the first key in the
       database. The argument db is a pointer to a database structure
       that has been returned from a call to dbm_open().

       The dbm_nextkey() function shall return the next key in the
       database. The argument db is a pointer to a database structure
       that has been returned from a call to dbm_open().  The
       application shall ensure that the dbm_firstkey() function is
       called before calling dbm_nextkey().  Subsequent calls to
       dbm_nextkey() return the next key until all of the keys in the
       database have been returned.

       The dbm_error() function shall return the error condition of the
       database. The argument db is a pointer to a database structure
       that has been returned from a call to dbm_open().

       The dbm_clearerr() function shall clear the error condition of
       the database. The argument db is a pointer to a database
       structure that has been returned from a call to dbm_open().

       The dptr pointers returned by these functions may point into
       static storage that may be changed by subsequent calls.

       These functions need not be thread-safe.

RETURN VALUE         top

       The dbm_store() and dbm_delete() functions shall return 0 when
       they succeed and a negative value when they fail.

       The dbm_store() function shall return 1 if it is called with a
       flags value of DBM_INSERT and the function finds an existing
       record with the same key.

       The dbm_error() function shall return 0 if the error condition is
       not set and return a non-zero value if the error condition is
       set.

       The return value of dbm_clearerr() is unspecified.

       The dbm_firstkey() and dbm_nextkey() functions shall return a key
       datum.  When the end of the database is reached, the dptr member
       of the key is a null pointer. If an error is detected, the dptr
       member of the key shall be a null pointer and the error condition
       of the database shall be set.

       The dbm_fetch() function shall return a content datum.  If no
       record in the database matches the key or if an error condition
       has been detected in the database, the dptr member of the content
       shall be a null pointer.

       The dbm_open() function shall return a pointer to a database
       structure. If an error is detected during the operation,
       dbm_open() shall return a (DBM *)0.

ERRORS         top

       No errors are defined.

       The following sections are informative.

EXAMPLES         top

       None.

APPLICATION USAGE         top

       The following code can be used to traverse the database:

           for(key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))

       The dbm_* functions provided in this library should not be
       confused in any way with those of a general-purpose database
       management system. These functions do not provide for multiple
       search keys per entry, they do not protect against multi-user
       access (in other words they do not lock records or files), and
       they do not provide the many other useful database functions that
       are found in more robust database management systems. Creating
       and updating databases by use of these functions is relatively
       slow because of data copies that occur upon hash collisions.
       These functions are useful for applications requiring fast lookup
       of relatively static information that is to be indexed by a
       single key.

       Note that a strictly conforming application is extremely limited
       by these functions: since there is no way to determine that the
       keys in use do not all hash to the same value (although that
       would be rare), a strictly conforming application cannot be
       guaranteed that it can store more than one block's worth of data
       in the database. As long as a key collision does not occur,
       additional data may be stored, but because there is no way to
       determine whether an error is due to a key collision or some
       other error condition (dbm_error() being effectively a Boolean),
       once an error is detected, the application is effectively limited
       to guessing what the error might be if it wishes to continue
       using these functions.

       The dbm_delete() function need not physically reclaim file space,
       although it does make it available for reuse by the database.

       After calling dbm_store() or dbm_delete() during a pass through
       the keys by dbm_firstkey() and dbm_nextkey(), the application
       should reset the database by calling dbm_firstkey() before again
       calling dbm_nextkey().  The contents of these files are
       unspecified and may not be portable.

       Applications should take care that database pathname arguments
       specified to dbm_open() are not prefixes of unrelated files. This
       might be done, for example, by placing databases in a separate
       directory.

       Since some implementations use three characters for a suffix and
       others use four characters for a suffix, applications should
       ensure that the maximum portable pathname length passed to
       dbm_open() is no greater than {PATH_MAX}-4 bytes, with the last
       component of the pathname no greater than {NAME_MAX}-4 bytes.

RATIONALE         top

       Previously the standard required the database to be stored in two
       files, one file being a directory containing a bitmap of keys and
       having .dir as its suffix. The second file containing all data
       and having .pag as its suffix. This has been changed not to
       specify the use of the files and to allow newer implementations
       of the Berkeley DB interface using a single file that have
       evolved while remaining compatible with the application
       programming interface. The standard developers considered
       removing the specific suffixes altogether but decided to retain
       them so as not to pollute the application file name space more
       than necessary and to allow for portable backups of the database.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       open(3p)

       The Base Definitions volume of POSIX.1‐2017, ndbm.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                  DBM_CLEARERR(3P)

Pages that refer to this page: ndbm.h(0p)