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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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DBM_CLEARERR(3P) POSIX Programmer's Manual DBM_CLEARERR(3P)
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.
dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch,
dbm_firstkey, dbm_nextkey, dbm_open, dbm_store — database
functions
#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);
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.
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.
No errors are defined.
The following sections are informative.
None.
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.
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.
None.
open(3p)
The Base Definitions volume of POSIX.1‐2017, ndbm.h(0p)
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)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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