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DBM_CLEARERR(P) DBM_CLEARERR(P)
NAME
dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch, dbm_firstkey, dbm_nex-
tkey, dbm_open, dbm_store - database functions
SYNOPSIS
#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
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 char-
acter strings, may be stored in the object pointed to by dptr.
The database is stored in two files. One file is a directory containing a bitmap of
keys and has .dir as its suffix. The second file contains all data and has .pag as
its suffix.
The dbm_open() function shall open a database. The file argument to the function is
the pathname of the database. The function opens two files named file.dir and
file.pag. 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_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 sup-
port 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 con-
tain 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 appli-
cation 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 argu-
ment 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 reentrant. A function that is not required to be reen-
trant is not required to be thread-safe.
RETURN VALUE
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
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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 pro-
vide 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 func-
tions are useful for applications requiring fast lookup of relatively static infor-
mation that is to be indexed by a single key.
Note that a strictly conforming application is extremely limited by these func-
tions: 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.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
open() , the Base Definitions volume of IEEE Std 1003.1-2001, <ndbm.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C)
2001-2003 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 .
POSIX 2003 DBM_CLEARERR(P)
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