path: root/man3p/dbm_open.3p

.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "DBM_CLEARERR" P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" dbm_clearerr 
.SH NAME
dbm_clearerr, dbm_close, dbm_delete, dbm_error, dbm_fetch, dbm_firstkey,
dbm_nextkey, dbm_open, dbm_store \- database
functions
.SH SYNOPSIS
.LP
\fB#include <ndbm.h>
.br
.sp
int dbm_clearerr(DBM *\fP\fIdb\fP\fB);
.br
void dbm_close(DBM *\fP\fIdb\fP\fB);
.br
int dbm_delete(DBM *\fP\fIdb\fP\fB, datum\fP \fIkey\fP\fB);
.br
int dbm_error(DBM *\fP\fIdb\fP\fB);
.br
datum dbm_fetch(DBM *\fP\fIdb\fP\fB, datum\fP \fIkey\fP\fB);
.br
datum dbm_firstkey(DBM *\fP\fIdb\fP\fB);
.br
datum dbm_nextkey(DBM *\fP\fIdb\fP\fB);
.br
DBM *dbm_open(const char *\fP\fIfile\fP\fB, int\fP \fIopen_flags\fP\fB,
mode_t\fP \fIfile_mode\fP\fB);
.br
int dbm_store(DBM *\fP\fIdb\fP\fB, datum\fP \fIkey\fP\fB, datum\fP
\fIcontent\fP\fB, int\fP
\fIstore_mode\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
These functions create, access, and modify a database.
.LP
A \fBdatum\fP consists of at least two members, \fIdptr\fP and \fIdsize\fP.
The \fIdptr\fP member points to an object that
is \fIdsize\fP bytes in length. Arbitrary binary data, as well as
character strings, may be stored in the object pointed to by
\fIdptr\fP.
.LP
The database is stored in two files. One file is a directory containing
a bitmap of keys and has \fB.dir\fP as its suffix. The
second file contains all data and has \fB.pag\fP as its suffix.
.LP
The \fIdbm_open\fP() function shall open a database. The \fIfile\fP
argument to the function is the pathname of the database.
The function opens two files named \fIfile\fP\fB.dir\fP and \fIfile\fP\fB.pag\fP.
The \fIopen_flags\fP argument has the same
meaning as the \fIflags\fP argument of \fIopen\fP() 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
\fIfile_mode\fP argument has the same meaning as the third argument
of \fIopen\fP().
.LP
The \fIdbm_close\fP() function shall close a database. The application
shall ensure that argument \fIdb\fP is a pointer to a
\fBdbm\fP structure that has been returned from a call to \fIdbm_open\fP().
.LP
These database functions shall support an internal block size large
enough to support key/content pairs of at least 1023
bytes.
.LP
The \fIdbm_fetch\fP() function shall read a record from a database.
The argument \fIdb\fP is a pointer to a database structure
that has been returned from a call to \fIdbm_open\fP(). The argument
\fIkey\fP is a \fBdatum\fP that has been initialized by the
application to the value of the key that matches the key of the record
the program is fetching.
.LP
The \fIdbm_store\fP() function shall write a record to a database.
The argument \fIdb\fP is a pointer to a database structure
that has been returned from a call to \fIdbm_open\fP(). The argument
\fIkey\fP is a \fBdatum\fP 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 \fIcontent\fP is a \fBdatum\fP that has been
initialized by the application to the value of the record the
program is writing. The argument \fIstore_mode\fP controls whether
\fIdbm_store\fP() replaces any pre-existing record that has
the same key that is specified by the \fIkey\fP argument. The application
shall set \fIstore_mode\fP to either DBM_INSERT or
DBM_REPLACE. If the database contains a record that matches the \fIkey\fP
argument and \fIstore_mode\fP is DBM_REPLACE, the
existing record shall be replaced with the new record. If the database
contains a record that matches the \fIkey\fP argument and
\fIstore_mode\fP 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 \fIkey\fP argument and \fIstore_mode\fP
is either DBM_INSERT or DBM_REPLACE, the new record
shall be inserted in the database.
.LP
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 \fIdbm_store\fP() function shall return an error
in the event that a disk block fills with inseparable data.
.LP
The \fIdbm_delete\fP() function shall delete a record and its key
from the database. The argument \fIdb\fP is a pointer to a
database structure that has been returned from a call to \fIdbm_open\fP().
The argument \fIkey\fP is a \fBdatum\fP that has been
initialized by the application to the value of the key that identifies
the record the program is deleting.
.LP
The \fIdbm_firstkey\fP() function shall return the first key in the
database. The argument \fIdb\fP is a pointer to a database
structure that has been returned from a call to \fIdbm_open\fP().
.LP
The \fIdbm_nextkey\fP() function shall return the next key in the
database. The argument \fIdb\fP is a pointer to a database
structure that has been returned from a call to \fIdbm_open\fP().
The application shall ensure that the \fIdbm_firstkey\fP()
function is called before calling \fIdbm_nextkey\fP(). Subsequent
calls to \fIdbm_nextkey\fP() return the next key until all of
the keys in the database have been returned.
.LP
The \fIdbm_error\fP() function shall return the error condition of
the database. The argument \fIdb\fP is a pointer to a
database structure that has been returned from a call to \fIdbm_open\fP().
.LP
The \fIdbm_clearerr\fP() function shall clear the error condition
of the database. The argument \fIdb\fP is a pointer to a
database structure that has been returned from a call to \fIdbm_open\fP().
.LP
The \fIdptr\fP pointers returned by these functions may point into
static storage that may be changed by subsequent calls.
.LP
These functions need not be reentrant. A function that is not required
to be reentrant is not required to be thread-safe.
.SH RETURN VALUE
.LP
The \fIdbm_store\fP() and \fIdbm_delete\fP() functions shall return
0 when they succeed and a negative value when they
fail.
.LP
The \fIdbm_store\fP() function shall return 1 if it is called with
a \fIflags\fP value of DBM_INSERT and the function finds an
existing record with the same key.
.LP
The \fIdbm_error\fP() function shall return 0 if the error condition
is not set and return a non-zero value if the error
condition is set.
.LP
The return value of \fIdbm_clearerr\fP() is unspecified.
.LP
The \fIdbm_firstkey\fP() and \fIdbm_nextkey\fP() functions shall return
a key \fBdatum\fP. When the end of the database is
reached, the \fIdptr\fP member of the key is a null pointer. If an
error is detected, the \fIdptr\fP member of the key shall be a
null pointer and the error condition of the database shall be set.
.LP
The \fIdbm_fetch\fP() function shall return a content \fBdatum\fP.
If no record in the database matches the key or if an error
condition has been detected in the database, the \fIdptr\fP member
of the content shall be a null pointer.
.LP
The \fIdbm_open\fP() function shall return a pointer to a database
structure. If an error is detected during the operation,
\fIdbm_open\fP() shall return a ( \fBDBM *\fP)0.
.SH ERRORS
.LP
No errors are defined.
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
The following code can be used to traverse the database:
.sp
.RS
.nf

\fBfor(key = dbm_firstkey(db); key.dptr != NULL; key = dbm_nextkey(db))
\fP
.fi
.RE
.LP
The \fIdbm_\fP* 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.
.LP
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 (
\fIdbm_error\fP() 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.
.LP
The \fIdbm_delete\fP() function need not physically reclaim file space,
although it does make it available for reuse by the
database.
.LP
After calling \fIdbm_store\fP() or \fIdbm_delete\fP() during a pass
through the keys by \fIdbm_firstkey\fP() and
\fIdbm_nextkey\fP(), the application should reset the database by
calling \fIdbm_firstkey\fP() before again calling
\fIdbm_nextkey\fP(). The contents of these files are unspecified and
may not be portable.
.SH RATIONALE
.LP
None.
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIopen\fP() , the Base Definitions volume of IEEE\ Std\ 1003.1-2001,
\fI<ndbm.h>\fP
.SH 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 .