gdbm.3

开源的数据库管理系统 说明文档！ gdbm 适用场合很多

.ds ve 1.8.3
.TH GDBM 3 10/15/2002
.SH NAME
GDBM - The GNU database manager.  Includes \fBdbm\fR and \fBndbm\fR
compatability. (Version \*(ve.)
.SH SYNOPSIS
.B #include <gdbm.h>
.PP
.SM
.B extern gdbm_error
.br
.B gdbm_errno
.PP
.B extern char
.br
.B *gdbm_version
.PP
.B GDBM_FILE
.br
.B gdbm_open (name, block_size, read_write, mode, fatal_func)
.br
.B char * name;
.br
.B int block_size, read_write, mode;
.br
.B void (*fatal_func) ();
.PP
.B void
.br
.B gdbm_close (dbf)
.br
.B GDBM_FILE dbf;
.PP
.B int
.br
.B gdbm_store (dbf, key, content, flag)
.br
.B GDBM_FILE dbf;
.br
.B datum key, content;
.br
.B int flag;
.PP
.B datum
.br
.B gdbm_fetch (dbf, key)
.br
.B GDBM_FILE dbf;
.br
.B datum key;
.PP
.B int
.br
.B gdbm_delete (dbf, key)
.br
.B GDBM_FILE dbf;
.br
.B datum key;
.PP
.B datum
.br
.B gdbm_firstkey (dbf)
.br
.B GDBM_FILE dbf;
.PP
.B datum
.br
.B gdbm_nextkey (dbf, key)
.br
.B GDBM_FILE dbf;
.br
.B datum key;
.PP
.B int
.br
.B gdbm_reorganize (dbf)
.br
.B GDBM_FILE dbf;
.PP
.B void
.br
.B gdbm_sync (dbf)
.br
.B GDBM_FILE dbf;
.PP
.B int
.br
.B gdbm_exists (dbf, key)
.br
.B GDBM_FILE dbf;
.br
.B datum key;
.PP
.B char *
.br
.B gdbm_strerror (errno)
.br
.B gdbm_error errno;
.PP
.B int
.br
.B gdbm_setopt (dbf, option, value, size)
.br
.B GDBM_FILE dbf;
.br
.B int option;
.br
.B int *value;
.br
.B int size;
.PP
.B int
.br
.B gdbm_fdesc (dbf)
.br
.B GDBM_FILE dbf;
.PP
.PP
.B DBM Compatability routines:
.PP
.B #include <dbm.h>
.PP
.SM
.B int
.br
.B dbminit (name)
.br
.B char *name;
.PP
.B int
.br
.B store (key, content)
.br
.B datum key, content;
.PP
.B datum
.br
.B fetch (key)
.br
.B datum key;
.PP
.B int
.br
.B delete (key)
.br
.B datum key;
.PP
.B datum
.br
.B firstkey ()
.PP
.B datum
.br
.B nextkey (key)
.br
.B datum key;
.PP
.B int
.br
.B dbmclose ()
.PP
.PP
.B NDBM Compatability routines:
.PP
.B #include <ndbm.h>
.PP
.SM
.B DBM
.br
.B *dbm_open (name, flags, mode)
.br
.B char *name;
.br
.B int flags, mode;
.PP
.B void
.br
.B dbm_close (file)
.br
.B DBM *file;
.PP
.B datum
.br
.B dbm_fetch (file, key)
.br
.B DBM *file;
.br
.B datum key;
.PP
.B int
.br
.B dbm_store (file, key, content, flags)
.br
.B DBM *file;
.br
.B datum key, content;
.br
.B int flags;
.PP
.B int
.br
.B dbm_delete (file, key)
.br
.B DBM *file;
.br
.B datum key;
.PP
.B datum
.br
.B dbm_firstkey (file)
.br
.B DBM *file;
.PP
.B datum
.br
.B dbm_nextkey (file)
.br
.B DBM *file;
.PP
.B int
.br
.B dbm_error (file)
.br
.B DBM *file;
.PP
.B int
.br
.B dbm_clearerr (file)
.br
.B DBM *file;
.PP
.B int
.br
.B dbm_pagfno (file)
.br
.B DBM *file;
.PP
.B int
.br
.B dbm_dirfno (file)
.br
.B DBM *file;
.PP
.B int
.br
.B dbm_rdonly (file)
.br
.B DBM *file;


.SH DESCRIPTION
GNU dbm is a library of routines that manages data files that contain
key/data pairs.  The access provided is that of storing, 
retrieval, and deletion by key and a non-sorted traversal of all
keys.  A process is allowed to use multiple data files at the
same time.

A process that opens a gdbm file is designated as a "reader" or a
"writer".  Only one writer may open a gdbm file and many readers may
open the file.  Readers and writers can not open the gdbm file at the
same time. The procedure for opening a gdbm file is:

  GDBM_FILE dbf;

  dbf = gdbm_open ( name, block_size, read_write, mode, fatal_func )

\fIName\fR is the name of the file (the complete name,
gdbm does not append any characters to this name).  \fIBlock_size\fR is
the size of a single transfer from disk to memory. This parameter is
ignored unless the file is a new file.  The minimum size is 512.  If
it is less than 512, dbm will use the stat block size for the file system.
\fIRead_write\fR can have one of the following values:
.br
.B GDBM_READER
reader
.br
.B GDBM_WRITER
writer
.br
.B GDBM_WRCREAT
writer - if database does not exist create new one
.br
.B GDBM_NEWDB
writer - create new database regardless if one exists
.br
For the last three (writers of the database) the following may be added
added to \fIread_write\fR by bitwise or:
.B GDBM_SYNC,
which causes all database operations to be synchronized to the disk, and
.B GDBM_NOLOCK,
which prevents the library from performing any locking on the database file.
The option
.B GDBM_FAST
is now obsolete, since gdbm defaults to no-sync mode.
.br
\fIMode\fR is the file mode (see \fBchmod(2)\fR and \fBopen(2)\fR) if the
file is created. \fI(*Fatal_func) ()\fR is a function for dbm to call
if it detects a fatal error. The only parameter of this function is a string.
If the value of 0 is provided, gdbm will use a default function.

The return value \fIdbf\fR is the pointer needed by all other routines to
access that gdbm file.  If the return is the NULL pointer, \fBgdbm_open\fR
was not successful.  The errors can be found in \fIgdbm_errno\fR for gdbm
errors and in \fIerrno\fR for system errors.  (For error codes, see
gdbmerrno.h.)

In all of the following calls, the parameter \fIdbf\fR refers to the pointer
returned from \fBgdbm_open\fR.

It is important that every file opened is also closed.  This is needed to
update the reader/writer count on the file.  This is done by:

  gdbm_close (dbf);


The database is used by 3 primary routines.  The first stores data in the
database.

  ret = gdbm_store ( dbf, key, content, flag )

\fIDbf\fR is the pointer returned by \fBgdbm_open\fR.  \fIKey\fR is the
key data.  \fIContent\fR is the data to be associated with the \fIkey\fR.
\fIFlag\fR can have one of the following values:
.br
.B GDBM_INSERT
insert only, generate an error if key exists
.br
.B GDBM_REPLACE
replace contents if key exists.

If a reader calls \fBgdbm_store\fR, the return value will be  -1.
If called with GDBM_INSERT and \fIkey\fR is in the database, the return
value will be 1.  Otherwise, the return value is 0.

\fINOTICE: 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.

NOTICE: The size in gdbm is not restricted like dbm or ndbm.  Your data
can be as large as you want.\fR


To search for some data:

  content = gdbm_fetch ( dbf, key )

\fIDbf\fR is the pointer returned by \fBgdbm_open\fR.  \fIKey\fR is
the key data.


If the \fIdptr\fR element of the return value is NULL, no data was
found.  Otherwise the return value is a pointer to the found data.
The storage space for the \fIdptr\fR element is allocated using
\fBmalloc(3C)\fR.  \fBGdbm\fI does not automatically free this data.
It is the programmer's responsibility to free this storage when it is
no longer needed.\fR


To search for some data, without retrieving it:

  ret = gdbm_exists ( dbf, key )

\fIDbf\fR is the pointer returned by \fBgdbm_open\fR.  \fIKey\fR is
the key data to search for.

If the \fIkey\fR is found within the database, the return value \fIret\fR
will be true.  If nothing appropiate is found, \fIret\fR will be false.
This routine is useful for checking for the existance of a record,
without performing the memory allocation done by \fBgdbm_fetch\fR.


To remove some data from the database:

  ret = gdbm_delete ( dbf, key )

\fIDbf\fR is the pointer returned by \fBgdbm_open\fR.  \fIKey\fR is the
key data.

The return value is -1 if the item is not present or the requester is a reader.
The return value is 0 if there was a successful delete.


The next two routines 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.)

  key = gdbm_firstkey ( dbf )

  nextkey = gdbm_nextkey ( dbf, key )

\fIDbf\fR is the pointer returned by \fBgdbm_open\fR. \fIKey\fR is the
key data.

The return values are both of type \fBdatum\fR.  If the \fIdptr\fR
element of the return value is NULL, there is no first key or next key.
Again notice that \fIdptr\fR points to data allocated by \fBmalloc(3C)\fR
and \fBgdbm\fR will not free it for you. 

These functions were intended to visit the database in read-only algorithms,
for instance, to validate the database or similar operations.

File `visiting' is based on a `hash table'.  \fIgdbm_delete\fR 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.  It is possible that some key will not be visited if a loop like
the following is executed:

   key = gdbm_firstkey ( dbf );
   while ( key.dptr ) {
      nextkey = gdbm_nextkey ( dbf, key );
      if ( some condition ) {
         gdbm_delete ( dbf, key );
         free ( key.dptr );
      }
      key = nextkey;
   }


The following routine should be used very infrequently.
  
  ret = gdbm_reorganize ( dbf )

If you have had a lot of deletions and would like to shrink the space
used by the \fBgdbm\fR file, this routine will reorganize the database.
\fBGdbm\fR will not shorten the length of a \fBgdbm\fR file except by
using this reorganization.  (Deleted file space will be reused.)


Unless your database was opened with the GDBM_SYNC flag, gdbm does not
wait for writes to be flushed to the disk before continuing.
The following routine can be used to guarantee that the database is
physically written to the disk file.

  gdbm_sync ( dbf )

It will not return until the disk file state is syncronized with the
in-memory state of the database.


To convert a \fBgdbm\fR error code into English text, use this routine:

  ret = gdbm_strerror ( errno )

Where \fIerrno\fR is of type \fIgdbm_error\fR, usually the global
variable \fIgdbm_errno\fR.  The appropiate phrase is returned.


\fBGdbm\fR now supports the ability to set certain options on an
already open database.

  ret = gdbm_setopt ( dbf, option, value, size )

Where \fIdbf\fR is the return value from a previous call to \fBgdbm_open\fR,
and \fIoption\fR specifies which option to set.  The valid options are
currently:

  \fBGDBM_CACHESIZE\fR - Set the size of the internal bucket
  cache. This option may only be set once on each \fIGDBM_FILE\fR
  descriptor, and is set automatically to 100 upon the first
  access to the database.

  \fBGDBM_FASTMODE\fR - Set \fBfast mode\fR to either on or off.  This
  allows \fBfast mode\fR to be toggled on an already open and
  active database. \fIvalue\fR (see below) should be set to either
  TRUE or FALSE.  \fIThis option is now obsolete.\fR

  \fBGDBM_SYNCMODE\fR - Turn on or off file system synchronization operations.
  This setting defaults to off; \fIvalue\fR (see below) should be set to either
  TRUE or FALSE.

  \fBGDBM_CENTFREE\fR - Set \fBcentral free block pool\fR to either on or off.
  The default is off, which is how previous versions of \fBGdbm\fR
  handled free blocks. If set, this option causes all subsequent free
  blocks to be placed in the \fBglobal\fR pool, allowing (in thoery)
  more file space to be reused more quickly. \fIvalue\fR (see below) should
  be set to either TRUE or FALSE.
  \fINOTICE: This feature is still under study.\fR

  \fBGDBM_COALESCEBLKS\fR - Set \fBfree block merging\fR to either on or off.
  The default is off, which is how previous versions of \fBGdbm\fR
  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 \fBGDBM_CENTFREE\fR. \fIvalue\fR
  (see below) should be set to either TRUE or FALSE.
  \fINOTICE: This feature is still under study.\fR

\fIvalue\fR is the value to set \fIoption\fR to, specified as an integer
pointer.  \fIsize\fR is the size of the data pointed to by \fIvalue\fR.
The return value will be -1 upon failure, or 0 upon success.  The global
variable \fIgdbm_errno\fR will be set upon failure.

For instance, to set a database to use a cache of 10, after opening it
with \fBgdbm_open\fR, but prior to accessing it in any way, the following
code could be used:

  int value = 10;
  
  ret = gdbm_setopt( dbf, GDBM_CACHESIZE, &value, sizeof(int));


If the database was opened with the \fBGDBM_NOLOCK\fR flag, the user may
wish 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 \fIgdbm_fdesc\fR routine is provided.

  ret = gdbm_fdesc ( dbf )

Where \fIdbf\fR is the return value from a previous call to \fBgdbm_open\fR.
The return value will be the file descriptor of the database.

The following two external variables may be useful:

\fIgdbm_errno\fR is the variable that contains more information about
gdbm errors.  (gdbm.h has the definitions of the error values and
defines gdbm_errno as an external variable.)
.br
\fIgdbm_version\fR is the string containing the version information.


There are a few more things of interest.  First, \fBgdbm\fR files are
not "sparse".  You can copy them with the UNIX \fBcp(1)\fR command and
they will not expand in the copying process.  Also, there is a
compatibility mode for use with programs that already use UNIX
\fBdbm\fR.  In this compatibility mode, no \fRgdbm\fR file pointer is
required by the programmer, and only one file may be opened at a time.
All users in compatibility mode are assumed to be writers.  If the
\fBgdbm\fR file is a read only, it will fail as a writer, but will
also try to open it as a reader.  All returned pointers in datum
structures point to data that \fBgdbm\fR WILL free.  They should be
treated as static pointers (as standard UNIX \fBdbm\fR does).


.SH LINKING
This library is accessed by specifying \fI-lgdbm\fR as the last
parameter to the compile line, e.g.:
.sp
	gcc -o prog prog.c -lgdbm

If you wish to use the \fBdbm\fR or \fBndbm\fR compatibility routines,
you must link in the \fIgdbm_compat\fR library as well.  For example:
.sp
	gcc -o prog proc.c -lgdbm -lgdbm_compat

.SH BUGS

.SH "SEE ALSO"
dbm, ndbm

.SH AUTHOR
by Philip A. Nelson and Jason Downs.
Copyright (C) 1990 - 1999 Free Software Foundation, Inc.

GDBM is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 1, or (at your option)
any later version.

GDBM is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GDBM; see the file COPYING.  If not, write to
the Free Software Foundation, 675 Mass Ave, Cambridge, MA 02139, USA.

You may contact the original author by:
.br
   e-mail:  [e-mail]
.br
  us-mail:  Philip A. Nelson
.br
Computer Science Department
.br
Western Washington University
.br
Bellingham, WA 98226

You may contact the current maintainer by:
.br
   e-mail:  [e-mail]