gdbm.texinfo

开源的数据库管理系统

@node Delete, Sequential, Fetch, Top
@chapter Removing records from the database.

To remove some data from the database:

@example
ret = gdbm_delete(dbf, key);
@end example

The parameters are:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@item datum key
The @code{key} data.
@end table

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

@code{gdbm_delete} removes the keyed item and the @code{key} from the database
@code{dbf}. The file on disk is updated to reflect the structure of the new
database before returning from this function.

@node Sequential, Reorganization, Delete, Top
@chapter Sequential access to records.

The next two functions allow for accessing all items in the database. This
access is not @code{key} sequential, but it is guaranteed to visit every
@code{key} in the database once. The order has to do with the hash values.
@code{gdbm_firstkey} starts the visit of all keys in the database.
@code{gdbm_nextkey} finds and reads the next entry in the hash structure for
@code{dbf}.

@example
key = gdbm_firstkey(dbf);

nextkey = gdbm_nextkey(dbf, key);
@end example

The parameters are:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@item datum @code{key}
@item datum nextkey
The @code{key} data.
@end table

The return values are both datum. If @code{key}.dptr or nextkey.dptr is NULL,
there is no first @code{key} or next @code{key}. Again notice that dptr points to
data allocated by malloc and @code{gdbm} 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 @code{visiting} is based on a @code{hash table}. @code{gdbm_delete}
re-arranges the hash table to make sure that any collisions in the table do not
leave some item @code{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:

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

@node Reorganization, Sync, Sequential, Top
@chapter Database reorganization.

The following function should be used very seldom.

@example
ret = gdbm_reorganize(dbf);
@end example

The parameter is:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@end table

If you have had a lot of deletions and would like to shrink the space
used by the @code{gdbm} file, this function will reorganize the database.
@code{gdbm} will not shorten the length of a @code{gdbm} file (deleted file space will be
reused) except by using this reorganization.

This reorganization requires creating a new file and inserting all the elements
in the old file @code{dbf} into the new file. The new file is then renamed to
the same name as the old file and @code{dbf} is updated to contain all the
correct information about the new file. If an error is detected, the return
value is negative. The value zero is returned after a successful
reorganization.

@node Sync, Errors, Reorganization, Top
@chapter Database Synchronization

Unless your database was opened with the GDBM_SYNC flag, @code{gdbm} does not
wait for writes to be flushed to the disk before continuing.  This allows
faster writing of databases at the risk of having a corrupted database if
the application terminates in an abnormal fashion.  The following function
allows the programmer to make sure the disk version of the 
database has been completely updated with all changes to the current time.

@example
gdbm_sync(dbf);
@end example

The parameter is:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@end table

This would usually be called after a complete set of changes have been
made to the database and before some long waiting time.
@code{gdbm_close} automatically calls the equivalent of @code{gdbm_sync}
so no call is needed if the database is to be closed immediately after
the set of changes have been made.

@node Errors, Options, Sync, Top
@chapter Error strings.

To convert a @code{gdbm} error code into English text, use this routine:

@example
ret = gdbm_strerror(errno)
@end example

The parameter is:

@table @asis
@item gdbm_error errno
The @code{gdbm} error code, usually @code{gdbm_errno}.
@end table

The appropiate phrase for reading by humans is returned.

@node Options, Locking, Errors, top
@chapter Seting options.

@code{Gdbm} supports the ability to set certain options on an already
open database.

@example
ret = gdbm_setopt(dbf, option, value, size);
@end example

The parameters are:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@item int option
The option to be set.
@item int *value
A pointer to the value to which @code{option} will be set.
@item int size
The length of the data pointed to by @code{value}.
@end table

The valid options are:

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

  GDBM_FASTMODE - Set fast mode to either on or off.  This allows
  fast mode to be toggled on an already open and active database.
  value (see below) should be set to either TRUE or FALSE.
  @emph{This option is now obsolete.}

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

  GDBM_CENTFREE - Set central free block pool to either on or off.
  The default is off, which is how previous versions of @code{Gdbm}
  handled free blocks.  If set, this option causes all subsequent free
  blocks to be placed in the @emph{global} pool, allowing (in theory)
  more file space to be reused more quickly.  value (see below) should
  be set to either TRUE or FALSE.
  @emph{NOTICE: This feature is still under study.}

  GDBM_COALESCEBLKS - Set free block merging to either on or off.
  The default is off, which is how previous versions of @code{Gdbm}
  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 GDBM_CENTFREE.  value (see below)
  should be set to either TRUE or FALSE.
  @emph{NOTICE: This feature is still under study.}

The return value will be -1 upon failure, or 0 upon success.  The global
variable @code{gdbm_errno} will be set upon failure.

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

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

@node Locking, Variables, Options, Top
@chapter File Locking.

With locking disabled (if @code{gdbm_open} was called with GDBM_NOLOCK),
the user may want 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 @code{gdbm_fdesc} routine is provided.

@example
ret = gdbm_fdesc(dbf);
@end example

The single valid parameter is:

@table @asis
@item GDBM_FILE dbf
The pointer returned by @code{gdbm_open}.
@end table

The return value will be the file descriptor of the database.

@node Variables, Compatibility, Locking, Top
@chapter Two useful variables.

The following two variables are variables that may need to be used:

@table @asis
@item gdbm_error gdbm_errno
The variable that contains more information about @code{gdbm} errors
(@code{gdbm.h} has the definitions of the error values).
@item char * gdbm_version
The string containing the version information.
@end table

@node Compatibility, Conversion, Variables, Top
@chapter Compatibility with standard @code{dbm} and @code{ndbm}.

GNU @code{dbm} files are not @code{sparse}. You can copy them with the UNIX
@code{cp} command and they will not expand in the copying process.

There is a compatibility mode for use with programs that already use UNIX
@code{dbm} and UNIX @code{ndbm}.

GNU @code{dbm} has compatibility functions for @code{dbm}. For @code{dbm}
compatibility functions, you need the include file @code{dbm.h}.

In this compatibility mode, no @code{gdbm} file pointer is required
by the user, and Only one file may be opened at a time. All users in
compatibility mode are assumed to be writers. If the @code{gdbm} 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 @code{gdbm} WILL
free. They should be treated as static pointers (as standard UNIX @code{dbm}
does). The compatibility function names are the same as the UNIX @code{dbm}
function names. Their definitions follow:

@example
int dbminit(name);
int store(key, content);
datum fetch(key);
int delete(key);
datum firstkey();
datum nextkey(key);
int dbmclose();
@end example

Standard UNIX @code{dbm} and GNU @code{dbm} do not have the same data
format in the file. You cannot access a standard UNIX @code{dbm} file with GNU
@code{dbm}!  If you want to use an old database with GNU @code{dbm}, you must
use the @code{conv2gdbm} program.

Also, GNU @code{dbm} has compatibility functions for @code{ndbm}. For
@code{ndbm} compatibility functions, you need the include file @code{ndbm.h}.

Again, just like @code{ndbm}, any returned datum can be assumed to be static
storage. You do not have to free that memory, the @code{ndbm} compatibility
functions will do it for you.

The functions are:

@example
DBM *dbm_open(name, flags, mode);
void dbm_close(file);
datum dbm_fetch(file, key);
int dbm_store(file, key, @code{content}, flags);
int dbm_delete(file, key);
datum dbm_firstkey(file);
datum dbm_nextkey(file);
int dbm_error(file);
int dbm_clearerr(file);
int dbm_dirfno(file);
int dbm_pagfno(file);
int dbm_rdonly(file);
@end example

If you want to compile an old C program that used UNIX @code{dbm} or @code{ndbm}
and want to use @code{gdbm} files, execute the following @code{cc} command:

@example
cc ... -L/usr/local/lib -lgdbm -lgdbm_compat
@end example

@node Conversion, Bugs, Compatibility, Top
@chapter Converting @code{dbm} files to @code{gdbm} format.

The program @code{conv2gdbm} has been provided to help you convert from @code{dbm}
databases to @code{gdbm}. The usage is:

@example
conv2gdbm [-q] [-b block_size] dbm_file [gdbm_file]
@end example

The options are:

@table @asis
@item -q
Causes @code{conv2gdbm} to work quietly.
@item block_size
Is the same as in @code{gdbm_open}.
@item dbm_file
Is the name of the @code{dbm} file without the @code{.pag} or @code{.dir}
extensions.
@item gdbm_file
Is the complete file name. If not included, the @code{gdbm} file name is the
same as the @code{dbm} file name without any extensions. That is
@code{conv2gdbm} @code{dbmfile} converts the files @code{dbmfile.pag} and
@code{dbmfile.dir} into a @code{gdbm} file called @code{dbmfile}.
@end table


@node Bugs, , Conversion, Top
@chapter Problems and bugs.

If you have problems with GNU @code{dbm} or think you've found a bug,
please report it. Before reporting a bug, make sure you've actually
found a real bug. Carefully reread the documentation and see if it
really says you can do what you're trying to do. If it's not clear
whether you should be able to do something or not, report that too; it's
a bug in the documentation!

Before reporting a bug or trying to fix it yourself, try to isolate it
to the smallest possible input file that reproduces the problem. Then
send us the input file and the exact results @code{gdbm} gave you. Also
say what you expected to occur; this will help us decide whether the
problem was really in the documentation.

Once you've got a precise problem, send e-mail to:

@example
Internet: @file{bug-gnu-utils@@prep.ai.mit.edu}.
UUCP: @file{mit-eddie!prep.ai.mit.edu!bug-gnu-utils}.
@end example

Please include the version number of GNU @code{dbm} you are using. You can get
this information by printing the variable @code{gdbm_version} (see Variables).

Non-bug suggestions are always welcome as well. If you have questions
about things that are unclear in the documentation or are just obscure
features, please report them too.

You may contact the author by:
@example
    e-mail:  phil@@cs.wwu.edu
   us-mail:  Philip A. Nelson
             Computer Science Department
             Western Washington University
             Bellingham, WA 98226
@end example

You may contact the current maintainer by:
@example
    e-mail:  downsj@@downsj.com
@end example

@bye