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16. Compatibility with standard dbm and ndbm.

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

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

GNU dbm has compatibility functions for dbm. For dbm compatibility functions, you need the include file dbm.h.

In this compatibility mode, no 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 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 gdbm WILL free. They should be treated as static pointers (as standard UNIX dbm does). The compatibility function names are the same as the UNIX dbm function names. Their definitions follow:

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

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

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

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

The functions are:

 
DBM *dbm_open(name, flags, mode);
void dbm_close(file);
datum dbm_fetch(file, key);
int dbm_store(file, key, 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);

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

 
cc ... -L/usr/local/lib -lgdbm -lgdbm_compat


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