dbmopen

dbmopen HASH,DBNAME,MASK

[This function has been largely superseded by the tie function.]

This binds a dbm(3), ndbm(3), sdbm(3), gdbm(3), or Berkeley DB file to a hash. HASH is the name of the hash. (Unlike normal open, the first argument is not a filehandle, even though it looks like one). DBNAME is the name of the database (without the .dir or .pag extension if any). If the database does not exist, it is created with protection specified by MASK (as modified by the umask). To prevent creation of the database if it doesn't exist, you may specify a MODE of 0, and the function will return a false value if it can't find an existing database. If your system supports only the older DBM functions, you may make only one dbmopen call in your program. In older versions of Perl, if your system had neither DBM nor ndbm, calling dbmopen produced a fatal error; it now falls back to sdbm(3).

If you don't have write access to the DBM file, you can only read hash variables, not set them. If you want to test whether you can write, either use file tests or try setting a dummy hash entry inside an eval to trap the error.

Note that functions such as keys and values may return huge lists when used on large DBM files. You may prefer to use the each function to iterate over large DBM files. Example:

# print out history file offsets
dbmopen(%HIST,'/usr/lib/news/history',0666);
while (($key,$val) = each %HIST) {
    print $key, ' = ', unpack('L',$val), "\n";
}
dbmclose(%HIST);

See also AnyDBM_File for a more general description of the pros and cons of the various dbm approaches, as well as DB_File for a particularly rich implementation.

You can control which DBM library you use by loading that library before you call dbmopen():

use DB_File;
dbmopen(%NS_Hist, "$ENV{HOME}/.netscape/history.db")
    or die "Can't open netscape history file: $!";

Portability issues: dbmopen in perlport.