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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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[This function has been largely superseded by the tie function.]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). 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
while (($key,$val) = each %HIST) {
    print $key, ' = ', unpack('L',$val), "\n";

See also "Provide framework for multiple DBMs" (AnyDBM_File) in the Perl Library Reference Manual (Volume 3) for a more general description of the pros and cons of the various dbm approaches, as well as "Perl5 access to Berkeley DB version 1.x" (DB_File) in the Perl Library Reference Manual (Volume 3) 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: $!";
ISBN 9781906966027Perl Language Reference ManualSee the print edition