What is GDBM_File and Fcntl ? Please comment this piece of code

[Ahmad]

Hi ,

I'm newbie in Perl, and I have some Perl scripts that contains some
text processing.

In the piece of Perl code shown below:
What do the following lines mean? What are the "Fcntl" and
"GDBM_File" ? Finally, what is the usage of "tie" and "untie"
functions, and what type of arguments are passed to them below?!

use Fcntl;
use GDBM_File;
tie %datain001,GDBM_File,"IRD_db/mainarea_db", O_RDWR | O_CREAT, 0644;
.....
.....
untie %datain001;


Thanks and regards
Ahmad

 

[RedGrittyBrick]

Hi ,

I'm newbie in Perl, and I have some Perl scripts that contains some
text processing.

In the piece of Perl code shown below:
What do the following lines mean? What are the "Fcntl" and
"GDBM_File" ?

They are Perl modules. A bit like subroutine/class/method libraries in
other programming languages.

You can read about them by opening a command prompt and typing#
perldoc Fcntl
perldoc GDBM_File

Finally, what is the usage of "tie" and "untie"
functions, and what type of arguments are passed to them below?!

See
perldoc -f tie
perldoc -f untie
perldoc perltie

use Fcntl;
use GDBM_File;
tie %datain001,GDBM_File,"IRD_db/mainarea_db", O_RDWR | O_CREAT, 0644;
....
....
untie %datain001;

You might want to read an introductory tutorial first ...
http://www.wdvl.com/Authoring/Languages/Perl/PerlfortheWeb/tie_a_hash.html

 

[xhoster]

Hi ,

I'm newbie in Perl, and I have some Perl scripts that contains some
text processing.

As a newbie, one of the first things you should do is learn how to
use the docs.

In the piece of Perl code shown below:
What do the following lines mean? What are the "Fcntl" and
"GDBM_File" ?

On a linux command line, I type "perldoc Fcntl" and get:

NAME
Fcntl - load the C Fcntl.h defines
.....

And for "perldoc GDBM_File":

NAME
GDBM_File - Perl5 access to the gdbm library.
.....

Finally, what is the usage of "tie" and "untie"
functions, and what type of arguments are passed to them below?!

perldoc -f tie
perldoc -f untie

use Fcntl;
use GDBM_File;
tie %datain001,GDBM_File,"IRD_db/mainarea_db", O_RDWR | O_CREAT, 0644;

The overall effect is that stuff put into the hash %datain001 is now stored
on disk in the file IRD_db/mainarea_db, rather than in RAM.

The purpose of Fcntl is to define O_RDWR and O_CREAT

Xho

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[Ben Morrow]

I'm newbie in Perl, and I have some Perl scripts that contains some
text processing.

In the piece of Perl code shown below:
What do the following lines mean? What are the "Fcntl" and
"GDBM_File" ? Finally, what is the usage of "tie" and "untie"
functions, and what type of arguments are passed to them below?!

use Fcntl;
use GDBM_File;

Fcntl and GDBM_File are modules; that is, pieces of Perl code (and
sometimes C code) that have already been written for you. 'use' is the
Perl keyword for pulling a module into your program: if you are familiar
with C, it's like a combination of '#include <foo.h>' and '-lfoo' on the
link line. Most other languages have some equivalent facility.

From your point of view, as a user of modules, all you need to know to
start with is that 'use' pulls a module in, and you can pass the module
arguments. To find out what the module does, and what arguments it
takes, you need to read 'perldoc Module'; that is, in this case, you
would type

perldoc Fcntl

at your command line and read the text it gives you. (Of course, if you

tie %datain001,GDBM_File,"IRD_db/mainarea_db", O_RDWR | O_CREAT, 0644;

'tie' attaches some 'magic' to a variable, meaning that when you read
and write its value something unusual happens. In this case, you're
using the magic provided by the GDBM_File module, which makes the
entries in the named database appear as elements in the hash.

untie %datain001;

'untie' undoes the 'tie' above; an explicit 'untie' is usually a
mistake.

Ben

 

[Ben Morrow]

You can read about them by opening a command prompt and typing#
perldoc Fcntl
perldoc GDBM_File

Except that those (particular) docs are *entirely useless* unless you
already know what the modules in question do. GDBM_File is
particularly bad: it doesn't even specify the arguments to 'tie' (and
I think the OP's original code was wrong, though maybe in a way that
happened to work).

Ben