rbc has asked for the wisdom of the Perl Monks concerning the following question:

Dear Monks,

Here is my perl script ...

#!/usr/bin/perl -w
use strict;

open( FH, ">junk.dat" ) or die "dang it!\n";

my $num = 1234;
syswrite ( FH, $num );
close FH;

[download]
... Here's what is does ...
$ ./bi.pl

$ cat junk.dat
1234
... but want it to behave more like this C program ...

#include <stdio.h>
main() {
        FILE *fh;
        int num = 1234;

        if(( fh = fopen( "junkc.dat", "w" ) ) == NULL ) {
                printf ("Cannot open it\n");
                exit();
        }
        fwrite( &num, sizeof(int), 1, fh);
        fclose( fh );
}

[download]
$ ./bi

$ cat junkc.dat
Ò

... why does my perl script write 1234 to junk.dat
and not the cool looking O thingy?

thanks

Replies are listed 'Best First'.
Re: writing binary data in perl
by dws (Chancellor) on Apr 11, 2002 at 22:24 UTC
    why does my perl script write 1234 to junk.dat and not the cool looking O thingy?

    You need to use pack() to get the number into a raw form.

    If you're on Win32, you'll also need to add   binmode(FH); to make binary work in general.

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      Or chr(), as in perl -le 'print chr(1234&0xff).chr(1234>>8)'
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        I think the chr(1234>>8) is superfluous.

        --
        John.

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Re: writing binary data in perl
by jmcnamara (Monsignor) on Apr 12, 2002 at 07:38 UTC

    To write binary data like this you should use pack: 
        #!/usr/bin/perl -w

    use strict;

    open( FH, ">junk.dat" ) or die "dang it!\n";

    binmode FH;

    my $num = pack 'V',1234;

    print FH $num;

    close FH;

    [download]

    If the program is going to be run on Windows as well you should also binmode the filehandle as shown above.

    --
    John.

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