Re: explanation for sysread and syswrite

[dbwiz]

Seriously, bahadur,

You should read How (Not) To Ask A Question.

It it seems too much, here's the bottom line: We are here to help, (also because many of us learn something new every day when we help solving problems). But to solve your problem we require that you play by the rules and show at least some effort.

Asking us to help without even giving a hint of what you have done, looks definitely out of order.

Please read the guidelines and try again.

[bahadur]

ok here is my code

print "Opening the file\n";                    open(F,"<$pair1[1]");
print "changing the mode to bin\n";
binmode F;
#binmode $new_sock;
my $bufsize = 100;
my $buffer=0;
my $n = sysread(F,$buffer,$bufsize);
print "the value of n is $n\n";                close(F);

my problem starts with the line my $n = sysread... the third last line. the $bufsize is the amount of bytes to be read from the file. because every file will be of a different length so i cant know how much the value will be. so how do i know the size of a file in bytes.
the next problem is receiving the data on the client side.

can any one write a piece of code how to receive the data on the client end.

[Joost]

You should re-read the docs more closely:

Returns the number of bytes actually read, 0 at end of file, or undef if there was an error (in the latter case $! is also set).

[dbwiz]

That's better.

A few comments:

• Always check the result of a file open or I/O operation.
• You don't need to know the size of your file to use sysread. You can say how much sysread should read, and that function will return the number of bytes read, or 0 when it's finished.
• If you really need to know the file size, use a -s check.

Here is an example.

#!/usr/bin/perl
use warnings;
use strict;

my $filename = "stable.tar.gz"; # it's the latest Perl source code

die "filename not found\n" unless -f $filename;

my $size = -s $filename;

my $total_read = 0;

open F, "< $filename"
        or die "can't open $filename\n";

my $bufsize = 2_000_000;
my $buffer;

while ( my $read = sysread(F , $buffer , $bufsize,  ) )
{
    printf "Read %8u bytes\n", $read;
    # do something with $buffer
    $total_read += $read;
}

print "initial size: $size\n";
print "total read:   $total_read\n";
__END__
output:
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read  2000000 bytes
Read   254916 bytes
initial size: 12254916
total read:   12254916

HTH

[bahadur]

thanks for that
so if i give the buffer size the same as the size of the file what would happen. would it write the whole thing to the scalar at once or what?

[bahadur]

and hey how do i change the mode back to normal.
like one mode is binmode. what is the other mode? how do i undo binmode?