string and sysread and read work differently

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

I need to read a file at a starting position ( != 0) and keep reading until I encounter a "\n" (ordinal 10), then stop. I am using sysopen, sysseek and sysread. The characters I am fetching are just fine (they display properly). The problem comes in when I try to determine the ordinal value of the just-read character ($c):

sysread(RCVQ,$c,1,$ptr);

my $x = ord($c); print "$c=$x\n"; yields T=0 (zero). Actually all characters are = ordinal 0, which is clearly wrong.

ord() will provide the correct ordinal number if I say ord('E') or $c = 'E'; ord($c). So it is not ord() that is broken.

per dox, ord() returns the ordinal value of the first character of the string it is passed. Does Perl infer that $c is a character and pass a 2 or more byte string (unicode??) to ord() ???

Comment on string and sysread and read work differently

Replies are listed 'Best First'.
Re: string and sysread and read work differently by seattlejohn (Deacon) on Dec 24, 2002 at 00:29 UTC
Is `$ptr` zero when you call `sysread`? What happens if you omit `$ptr` from the call? If `$ptr` is nonzero, `sysread` is going to insert the characters into `$c` someplace other than the beginning of the string. I would suspect that `$c` gets filled from the left with `chr(0)` if needed, which is why `ord` then returns `0`. If you wanted to reliably get the ordinal of the string you just read, I think you'd need to use something like `my $x = ord(substr($c,$ptr,1));`. Or get rid of `$ptr` (or make sure it's really `0`) so you are actually reading into the beginning of `$c`. $perlmonks{seattlejohn} = 'John Clyman';	[reply]
Re: Re: string and sysread and read work differently by gary kuipers (Beadle) on Dec 24, 2002 at 01:18 UTC
DOH! I am using the OFFSET $ptr as an offset in the file I am reading instead of what it really is ... an offset into the string I am placing the data into. ARRRGH! Too tired! Thanks for your help!!!	[reply]
Re: string and sysread and read work differently by Zaxo (Archbishop) on Dec 24, 2002 at 00:36 UTC
It would be simpler to use the buffered operations for this. That gives you automatic record seperator detection: `my $string { local $/ = "\n"; open my $fle, '<', $filepath or die $!; seek $fle, $offset, 0 or die $!; $string = <$fle>; close $fle or die $!; }` [download] You could position without seek by first reading a dummy line with `local $/ = \$offset;`, but that is unnecessarily opaque. After Compline, Zaxo	[reply] [d/l]