seek function always returns true

rudyg123 has asked for the wisdom of the Perl Monks concerning the following question:

Hi, I'm very new to Perl programming so please excuse me if this problem I'm having sounds basic. I am writing a script to parse a binary file. The file contains hex data that is formatted in such a way that I can jump anywhere in the file to read data. The program prompts the user for input asking which record in the file to read. There is a possibility that the record is not contained in the file. This can happen if the file is too small (not enough records to satisfy the input parameter request). The following code is executed and my problem is that the die is never executed. The seek is always returning a 1. I have run test cases where I know the record I am asking for is greater than the contents of the file. In other words the offset in bytes from the beginning of the file is greater than the number of bytes in the file. Why doesn't the seek return 0.

seek(FIN, $start_pc, 0) or die "seek:$!";
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Re: seek function always returns true by Joost (Canon) on Jun 19, 2007 at 22:24 UTC
UNIX at least, it's explicitly allowed to seek past the end of a file (so that you can create files with holes in them). You'll probably only get an error if you try to read at that point. "What should it profit a man, if he should win a flame war, yet lose his cool?"	[reply]
Re^2: seek function always returns true by johngg (Canon) on Jun 19, 2007 at 22:31 UTC
I had heard of files with holes but didn't realise the implicatons for `seek`. Looks like you do get the read error on Solaris. `$ ls -l xxxx -rw-r--r-- 1 johngg users 639 Jun 19 23:26 xxxx $ perl -le ' > open $in, q{<}, q{xxxx} or die $!; > seek $in, 1000, 0 == tell $in or die q{bad seek}; > print tell $in; > 10 == read $in, $buf, 10 or die q{bad read}; > print tell $in; > print $buf;' 1000 bad read at -e line 5. $` [download] Cheers, JohnGG	[reply] [d/l] [select]
Re^2: seek function always returns true by cdarke (Prior) on Jun 20, 2007 at 07:40 UTC
Same on Windows (SetFilePointerEx API).	[reply]
Re: seek function always returns true by graff (Chancellor) on Jun 20, 2007 at 02:20 UTC
It's easy enough to get the current size of the file: `my $current_file_size = -s $file_name;` [download] So you just need to have that value, and check it against the number that the user provides as the offset to go to: `if ( $start_pc >= $current_file_size ) { warn sprintf( "Your requested offset (%d) is past EOF (%d)\n", $start_pc, $current_file_size ); # and do something else, like prompt for new input }` [download] Of course, if the user is asking for fixed-length-record $n, you'd want to say that the requested $n is greater than the number of records in the file (file size divided by record size, which, in an ideal world, would always be an integer value).	[reply] [d/l] [select]
Re^2: seek function always returns true by rudyg123 (Initiate) on Jun 20, 2007 at 14:10 UTC
Thanks for all who responded to my question. I didn't realize how seek actually worked until you guys posted these responses. I didn't know that seek() could actually go beyond the EOF. I tried the code that you guys provided and it worked for me. The key for me was doing the read(). If you try and read past the EOF you do get an error. RG	[reply]
Re: seek function always returns true by crouchingpenguin (Priest) on Jun 20, 2007 at 13:17 UTC
See eof: `open(my $fh, '>', 'f00') or die $!; seek($fh, 100, 0) or die "seek:$!"; print "eof!" if eof($fh); close($fh);` [download] cp ---- "Never be afraid to try something new. Remember, amateurs built the ark. Professionals built the Titanic."	[reply] [d/l]