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Full Discussion: Understanding lseek
Top Forums UNIX for Dummies Questions & Answers Understanding lseek Post 302705313 by alister on Monday 24th of September 2012 02:32:52 PM
Old 09-24-2012
Quote:
Originally Posted by Deepak Raj
I tried to use lseek system call to determine the number of bytes in a file. To do so, I used open system call with O_APPEND flag to open a file. As lseek returns the current offset so I called lseek for opened file with offset as zero and whence as SEEK_CUR. So I guess it must return the number of bytes as the file is ready to append and lseek seeks only 0 bytes. But result is showing 0 bytes. Please correct me if I have understood wrong.
Your entire line of reasoning depends on the open() syscall placing the file pointer at the end of the file. That is not a given.

From the POSIX open() system call manual page
Quote:
O_APPEND
If set, the file offset shall be set to the end of the file prior to each write.
Additionally, keep in mind that writes from other processes won't adjust your descriptor's offset.

Regards,
Alister

---------- Post updated at 02:32 PM ---------- Previous update was at 02:18 PM ----------

Quote:
Originally Posted by Deepak Raj
Code:
   printf("File size in bytes: %ld\n", lseek(fd, 0, SEEK_CUR));

That's not a safe use of printf. You're assuming that off_t and long are the same width. I suggest something like:
Code:
printf("lseek return value: %jd\n", (intmax_t) lseek(fd, 0, SEEK_CUR));

Regards,
Alister
 

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LEARN ABOUT DEBIAN

explain_lseek_or_die

explain_lseek_or_die(3) 				     Library Functions Manual					   explain_lseek_or_die(3)

NAME

       explain_lseek_or_die - reposition file offset and report errors

SYNOPSIS

       #include <libexplain/lseek.h>
       long long explain_lseek_or_die(int fildes, long long offset, int whence);

DESCRIPTION

       The  explain_lseek_or_die function is used to call the lseek(2) system call.  On failure an explanation will be printed to stderr, obtained
       from explain_lseek(3), and then the process terminates by calling exit(EXIT_FAILURE).

       This function is intended to be used in a fashion similar to the following example:
	      long long result = explain_lseek_or_die(fildes, offset, whence);

       fildes  The fildes, exactly as to be passed to the lseek(2) system call.

       offset  The offset, exactly as to be passed to the lseek(2) system call.

       whence  The whence, exactly as to be passed to the lseek(2) system call.

       Returns:
	       On successful, returns the resulting offset location as measured in bytes from the beginning of the file.  On  failure,	prints	an
	       explanation and exits.

SEE ALSO

       lseek(2)
	       reposition file offset

       explain_lseek(3)
	       explain lseek(2) errors

       exit(2) terminate the calling process

COPYRIGHT

       libexplain version 0.52
       Copyright (C) 2008 Peter Miller

															   explain_lseek_or_die(3)
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