Understanding lseek Post: 302705501

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Top Forums UNIX for Dummies Questions & Answers Understanding lseek Post 302705501 by Deepak Raj on Tuesday 25th of September 2012 04:54:57 AM

09-25-2012

Registered User

Thanks Jim. It's an nice and simple idea for identifying the file size.

Quote:

Originally Posted by jim mcnamara

File pointers do not necesarily have any relevance to file size. Use fstat() on an open file. lseek() is meant to move file pointers, other uses may not work as you found out.

Code:

#include <sys/stat.h>
size_t filesize(int fd)
{
    struct stat st;
    if(fstat(fd, &st)==-1)
    {
        perror("Cannot stat file");
        exit(1);
    }
    return st.st_size;
}

// usage someplace else in your code

FILE *in=fopen(somefile.dat, "a");
size_t sz=filesize(fileno(in));

---------- Post updated at 02:17 PM ---------- Previous update was at 01:19 PM ----------

Thanks Alister

That was helpful. Open syscall used with O_APPEND option doesn't position the offset to end of the file. Rather offset is positioned to the 'seeked' place just before any write operation occurs. This is what I observed with with the following code.

#include <unistd.h>
#include <stdio.h>
#include <fcntl.h>

int main(void)
{
int fd;

if((fd = open("./example.c", O_WRONLY | APPEND)) < 0) {
perror("Open failed: ");
exit(1);
}
printf("Offset after append flag: %jd\n", (intmax_t) lseek(fd, 0, SEEK_CUR));
if(write(fd, "end", 3) != 3)
perror("Write failed: ");
printf("Offset after write: %jd\n", (intmax_t) lseek(fd, 0, SEEK_CUR));
close(fd);

exit(0);
}

And Thanks for correcting me with format specifier %jd and type conversion of lseek's return value. It helps with compatibility issue, that's what I found out. It will be appreciable if you can explain it.

Regards
Deepak

---------- Post updated at 02:24 PM ---------- Previous update was at 02:17 PM ----------

Hi

Okay, now I thought of a way to use lseek to know number of bytes in file. I only need to simply replace whence SEEK_CUR to SEEK_END. I guess, it works because we can't make sure that offset is set to EOF using O_APPEND, but SEEK_END with zero as offset will set it to EOF, surly. Correct me if I'm incorrect.

Regards
Deepak

Deepak Raj

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LEARN ABOUT SUSE

lseek

LSEEK(2)						     Linux Programmer's Manual							  LSEEK(2)

NAME

       lseek - reposition read/write file offset

SYNOPSIS

       #include <sys/types.h>
       #include <unistd.h>

       off_t lseek(int fd, off_t offset, int whence);

DESCRIPTION

       The lseek() function repositions the offset of the open file associated with the file descriptor fd to the argument offset according to the
       directive whence as follows:

       SEEK_SET
	      The offset is set to offset bytes.

       SEEK_CUR
	      The offset is set to its current location plus offset bytes.

       SEEK_END
	      The offset is set to the size of the file plus offset bytes.

       The lseek() function allows the file offset to be set beyond the end of the file (but this does not change the size of the file).  If  data
       is  later written at this point, subsequent reads of the data in the gap (a "hole") return null bytes ('') until data is actually written
       into the gap.

RETURN VALUE

       Upon successful completion, lseek() returns the resulting offset location as measured in bytes from the beginning of the file.	Otherwise,
       a value of (off_t) -1 is returned and errno is set to indicate the error.

ERRORS

       EBADF  fd is not an open file descriptor.

       EINVAL whence  is  not one of SEEK_SET, SEEK_CUR, SEEK_END; or the resulting file offset would be negative, or beyond the end of a seekable
	      device.

       EOVERFLOW
	      The resulting file offset cannot be represented in an off_t.

       ESPIPE fd is associated with a pipe, socket, or FIFO.

CONFORMING TO

       SVr4, 4.3BSD, POSIX.1-2001.

NOTES

       This document's use of whence is incorrect English, but maintained for historical reasons.

       Some devices are incapable of seeking and POSIX does not specify which devices must support lseek().

       On Linux, using lseek() on a tty device returns ESPIPE.

       When converting old code, substitute values for whence with the following macros:

	old	  new
       0	SEEK_SET
       1	SEEK_CUR
       2	SEEK_END
       L_SET	SEEK_SET
       L_INCR	SEEK_CUR
       L_XTND	SEEK_END

       SVr1-3 returns long instead of off_t, BSD returns int.

       Note that file descriptors created by dup(2) or fork(2) share the current file position pointer, so seeking on such files may be subject to
       race conditions.

SEE ALSO

       dup(2), fork(2), open(2), fseek(3), lseek64(3), posix_fallocate(3)

COLOPHON

       This  page is part of release 3.25 of the Linux man-pages project.  A description of the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux								    2001-09-24								  LSEEK(2)