Why does lseek return 0?

Question

lseek() is supposed to return the position of the file descriptor.

The documentation says:

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

Trouble is, not even this works:

#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
printf("size off_t: %i\n", sizeof(off_t));

off_t pos;
pos = lseek(file, (off_t)0, SEEK_CUR);
printf("pos: %lli\n", pos);

// same result for SEEK_SET and SEEK_END
pos = lseek(file, (off_t)2352, SEEK_CUR);
printf("pos: %lli\n", pos);

This gives me:

size off_t: 8
pos: 0
pos: 0

Why is this? Is there an alternative to find the current offset, using the raw I/O functions? (read, open, lseek, …)

Edit 1:

I tried to make the example simpler.

Answer 1

You might want to change the test to:

if ( (pos = lseek(file, (off_t)i, SEEK_CUR)) != -1 ) {

You are probably hitting a -1 somewhere, but you're testing for 0 here.

Answer 2

I'm not sure I understand your question, but here are a few thoughts which might help.

• Offset 0 is valid; it means you are at the beginning of the file
• Depending on your platform, off_t may well be limited to 32 bits unsigned.
• Are you intending to seek relative to your current position?

-- MarkusQ

Answer 3

What kind of file is it? Is it a pipe by any chance? Because if it's anything but a regular file, chances are it doesn't support seeking:

The behavior of lseek() on devices which are incapable of seeking is implementation-defined. The value of the file offset associated with such a device is undefined.

Answer 4

Something else is up, probably something silly. I tried your code, as here:

#include <fcntl.h>
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>

int main(int argc, char *argv[]){
    off_t pos;
    int file ;
    if((file = open("/Users/chasrmartin/.bash_history",O_RDONLY)) == -1){
        perror(argv[0]);
        exit(1);
    }
    printf("size off_t: %i\n", sizeof(off_t));

    pos = lseek(file, (off_t)0, SEEK_CUR);
    printf("pos: %lli\n", pos);

    // same result for SEEK_SET and SEEK_END
    pos = lseek(file, (off_t)2352, SEEK_CUR);
    printf("pos: %lli\n", pos);

    exit(0);
}

and get this result:

bash $ gcc foo.c
bash $ ./a.out
size off_t: 8
pos: 0
pos: 2352

(Just to be definite, this is on Mac OS/X 10.5.6 on Intel.)

Update.

Or maybe it's not silly. I just tried it on a PPC G5, and get the results you do.

Update 2

Okay, here's the result on a PPC:

$ gcc foo.c
$ ./a.out
size off_t: 8
pos: 0
pos: 0

Answer 5

Try adding #include <unistd.h> to the top.

See: http://forums.macosxhints.com/archive/index.php/t-35508.html

Basically, since you didn't #include <unistd.h>, the compiler is "guessing" that lseek() returns an int.

Probably an int is 4-bytes long, and since PPC is "big-endian" byte order, you're getting the "top" 4 bytes, which are all zero.

Include unistd.h lets the compiler realize that lseek() is returning an off_t, so you don't lose half the bytes.