Why does lseek return 0? - api

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

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.

Answer 2

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 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

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 5

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.