[SOLVED] lseek failing for filesize 4GB

kvm1983 · 09-21-2010, 01:18 PM

Hello all,
I am trying to lseek into a file till offset 4GB (4294967296 bytes).

Code:

off64_t FILESIZE = 4294967296;
result = lseek64(fd, FILESIZE-1, SEEK_SET);

Please note that fd is a valid file descriptor, I have made sure that the file open call is successful.

Heres what I observe:
1. If FILESIZE < 4GB, lseek works.
2. If FILESIZE = 4GB, lseek fails with errno 29 (ESPIPE), which states that fd is associated with a pipe, socket, or FIFO.
3. If FILESIZE > 4GB, lseek works again.

So I am confused why lseek fails when FILESIZE is exactly 4GB. I am running this program on a dual core 64 bit Opteron machine running OpenSuse. Any pointers into this error are greatly appreciated.

Heres the complete program:

Code:

#include <stdio.h>
#include <stdlib.h>
#define _LARGEFILE64_SOURCE 1
#include <sys/types.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/mman.h>
#include <string.h>
#include <errno.h>

#define FILEPATH "/tmp/km_mmap.txt"

off64_t FILESIZE = 4294967296; //2147483648;

int main(int argc, char *argv[])
{
    int fd; 
    int result;

    fd = open64(FILEPATH, O_RDWR | O_CREAT | O_TRUNC, (mode_t)0600);
    if (fd == -1) {
        perror("Error opening file for writing");
        exit(EXIT_FAILURE);
    }   

    result = lseek64(fd, (off64_t)(FILESIZE-1), SEEK_SET);
    if (result == -1) {
        close(fd);
        perror("Error calling lseek() to 'stretch' the file");
        fprintf(stdout, "errno: %d\n", errno);
        exit(EXIT_FAILURE);
    }   
    
    return 0;
}

- Kshitij

paulsm4 · 09-22-2010, 03:27 AM

Try adding "LLU" or "LL" suffix to your integer literal.

EXAMPLE:

Code:

off64_t FILESIZE = 4294967296LLU;

'Hope that helps

kvm1983 · 09-22-2010, 11:31 AM

Nopes, that didnt work. I tried LU, LLU, U. None of these solved it

- Kshitij

Sergei Steshenko · 09-22-2010, 12:09 PM

Quote:

Originally Posted by kvm1983

Nopes, that didnt work. I tried LU, LLU, U. None of these solved it

- Kshitij

Did you try to print FILESIZE to make sure the compiler correctly understood your intent ?

kvm1983 · 09-22-2010, 12:15 PM

yes, i printf filesize and it correctly prints the value 4294967296.

- Kshitij

Sergei Steshenko · 09-22-2010, 12:37 PM

Quote:

Originally Posted by kvm1983

yes, i printf filesize and it correctly prints the value 4294967296.

- Kshitij

For no good reason try to modify your code this way:

Code:

off64_t offset = (off64_t)FILESIZE - (off64_t)1;
fprintf(stderr, "offset=%llu\n", (unsigned long long)offset);
result = lseek64(fd, offset, SEEK_SET);

kvm1983 · 09-22-2010, 12:52 PM

Well that didnt work either. Now it prints the offset correctly as 4294967295, but the program still crashes with errno=29 (ESPIPE).

- Kshitij

paulsm4 · 09-22-2010, 01:20 PM

Hi -

Several suggestions:
1. Make sure nothing in any of your expressions casts the value down to a 32-bit int (that's what I was suggesting with "LL"; that's what Sergei Steshenko was suggestiong with "off64_t offset = (off64_t)FILESIZE - (off64_t)1").

2. Lose the "open64()" and substitute "open()" with "O_LARGEFILE".

3. Read this man page on "lseek64()" and see if any of the "gotcha's" mentioned in it might be applicable to your system:

http://linux.die.net/man/3/lseek64

kvm1983 · 09-22-2010, 02:17 PM

Ok, got it.
This is the culprit:

Code:

result = lseek64(fd, offset, SEEK_SET);

Like an idiot, I declared result as an int, instead of off64_t. Note that lseek returns the offset at which the file pointer was positioned, hence the program fails.

What I noticed was that
For filesize 4294967298 (4GB + 2), the program did not crash, but the result was being set to 2.
For filesize 4294967295 (4GB - 2), the result was being set to -1.
It crashed when filesize was exactly 4GB.

Since the int could hold a maximum value of 2147483647, any value over it was making the result a -ve number.

Thanks a lot for all your help.

- Kshitij