Question about the fallocate system call

Question about the fallocate system call

[Jidong Xiao]

Hi,

I just have a simple question about fallocate.

I want to test the punch hole function of fallocate(). So I wrote such
a simple program:

yosemite:/mnt # cat test.c
#include <fcntl.h>
#include <errno.h>
#include <stdio.h>
#include <stdint.h>
#include <linux/fs.h>
#include <linux/falloc.h>

int main(void)
{
        int fd;

        fd = open("testfile", O_RDWR);
        fallocate(fd,FALLOC_FL_PUNCH_HOLE,0,500*1024*1024);
        close(fd);

        return 0;
}

I created a file called "testfile" whose size is 1GB, however, when I
run the above program, the size of the testfile simply won't change,
if I use stat command to check the file status, nothing is changed when I
execute the above program. My filesystem is ext4, as I understand,
ideally when I run the above program, the file size should decrease
from 1GB to 512MB, is there anything wrong with the program or I just
understood incorrectly?

Thank you for any inputs/comments.

-Jidong

Re: Question about the fallocate system call

[Pádraig Brady]

On 07/26/2012 03:30 PM, Jidong Xiao wrote:
> Hi,
> 
> I just have a simple question about fallocate.
> 
> I want to test the punch hole function of fallocate(). So I wrote such
> a simple program:
> 
> yosemite:/mnt # cat test.c
> #include <fcntl.h>
> #include <errno.h>
> #include <stdio.h>
> #include <stdint.h>
> #include <linux/fs.h>
> #include <linux/falloc.h>
> 
> int main(void)
> {
>         int fd;
> 
>         fd = open("testfile", O_RDWR);
>         fallocate(fd,FALLOC_FL_PUNCH_HOLE,0,500*1024*1024);
>         close(fd);
> 
>         return 0;
> }
> 
> I created a file called "testfile" whose size is 1GB, however, when I
> run the above program, the size of the testfile simply won't change,
> if I use stat command to check the file status, nothing is changed when I
> execute the above program. My filesystem is ext4, as I understand,
> ideally when I run the above program, the file size should decrease
> from 1GB to 512MB, is there anything wrong with the program or I just
> understood incorrectly?
> 
> Thank you for any inputs/comments.

code looks OK,
but you're not checking the return from fallocate().
I'm guessing it's returning -1 with errno = ENOTSUP

cheers,
Pádraig.