Question about the fallocate system call

Jidong Xiao
2012-07-26T14:30:15Z

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