linux-kernel - Question about the fallocate system call

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Message-ID: <CAG4AFWa5qKzUqmR+3o4Od_jBt0PT-osUPczbh=VhvLh45+E6ng@mail.gmail.com>
Date:	Thu, 26 Jul 2012 10:30:09 -0400
From:	Jidong Xiao <jidong.xiao@...il.com>
To:	Kernel development list <linux-kernel@...r.kernel.org>
Subject: Question about the fallocate system call

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
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