open() not setting file permissions correctly

Question

I create a file using the code below:

#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <errno.h>
#include <fcntl.h>
#include <unistd.h>

int main()
{
    const char* filename = "./test.out";
    int fd;
    if(-1 == (fd = open(filename, O_CREAT|O_RDWR, 0666)))
    {
        perror("Error");
        errno = 0;
    }       
    else
        puts("File opened");

    if(-1 == (close(fd)))
    {
        perror("Error");
        errno = 0;
    }
    else
        puts("File closed");

    return 0;
}

I specify the mode argument as 0666, which should grant read,write access to everyone. However, an ls -l shows

-rw-r--r-- 1 kmehta users 0 2012-01-29 16:29 test.out

As you can see, write permissions are only granted to the owner of the file. I do not know why everyone else is not granted permissions correctly. chmod a+w test.out sets the permissions correctly though.

Code compiled as gcc -Wall test.c

Specs: gcc v 4.5.0 on Opensuse 11.3 64 bit

Answer 1

The mode argument to open specifies the maximum allowed permissions. The umask setting is then applied to further restrict the permissions.

If you need to make the permissions be 0666 specifically you will need to use fchmod on the file handle after the open succeeds or use umask to set the process’ permissions mask before the open.