c - Why are the file permissions changed when creating a file with the open system call on Linux?

Question

I am creating a file with full permission (777) using the open system call, but when I do ls -l I can see only permission as (755). Could you please tell why file permission is not 777?

Code

#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>

int main()
{
        int fd;

        /* Creates a file with full permission*/
        fd = open("test", O_CREAT | O_RDWR | O_APPEND, 0777);

        if (fd = -1)
        {
                return -1;
        }

        close(fd);
}

Output

     $ ls -l
    -rwxr-xr-x 1 ubuntu ubuntu    0 2012-09-19 11:55 test

Answer 1

There's a value maintained by the system called the umask; it is a property of the process, just like the PID (process ID) or EUID (effective user ID) is. It will be set to 022 (octal), which indicates that the system should remove the group and other write permission from files that are created.

You can call umask(0); before using open() so that the mode you specify in open() won't be altered. You should certainly do this to demonstrate that umask is the issue. However, it is generally best to let the user's choice of umask prevail — I for one get very stroppy if a program doesn't obey my umask setting; it tends not to be used again after I spot and verify the problem.

The shell also has a (built-in) command umask which you can use. The 022 value is a sensible default; most of the time, you do not want just anybody writing to your files.