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 pleas

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.

Answer 2

The permissions of a created file are restricted by the process's current umask -- your current umask is 022, so group and world write are always disabled by default. (Which is a good thing, in most cases.) If you really want a group- and world-writable file, you will need to temporarily set umask(0) while creating this file (make sure to save the old value returned by the system call, and set it back afterwards!), or "manually" set the file's permissions using chmod().

Answer 3

umask will return the original value of the mask, so to reset it temporarily you just need to do

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

mode_t old_mask = umask(0);
...
umask(old_mask);

though it would be perhaps preferable to use fchmod after opening the file - this will avoid the problem of changing process-global state:

fd = open("test", O_CREAT | O_RDWR | O_APPEND, 0777);
if (fd < 0) { ... }
int rv = fchmod(fd, 0777);
if (rv < 0) { /* fchmod errored */ }