From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Richard W.M. Jones" <rjones-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
Subject: [PATCH 0/2] vfs: Define new syscall getumask.
Date: Wed, 13 Apr 2016 12:43:04 +0100
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It's not possible to read the process umask without also modifying it,
which is what umask(2) does.  A library cannot read umask safely,
especially if the main program might be multithreaded.

This patch series adds a trivial system call "getumask" which returns
the umask of the current process.

Another approach to this has been attempted before, adding something
to /proc, although it didn't go anywhere.  See:

  http://comments.gmane.org/gmane.linux.kernel/1292109

Another way to solve this would be to add a thread-safe getumask to
glibc.  Since glibc could own the mutex, this would permit libraries
linked to this glibc to read umask safely.

I should also note that man-pages documents getumask(3), but no
version of glibc has ever implemented it.

Typical test script:

#include <stdio.h>
#include <stdlib.h>
#include <linux/unistd.h>
#include <sys/syscall.h>

int main(int argc, char *argv[])
{
  int r = syscall(329);
  if (r == -1) {
    perror("getumask");
    exit(1);
  }
  printf("umask = %o\n", r);
  exit(0);
}

$ ./getumask 
umask = 22

Rich.