Setting group/other write flags not honoured in open

[Nicky Chorley]

Hi folks,

I hope it's OK to ask this question here. I'm reading The Linux
Programming Interface and am beginning to get to grips with the file
oriented system calls.

I have a program that calls open to create a file, specifying the mode
flags for owner, group, other read and write. The file is created
sucessfully, but when I look at its permissions, I only see the read
perms set for group and others:

> ./create_file foo
> ls -l foo
-rw-r--r-- 1 nick users 0 Oct 31 14:52 foo

Why would that be? The entirety of my program is

#include <fcntl.h>
#include <unistd.h>

int main(int argc, char *argv[]) {
  int open_flags = O_CREAT | O_WRONLY | O_TRUNC;
  mode_t file_perms =
    S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH;

  int output_fd = open(argv[1], open_flags, file_perms);

  close(output_fd);
}

I also ran the program with strace and the perms in the openat call are
as I would expect:

openat(AT_FDCWD, "foo", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3

I don't understand why the group and other write flags aren't being
set. Could someone help shed some light on this please?

I'm not sure what factors influence this, but I'm running openSUSE Leap
15.2, with kernel 5.3.18-lp152.47-default, gcc 7.5.0 and glibc 2.26.

Thanks,

Nicky