Question

Just starting to learn some basic C and messing around with the stat() system call on Linux.

So st_mode returned in the struct from stat() is a bitfield. And I can get the octal permissions by simply printf("octal %o", st.st_mode) but I’m only interested in certain flags in the bitfield, namely S_IRWXU, S_IRWXG and S_IRWXO to send that value to another function, eg: mkdir.

Here’s my sample program.
Ignore the fact that this is a rubbish program with the directories hard-coded, not passed as an argument to the create_dir() function and with no error checking.

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

static int create_dir(mode_t mode) {
  mkdir("/home/user/blahnew", mode);
}

main() {
  struct stat st;
  int res;
  res = stat("/home/user/blah", &st);

  printf("user %o\n", st.st_mode & S_IRWXU);
  printf("group %o\n", st.st_mode & S_IRWXG);
  printf("other %o\n", st.st_mode & S_IRWXO);

  create_dir(mode);

  return 0;
}

How can I take only the above flages in st.st_mode and pass it as an octal to create_dir() to be used by an mkdir() function?

If I just did create_dir(st.st_mode) would that not also pass all the other flags that mkdir() doesn’t need… uid, gid, sticky bit etc?

Cheers, B

Answer 1

You already did 99% of the work in your code. When you say

st.st_mode & S_IRWXU

only the bits in st.st_mode that are present in the S_IRWXU bitmask remain. So what you want is a different bitmask that combines the other three:

st.st_mode & (S_IRWXU | S_IRWXG | S_IRWXO)