c - When non-blocking I/O is turned on for stdout, is it correct for the OS to turn it on for stdin too?

Question

I have noticed some unexpected behaviour on both OS X and Linux. Turning on non-blocking I/O (using O_NONBLOCK) for standard output turns it on for standard input too!

Are these OSes behaving correctly? If so, is this behaviour defined by POSIX? Please point me to the relevant documentation if this is the case.

Here's a example program I used to test this:

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

int main (int argc, char *argv[]) {
  int flags = fcntl(STDOUT_FILENO, F_GETFL);
  if (argc > 1 && strcmp(argv[1], "1") == 0) {
    fcntl(STDOUT_FILENO, F_SETFL, flags | O_NONBLOCK);
  }
  printf("stdout O_NONBLOCK is: %d
", fcntl(STDOUT_FILENO, F_GETFL) & O_NONBLOCK);
  printf("stdin O_NONBLOCK is: %d
", fcntl(STDIN_FILENO, F_GETFL) & O_NONBLOCK);
  return 0;
}

On OS X:

$ clang -o fd-test fd-test.c
$ ./fd-test
stdout O_NONBLOCK is: 0
stdin O_NONBLOCK is: 0
$ ./fd-test 1
stdout O_NONBLOCK is: 4
stdin O_NONBLOCK is: 4

On Linux:

$ gcc -o fd-test fd-test.c
$ ./fd-test
stdout O_NONBLOCK is: 0
stdin O_NONBLOCK is: 0
$ ./fd-test 1
stdout O_NONBLOCK is: 2048
stdin O_NONBLOCK is: 2048

Answer 1

When a process is started from the shell, stdin, stdout and stderr point to the same file description. This file description is marked as O_NONBLOCK by your fcntl(1) call and therefore expectedly both file descriptors are marked as O_NONBLOCK.

If you want to indeed write to the same file from two file descriptors but want one to be marked as O_NONBLOCK, you need to create a new file description for the same file. If you know the file name of the file in question, you can achieve this by simply calling open() with the desired file name and flags. On some operating systems, you can find the file name using a platform-specific API, such as the /proc/fd virtual file system (many Unices including Linux) or the fd2path() function from Plan 9. Refer to this question for more details.

Note that simply calling open("/dev/fd/0", ...) may not work as intended as it returns a pointer to the same file description on some platforms if I recall correctly.