In c, how to make a file descriptor blocking?

Question

Given an arbitrary file descriptor, can I make it blocking if it is non-blocking? If so, how?

Answer 1

Its been a while since I played with C, but you could use the fcntl() function to change the flags of a file descriptor:

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

// Save the existing flags

saved_flags = fcntl(fd, F_GETFL);

// Set the new flags with O_NONBLOCK masked out

fcntl(fd, F_SETFL, saved_flags & ~O_NONBLOCK);

Answer 2

I would expect simply non-setting the O_NONBLOCK flag should revert the file descriptor to the default mode, which is blocking:

/* Makes the given file descriptor non-blocking.
 * Returns 1 on success, 0 on failure.
*/
int make_blocking(int fd)
{
  int flags;

  flags = fcntl(fd, F_GETFL, 0);
  if(flags == -1) /* Failed? */
   return 0;
  /* Clear the blocking flag. */
  flags &= ~O_NONBLOCK;
  return fcntl(fd, F_SETFL, flags) != -1;
}