Why fcntl(fd, F_SETFL, 0) use in serial port programming

Question

I am starting serial port programming in Linux. After reading several examples on the web, I don't understand exact effect of fcntl(fd, F_SETFL, 0)? It is clearing bits, but what flags does it affect? What does it set and or clear?

Answer 1

Take one by one

1) Function call used

fcntl() - It perform the operation on file descriptor passed in argument.

2) 2nd argument in call

F_SETFL (int)

Set the file status flags to the value specified by arg. File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC) in arg are ignored. On Linux this command can change only the O_APPEND, O_ASYNC, O_DIRECT, O_NOATIME, and O_NONBLOCK flags.

3) 3rd argument in call

It is 0 means, It set file status flag to zero. As Jean-Baptiste Yunès said in comment.

file access mode and file creation flags are ignored. This command reset every other flags: no append, no async, no direct, no atime, and no nonblocking

---

So finally

fcntl(fd, F_SETFL, 0)

This call will set opened file desciptor's file status flag to value 0.

---

But idealy this way we should not change file status flag. Best way is to first get the current file status flag using F_GETFL and then just change required bit in that. See example:

If you want to modify the file status flags, you should get the current flags with F_GETFL and modify the value. Don’t assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag O_NONBLOCK without altering any other flags:

/* Set the O_NONBLOCK flag of desc if value is nonzero,
   or clear the flag if value is 0.
   Return 0 on success, or -1 on error with errno set. */

int
set_nonblock_flag (int desc, int value)
{
  int oldflags = fcntl (desc, F_GETFL, 0);
  /* If reading the flags failed, return error indication now. */
  if (oldflags == -1)
    return -1;
  /* Set just the flag we want to set. */
  if (value != 0)
    oldflags |= O_NONBLOCK;
  else
    oldflags &= ~O_NONBLOCK;
  /* Store modified flag word in the descriptor. */
  return fcntl (desc, F_SETFL, oldflags);
}