The GNU C Library - Getting File Status Flags

Getting and Setting File Status Flags

The fcntl function can fetch or change file status flags.

Macro int F_GETFL

This macro is used as the command argument to fcntl , to read the file status flags for the open file with descriptor filedes.

The normal return value from fcntl with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags. Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with O_ACCMODE to compare them.

In case of an error, fcntl returns -1 . The following errno error conditions are defined for this command:

EBADF

The filedes argument is invalid.

Macro int F_SETFL

This macro is used as the command argument to fcntl , to set the file status flags for the open file corresponding to the filedes argument. This command requires a third int argument to specify the new flags, so the call looks like this:

	fcntl (filedes, F_SETFL, new-flags)

You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing.

The normal return value from fcntl with this command is an unspecified value other than -1 , which indicates an error. The error conditions are the same as for the F_GETFL command.

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);
	}