first to get the correct function definitions.
SCALAR will be read and/or written depending on the
FUNCTION - a pointer to the string value of
SCALAR will be passed as the third argument of the
actual fcntl call. (If
has no string value but does have a numeric value, that value will be passed
directly rather than a pointer to the string value.)
|System call returns||Perl returns|
|anything else||that number|
Thus Perl returns true on success and false on failure, yet you can still easily determine the actual value returned by the operating system:
$retval = fcntl(...) or $retval = -1; printf "System returned %d\n", $retval;
Here, even the string "
0 but true" prints as 0, thanks to the
For example, since Perl always sets the close-on-exec flag for file descriptors above 2, if you wanted to pass file descriptor 3 to a subprocess, you might want to clear the flag like this:
use Fcntl; open TTY,"+>/dev/tty" or die "Can't open /dev/tty: $!\n"; fileno TTY == 3 or die "Internal error: fd mixup"; fcntl TTY, &F_SETFL, 0 or die "Can't clear the close-on-exec flag: $!\n";
fcntl will produce a fatal error if used on a machine
that doesn't implement fcntl(2). On machines that do implement
it, you can do such things as modify the close-on-exec flags, modify
the non-blocking I/O flags, emulate the lockf(3) function, and
arrange to receive the
SIGIO signal when I/O is pending. You
might even have record-locking facilities.