NAME

flockfile, ftrylockfile, funlockfile — stdio stream locking functions

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <stdio.h>

void
flockfile(FILE *file);

int
ftrylockfile(FILE *file);

void
funlockfile(FILE *file);

DESCRIPTION

The flockfile(), ftrylockfile(), and funlockfile() functions provide applications with explicit control of locking of stdio stream objects. They can be used by a thread to execute a sequence of I/O operations as a unit, without interference from another thread.

Locks on stdio streams are recursive, and a lock count is maintained. stdio streams are created unlocked, with a lock count of zero. After successful acquisition of the lock, its count is incremented to one, indicating locked state of the stdio stream. Each subsequent relock operation performed by the owner thread increments the lock count by one, and each subsequent unlock operation performed by the owner thread decrements the lock count by one, allowing matching lock and unlock operations to be nested. After its lock count is decremented to zero, the stdio stream returns to unlocked state, and ownership of the stdio stream is relinquished.

The flockfile() function acquires the ownership of file for the calling thread. If file is already owned by another thread, the calling thread is suspended until the acquisition is possible (i.e., file is relinquished again and the calling thread is scheduled to acquire it).

The ftrylockfile() function acquires the ownership of file for the calling thread only if file is available.

The funlockfile() function relinquishes the ownership of file previously granted to the calling thread. Only the current owner of file may funlockfile() it.

RETURN VALUES

If successful, the ftrylockfile() function returns 0. Otherwise, it returns non-zero to indicate that the lock cannot be acquired.

SEE ALSO

flock(2), getc_unlocked(3), getchar_unlocked(3), lockf(3), putc_unlocked(3), putchar_unlocked(3)

STANDARDS

The flockfile(), ftrylockfile() and funlockfile() functions conform to IEEE Std 1003.1-2001 (“POSIX.1”).

HISTORY

The flockfile() function first appeared in FreeBSD 2.0.

BUGS

The design of these interfaces does not allow for addressing the problem of priority inversion.