FLOCKFILE(3) | Library Functions Manual | FLOCKFILE(3)

# 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.

October 15, 2011 | NetBSD 9.4