NAME
flockfile,
ftrylockfile, funlockfile
— stdio stream locking
functions
LIBRARY
library “libc”
SYNOPSIS
#include
<stdio.h>
void
flockfile(FILE
*file);
int
ftrylockfile(FILE
*file);
void
funlockfile(FILE
*file);
DESCRIPTION
Theflockfile(),
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.