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.