FLOCKFILE(3C) Standard C Library Functions FLOCKFILE(3C)

NAME

flockfile, funlockfile, ftrylockfile - acquire and release stream
lock

SYNOPSIS

#include <stdio.h>

void flockfile(FILE *stream);


void funlockfile(FILE *stream);


int ftrylockfile(FILE *stream);

DESCRIPTION

The flockfile() function acquires an internal lock of a stream
stream. If the lock is already acquired by another thread, the thread
calling flockfile() is suspended until it can acquire the lock. In
the case that the stream lock is available, flockfile() not only
acquires the lock, but keeps track of the number of times it is being
called by the current thread. This implies that the stream lock can
be acquired more than once by the same thread.


The funlockfile() function releases the lock being held by the
current thread. In the case of recursive locking, this function must
be called the same number of times flockfile() was called. After the
number of funlockfile() calls is equal to the number of flockfile()
calls, the stream lock is available for other threads to acquire.


The ftrylockfile() function acquires an internal lock of a stream
stream, only if that object is available. In essence ftrylockfile()
is a non-blocking version of flockfile().

RETURN VALUES

The ftrylockfile() function returns 0 on success and non-zero to
indicate a lock cannot be acquired.

EXAMPLES

Example 1: A sample program of flockfile().

The following example prints everything out together, blocking other
threads that might want to write to the same file between calls to
fprintf(3C):


FILE iop;
flockfile(iop);
fprintf(iop, "hello ");
fprintf(iop, "world);
fputc(iop, 'a');
funlockfile(iop);


An unlocked interface is available in case performance is an issue.
For example:


flockfile(iop);
while (!feof(iop)) {
*c++ = getc_unlocked(iop);
}
funlockfile(iop);

ATTRIBUTES

See attributes(7) for descriptions of the following attributes:


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO

Intro(3), __fsetlocking(3C), ferror(3C), fprintf(3C), getc(3C),
putc(3C), stdio(3C), ungetc(3C), attributes(7), standards(7)

NOTES

The interfaces on this page are as specified in IEEE Std
1003.1:2001. See standards(7).

September 10, 2003 FLOCKFILE(3C)