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)