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.TH FLOCKFILE 3C "Sep 10, 2003"
.SH NAME
flockfile, funlockfile, ftrylockfile \- acquire and release stream lock
.SH SYNOPSIS
.LP
.nf
#include <stdio.h>

\fBvoid\fR \fBflockfile\fR(\fBFILE *\fR\fIstream\fR);
.fi

.LP
.nf
\fBvoid\fR \fBfunlockfile\fR(\fBFILE *\fR\fIstream\fR);
.fi

.LP
.nf
\fBint\fR \fBftrylockfile\fR(\fBFILE *\fR\fIstream\fR);
.fi

.SH DESCRIPTION
.sp
.LP
The \fBflockfile()\fR function acquires an internal lock of a stream
\fIstream\fR. If the lock is already acquired by another thread, the thread
calling \fBflockfile()\fR is suspended until it can acquire the lock.  In the
case that the stream lock is available, \fBflockfile()\fR 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.
.sp
.LP
The \fBfunlockfile()\fR 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 \fBflockfile()\fR was called.  After the number of
\fBfunlockfile()\fR calls is equal to the number of \fBflockfile()\fR calls,
the stream lock is available for other threads to acquire.
.sp
.LP
The \fBftrylockfile()\fR function acquires an internal lock of a stream
\fIstream\fR, only if that object is available.  In essence
\fBftrylockfile()\fR is a non-blocking version of \fBflockfile()\fR.
.SH RETURN VALUES
.sp
.LP
The \fBftrylockfile()\fR function returns \fB0\fR on success and non-zero to
indicate a lock cannot be acquired.
.SH EXAMPLES
.LP
\fBExample 1 \fRA sample program of \fBflockfile()\fR.
.sp
.LP
The following example prints everything out together, blocking other threads
that might want to write to the same file between calls to \fBfprintf\fR(3C):

.sp
.in +2
.nf
FILE iop;
flockfile(iop);
fprintf(iop, "hello ");
fprintf(iop, "world);
fputc(iop, 'a');
funlockfile(iop);
.fi
.in -2

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

.sp
.in +2
.nf
flockfile(iop);
while (!feof(iop)) {
        *c++ = getc_unlocked(iop);
}
funlockfile(iop);
.fi
.in -2

.SH ATTRIBUTES
.sp
.LP
See \fBattributes\fR(5) for descriptions of the following attributes:
.sp

.sp
.TS
box;
c | c
l | l .
ATTRIBUTE TYPE	ATTRIBUTE VALUE
_
Interface Stability	Standard
_
MT-Level	MT-Safe
.TE

.SH SEE ALSO
.sp
.LP
\fBIntro\fR(3), \fB__fsetlocking\fR(3C), \fBferror\fR(3C), \fBfprintf\fR(3C),
\fBgetc\fR(3C), \fBputc\fR(3C), \fBstdio\fR(3C), \fBungetc\fR(3C),
\fBattributes\fR(5), \fBstandards\fR(5)
.SH NOTES
.sp
.LP
The interfaces on this page  are as specified in IEEE Std 1003.1:2001.  See
\fBstandards\fR(5).