NAME

flockfile, funlockfile − acquire and release stream lock

SYNOPSIS

#include <stdio.h>

void flockfile(FILE ∗stream);

void funlockfile(FILE ∗stream);

MT-LEVEL

MT-Safe

DESCRIPTION

flockfile() 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 as long as it is the same thread. 

funlockfile() 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. 

EXAMPLES

For example:

FILE iop;
 .
 .
 .
flockfile(iop);
fprintf(iop, "hello ");
fprintf(iop, "world0);
fputc(iop, ’a’);
funlockfile(iop);

will print everything out together, blocking other threads that might want to write to the same file between fprintf’s. 

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

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

SEE ALSO

intro(3), ferror(3S), getc(3S), putc(3S), ungetc(3S), stdio(3S)

NOTES

The interfaces on this page are as proposed in the POSIX.4a Draft #6 document, and are subject to change to be compliant to the standard when it is accepted. 

When compiling multi-thread applications, the _REENTRANT flag must be defined on the compile line.  This flag should only be used in multi-thread applications. 

Sun Microsystems  —  Last change: 3 Jul 1990