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Linux 2.6 - man page for flockfile (linux section 3)

FLOCKFILE(3)						     Linux Programmer's Manual						      FLOCKFILE(3)

flockfile, ftrylockfile, funlockfile - lock FILE for stdio
#include <stdio.h> void flockfile(FILE *filehandle); int ftrylockfile(FILE *filehandle); void funlockfile(FILE *filehandle); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): All functions shown above: _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE || _SVID_SOURCE || _POSIX_SOURCE
The stdio functions are thread-safe. This is achieved by assigning to each FILE object a lockcount and (if the lockcount is nonzero) an owning thread. For each library call, these functions wait until the FILE object is no longer locked by a different thread, then lock it, do the requested I/O, and unlock the object again. (Note: this locking has nothing to do with the file locking done by functions like flock(2) and lockf(3).) All this is invisible to the C-programmer, but there may be two reasons to wish for more detailed control. On the one hand, maybe a series of I/O actions by one thread belongs together, and should not be interrupted by the I/O of some other thread. On the other hand, maybe the locking overhead should be avoided for greater efficiency. To this end, a thread can explicitly lock the FILE object, then do its series of I/O actions, then unlock. This prevents other threads from coming in between. If the reason for doing this was to achieve greater efficiency, one does the I/O with the nonlocking versions of the stdio functions: with getc_unlocked(3) and putc_unlocked(3) instead of getc(3) and putc(3). The flockfile() function waits for *filehandle to be no longer locked by a different thread, then makes the current thread owner of *file- handle, and increments the lockcount. The funlockfile() function decrements the lock count. The ftrylockfile() function is a nonblocking version of flockfile(). It does nothing in case some other thread owns *filehandle, and it obtains ownership and increments the lockcount otherwise.
The ftrylockfile() function returns zero for success (the lock was obtained), and nonzero for failure.
These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is defined. They are in libc since libc 5.1.1 and in glibc since glibc 2.0.
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at 2008-08-29 FLOCKFILE(3)