FLOCKFILESection: Linux Programmer's Manual (3)
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NAMEflockfile, 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)):
DESCRIPTIONThe stdio functions are thread-safe. This is achieved by assigning to each FILE object a lockcount and (if the lockcount is non-zero) 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.
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 non-locking 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 *filehandle, and increments the lockcount.
The funlockfile() function decrements the lock count.
RETURN VALUEThe ftrylockfile() function returns zero for success (the lock was obtained), and non-zero for failure.
AVAILABILITYThese 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.
COLOPHONThis page is part of release 3.21 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.