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:
/* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L || /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
The stdio functions are thread-safe. This is achieved by assigning to
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.
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
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
*filehandle, and increments
The funlockfile() function decrements the lock count.
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.
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