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NAMEflockfile, ftrylockfile, funlockfile - lock FILE for stdio
#include <stdio.h> void flockfile(FILE *filehandle);
DESCRIPTIONThe 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.
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 fIgetc_unlockedfP() and fIputc_unlockedfP() instead of fIgetcfP() and fIputcfP().
The fBflockfile()fP function waits for *fIfilehandlefP to be no longer locked by a different thread, then makes the current thread owner of *fIfilehandlefP, and increments the lockcount.
The fBfunlockfile()fP function decrements the lock count.
The fBftrylockfile()fP function is a non-blocking version of fBflockfile()fP. It does nothing in case some other thread owns *fIfilehandlefP, and it obtains ownership and increments the lockcount otherwise.
RETURN VALUEThe fBftrylockfile()fP function returns zero for success (the lock was obtained), and nonzero 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.