funlockfile(3) -- Linux man page
NAMEflockfile, ftrylockfile, funlockfile - lock FILE for stdio
#include <stdio.h> void flockfile(FILE *filehandle);
int ftrylockfile(FILE *filehandle);
void funlockfile(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 getc_unlocked() and putc_unlocked() instead of getc() and putc().
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 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.