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FLOCKFILE(3) Linux Programmer's Manual FLOCKFILE(3)
flockfile, ftrylockfile, funlockfile - lock FILE for stdio
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 non-zero) an own-
ing 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 inter-
rupted 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 *filehan-
dle, and increments the lockcount.
The funlockfile() function decrements the lock count.
The ftrylockfile() function is a non-blocking 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 non-zero 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
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