Section: Linux Programmer's Manual (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
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
object a lockcount and (if the lockcount is nonzero)
an owning thread.
For each library call, these functions wait until the
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
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.
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
function waits for
no longer locked by a different thread, then makes the
current thread owner of
function decrements the lock count.
function is a nonblocking version
It does nothing in case some other thread
and it obtains ownership and increments
the lockcount otherwise.
function returns zero for success
(the lock was obtained), and nonzero for failure.
For an explanation of the terms used in this section, see
These functions are available when
This page is part of release 5.02 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at