[MAN] flockfile

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FLOCKFILE

Section: Linux Programmer's Manual (3)
Updated: 2017-07-13
Index Return to Main Contents

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NAME

flockfile, ftrylockfile, funlockfile - lock FILE for stdio  

SYNOPSIS

#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

 

DESCRIPTION

The 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.

(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 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 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 lockcount.

The funlockfile() function decrements the lock count.

The ftrylockfile() function is a nonblocking version of flockfile(). It does nothing in case some other thread owns *filehandle, and it obtains ownership and increments the lockcount otherwise.  

RETURN VALUE

The ftrylockfile() function returns zero for success (the lock was obtained), and nonzero for failure.  

ERRORS

None.  

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
flockfile(), ftrylockfile(), funlockfile()	Thread safety	MT-Safe

 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.  

AVAILABILITY

These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is defined.  

SEE ALSO

unlocked_stdio(3)  

COLOPHON

This page is part of release 4.15 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

ERRORS

ATTRIBUTES

CONFORMING TO

AVAILABILITY

SEE ALSO

COLOPHON

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Time: 04:45:39 GMT, September 16, 2022