Manpage of FLOCKFILE

FLOCKFILE

Section: Linux Programmer's Manual (3)
Updated: 2016-03-15
Index
 

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:

_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 FILEobject a lockcount and (if the lockcount is nonzero) an owning thread. For each library call, these functions wait until the FILEobject 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 FILEobject, 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 *filehandleto 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).
InterfaceAttributeValue
flockfile(), ftrylockfile(), funlockfile() Thread safetyMT-Safe
 

CONFORMING TO

POSIX.1-2001, POSIX.1-2008.  

AVAILABILITY

These functions are available when _POSIX_THREAD_SAFE_FUNCTIONSis defined.  

SEE ALSO

unlocked_stdio(3)


 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
ERRORS
ATTRIBUTES
CONFORMING TO
AVAILABILITY
SEE ALSO

This document was created by man2html, using the manual pages.
Time: 22:27:53 GMT, June 20, 2016