1FLOCKFILE(3)               Linux Programmer's Manual              FLOCKFILE(3)
2
3
4

NAME

6       flockfile, ftrylockfile, funlockfile - lock FILE for stdio
7

SYNOPSIS

9       #include <stdio.h>
10
11       void flockfile(FILE *filehandle);
12       int ftrylockfile(FILE *filehandle);
13       void funlockfile(FILE *filehandle);
14
15   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
16
17       All functions shown above:
18           _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _BSD_SOURCE ||
19           _SVID_SOURCE || _POSIX_SOURCE
20

DESCRIPTION

22       The stdio functions are thread-safe.  This is achieved by assigning  to
23       each  FILE object a lockcount and (if the lockcount is nonzero) an own‐
24       ing thread.  For each library call, these functions wait until the FILE
25       object  is no longer locked by a different thread, then lock it, do the
26       requested I/O, and unlock the object again.
27
28       (Note: this locking has nothing to do with the  file  locking  done  by
29       functions like flock(2) and lockf(3).)
30
31       All this is invisible to the C-programmer, but there may be two reasons
32       to wish for more detailed control.  On the one hand, maybe a series  of
33       I/O  actions  by  one thread belongs together, and should not be inter‐
34       rupted by the I/O of some other thread.  On the other hand,  maybe  the
35       locking overhead should be avoided for greater efficiency.
36
37       To  this end, a thread can explicitly lock the FILE object, then do its
38       series of I/O actions, then unlock.  This prevents other  threads  from
39       coming in between.  If the reason for doing this was to achieve greater
40       efficiency, one does the I/O with the nonlocking versions of the  stdio
41       functions:   with  getc_unlocked(3)  and  putc_unlocked(3)  instead  of
42       getc(3) and putc(3).
43
44       The flockfile() function waits for *filehandle to be no  longer  locked
45       by a different thread, then makes the current thread owner of *filehan‐
46       dle, and increments the lockcount.
47
48       The funlockfile() function decrements the lock count.
49
50       The ftrylockfile() function is a nonblocking  version  of  flockfile().
51       It  does  nothing  in  case  some other thread owns *filehandle, and it
52       obtains ownership and increments the lockcount otherwise.
53

RETURN VALUE

55       The ftrylockfile() function returns zero  for  success  (the  lock  was
56       obtained), and nonzero for failure.
57

ERRORS

59       None.
60

ATTRIBUTES

62   Multithreading (see pthreads(7))
63       The   flockfile(),  ftrylockfile(),  and  funlockfile()  functions  are
64       thread-safe.
65

CONFORMING TO

67       POSIX.1-2001.
68

AVAILABILITY

70       These functions  are  available  when  _POSIX_THREAD_SAFE_FUNCTIONS  is
71       defined.   They  are  in libc since libc 5.1.1 and in glibc since glibc
72       2.0.
73

SEE ALSO

75       unlocked_stdio(3)
76

COLOPHON

78       This page is part of release 3.53 of the Linux  man-pages  project.   A
79       description  of  the project, and information about reporting bugs, can
80       be found at http://www.kernel.org/doc/man-pages/.
81
82
83
84                                  2013-07-23                      FLOCKFILE(3)
Impressum