1FLOCKFILE(3)               Linux Programmer's Manual              FLOCKFILE(3)
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NAME

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

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

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

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

58       None.
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CONFORMING TO

61       POSIX.1-2001.
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AVAILABILITY

64       These  functions  are  available  when  _POSIX_THREAD_SAFE_FUNCTIONS is
65       defined.  They are in libc since libc 5.1.1 and in  glibc  since  glibc
66       2.0.
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SEE ALSO

69       unlocked_stdio(3)
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COLOPHON

72       This  page  is  part of release 3.25 of the Linux man-pages project.  A
73       description of the project, and information about reporting  bugs,  can
74       be found at http://www.kernel.org/doc/man-pages/.
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78                                  2008-08-29                      FLOCKFILE(3)
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