flockfile(3)

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:
18           /* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
19               || /* Glibc <= 2.23: */ _POSIX_C_SOURCE
20               || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

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

56       The  ftrylockfile() function returns zero for success (the lock was ob‐
57       tained), and nonzero for failure.
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ERRORS

60       None.
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ATTRIBUTES

63       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
64       tributes(7).
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66       ┌────────────────────────────────────────────┬───────────────┬─────────┐
67       │Interface                                   │ Attribute     │ Value   │
68       ├────────────────────────────────────────────┼───────────────┼─────────┤
69       │flockfile(), ftrylockfile(), funlockfile()  │ Thread safety │ MT-Safe │
70       └────────────────────────────────────────────┴───────────────┴─────────┘
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CONFORMING TO

73       POSIX.1-2001, POSIX.1-2008.
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75       These  functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is de‐
76       fined.
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COLOPHON

82       This page is part of release 5.12 of the Linux  man-pages  project.   A
83       description  of  the project, information about reporting bugs, and the
84       latest    version    of    this    page,    can     be     found     at
85       https://www.kernel.org/doc/man-pages/.
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89                                  2021-03-22                      FLOCKFILE(3)