flockfile(3)

1flockfile(3)               Library Functions Manual               flockfile(3)
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NAME

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

9       Standard C library (libc, -lc)
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SYNOPSIS

12       #include <stdio.h>
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14       void flockfile(FILE *filehandle);
15       int ftrylockfile(FILE *filehandle);
16       void funlockfile(FILE *filehandle);
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18   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
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20       All functions shown above:
21           /* Since glibc 2.24: */ _POSIX_C_SOURCE >= 199309L
22               || /* glibc <= 2.23: */ _POSIX_C_SOURCE
23               || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
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DESCRIPTION

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

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

63       None.
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ATTRIBUTES

66       For an  explanation  of  the  terms  used  in  this  section,  see  at‐
67       tributes(7).
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69       ┌────────────────────────────────────────────┬───────────────┬─────────┐
70       │Interface                                   │ Attribute     │ Value   │
71       ├────────────────────────────────────────────┼───────────────┼─────────┤
72       │flockfile(), ftrylockfile(), funlockfile()  │ Thread safety │ MT-Safe │
73       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

76       POSIX.1-2008.
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HISTORY

79       POSIX.1-2001.
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81       These  functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is de‐
82       fined.
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