1flockfile(3) Library Functions Manual flockfile(3)
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6 flockfile, ftrylockfile, funlockfile - lock FILE for stdio
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9 Standard C library (libc, -lc)
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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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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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59 The ftrylockfile() function returns zero for success (the lock was ob‐
60 tained), and nonzero for failure.
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63 None.
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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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76 POSIX.1-2008.
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79 POSIX.1-2001.
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81 These functions are available when _POSIX_THREAD_SAFE_FUNCTIONS is
82 defined.
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85 unlocked_stdio(3)
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89Linux man-pages 6.05 2023-07-20 flockfile(3)