1FLOCKFILE(3P) POSIX Programmer's Manual FLOCKFILE(3P)
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6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
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13 flockfile, ftrylockfile, funlockfile — stdio locking functions
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16 #include <stdio.h>
17 void flockfile(FILE *file);
19 int ftrylockfile(FILE *file);
20 void funlockfile(FILE *file);
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23 These functions shall provide for explicit application-level locking of
24 stdio (FILE *) objects. These functions can be used by a thread to
25 delineate a sequence of I/O statements that are executed as a unit.
26 The flockfile() function shall acquire for a thread ownership of a
28 (FILE *) object.
29 The ftrylockfile() function shall acquire for a thread ownership of a
31 (FILE *) object if the object is available; ftrylockfile() is a non-
32 blocking version of flockfile().
33 The funlockfile() function shall relinquish the ownership granted to
35 the thread. The behavior is undefined if a thread other than the cur‐
36 rent owner calls the funlockfile() function.
37 The functions shall behave as if there is a lock count associated with
39 each (FILE *) object. This count is implicitly initialized to zero when
40 the (FILE *) object is created. The (FILE *) object is unlocked when
41 the count is zero. When the count is positive, a single thread owns the
42 (FILE *) object. When the flockfile() function is called, if the count
43 is zero or if the count is positive and the caller owns the (FILE *)
44 object, the count shall be incremented. Otherwise, the calling thread
45 shall be suspended, waiting for the count to return to zero. Each call
46 to funlockfile() shall decrement the count. This allows matching calls
47 to flockfile() (or successful calls to ftrylockfile()) and funlock‐
48 file() to be nested.
49 All functions that reference (FILE *) objects, except those with names
51 ending in _unlocked, shall behave as if they use flockfile() and fun‐
52 lockfile() internally to obtain ownership of these (FILE *) objects.
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55 None for flockfile() and funlockfile().
56 The ftrylockfile() function shall return zero for success and non-zero
58 to indicate that the lock cannot be acquired.
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61 No errors are defined.
62 The following sections are informative.
64
66 None.
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69 Applications using these functions may be subject to priority inver‐
70 sion, as discussed in the Base Definitions volume of POSIX.1‐2008, Sec‐
71 tion 3.287, Priority Inversion.
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74 The flockfile() and funlockfile() functions provide an orthogonal
75 mutual-exclusion lock for each FILE. The ftrylockfile() function pro‐
76 vides a non-blocking attempt to acquire a file lock, analogous to
77 pthread_mutex_trylock().
78 These locks behave as if they are the same as those used internally by
80 stdio for thread-safety. This both provides thread-safety of these
81 functions without requiring a second level of internal locking and
82 allows functions in stdio to be implemented in terms of other stdio
83 functions.
84 Application developers and implementors should be aware that there are
86 potential deadlock problems on FILE objects. For example, the line-
87 buffered flushing semantics of stdio (requested via {_IOLBF}) require
88 that certain input operations sometimes cause the buffered contents of
89 implementation-defined line-buffered output streams to be flushed. If
90 two threads each hold the lock on the other's FILE, deadlock ensues.
91 This type of deadlock can be avoided by acquiring FILE locks in a con‐
92 sistent order. In particular, the line-buffered output stream deadlock
93 can typically be avoided by acquiring locks on input streams before
94 locks on output streams if a thread would be acquiring both.
95 In summary, threads sharing stdio streams with other threads can use
97 flockfile() and funlockfile() to cause sequences of I/O performed by a
98 single thread to be kept bundled. The only case where the use of flock‐
99 file() and funlockfile() is required is to provide a scope protecting
100 uses of the *_unlocked functions/macros. This moves the cost/perfor‐
101 mance tradeoff to the optimal point.
102
104 None.
105
107 getc_unlocked()
108 The Base Definitions volume of POSIX.1‐2008, Section 3.287, Priority
110 Inversion, <stdio.h>
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113 Portions of this text are reprinted and reproduced in electronic form
114 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
115 -- Portable Operating System Interface (POSIX), The Open Group Base
116 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
117 cal and Electronics Engineers, Inc and The Open Group. (This is
118 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
119 event of any discrepancy between this version and the original IEEE and
120 The Open Group Standard, the original IEEE and The Open Group Standard
121 is the referee document. The original Standard can be obtained online
122 at http://www.unix.org/online.html .
123 Any typographical or formatting errors that appear in this page are
125 most likely to have been introduced during the conversion of the source
126 files to man page format. To report such errors, see https://www.ker‐
127 nel.org/doc/man-pages/reporting_bugs.html .
128IEEE/The Open Group 2013 FLOCKFILE(3P)