1FLOCKFILE(P) POSIX Programmer's Manual FLOCKFILE(P)
2
6 flockfile, ftrylockfile, funlockfile - stdio locking functions
7
9 #include <stdio.h>
10 void flockfile(FILE *file);
12 int ftrylockfile(FILE *file);
13 void funlockfile(FILE *file);
14
17 These functions shall provide for explicit application-level locking of
18 stdio ( FILE *) objects. These functions can be used by a thread to
19 delineate a sequence of I/O statements that are executed as a unit.
20 The flockfile() function shall acquire for a thread ownership of a (
22 FILE *) object.
23 The ftrylockfile() function shall acquire for a thread ownership of a (
25 FILE *) object if the object is available; ftrylockfile() is a non-
26 blocking version of flockfile().
27 The funlockfile() function shall relinquish the ownership granted to
29 the thread. The behavior is undefined if a thread other than the cur‐
30 rent owner calls the funlockfile() function.
31 The functions shall behave as if there is a lock count associated with
33 each ( FILE *) object. This count is implicitly initialized to zero
34 when the ( FILE *) object is created. The ( FILE *) object is unlocked
35 when the count is zero. When the count is positive, a single thread
36 owns the ( FILE *) object. When the flockfile() function is called, if
37 the count is zero or if the count is positive and the caller owns the (
38 FILE *) object, the count shall be incremented. Otherwise, the calling
39 thread shall be suspended, waiting for the count to return to zero.
40 Each call to funlockfile() shall decrement the count. This allows
41 matching calls to flockfile() (or successful calls to ftrylockfile())
42 and funlockfile() to be nested.
43 All functions that reference ( FILE *) objects shall behave as if they
45 use flockfile() and funlockfile() internally to obtain ownership of
46 these ( FILE *) objects.
47
49 None for flockfile() and funlockfile().
50 The ftrylockfile() function shall return zero for success and non-zero
52 to indicate that the lock cannot be acquired.
53
55 No errors are defined.
56 The following sections are informative.
58
60 None.
61
63 Applications using these functions may be subject to priority inver‐
64 sion, as discussed in the Base Definitions volume of
65 IEEE Std 1003.1-2001, Section 3.285, Priority Inversion.
66
68 The flockfile() and funlockfile() functions provide an orthogonal
69 mutual-exclusion lock for each FILE. The ftrylockfile() function pro‐
70 vides a non-blocking attempt to acquire a file lock, analogous to
71 pthread_mutex_trylock().
72 These locks behave as if they are the same as those used internally by
74 stdio for thread-safety. This both provides thread-safety of these
75 functions without requiring a second level of internal locking and
76 allows functions in stdio to be implemented in terms of other stdio
77 functions.
78 Application writers and implementors should be aware that there are
80 potential deadlock problems on FILE objects. For example, the line-
81 buffered flushing semantics of stdio (requested via {_IOLBF}) require
82 that certain input operations sometimes cause the buffered contents of
83 implementation-defined line-buffered output streams to be flushed. If
84 two threads each hold the lock on the other's FILE, deadlock ensues.
85 This type of deadlock can be avoided by acquiring FILE locks in a con‐
86 sistent order. In particular, the line-buffered output stream deadlock
87 can typically be avoided by acquiring locks on input streams before
88 locks on output streams if a thread would be acquiring both.
89 In summary, threads sharing stdio streams with other threads can use
91 flockfile() and funlockfile() to cause sequences of I/O performed by a
92 single thread to be kept bundled. The only case where the use of
93 flockfile() and funlockfile() is required is to provide a scope pro‐
94 tecting uses of the *_unlocked() functions/macros. This moves the
95 cost/performance tradeoff to the optimal point.
96
98 None.
99
101 getc_unlocked() , putc_unlocked() , the Base Definitions volume of
102 IEEE Std 1003.1-2001, <stdio.h>
103
105 Portions of this text are reprinted and reproduced in electronic form
106 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
107 -- Portable Operating System Interface (POSIX), The Open Group Base
108 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
109 Electrical and Electronics Engineers, Inc and The Open Group. In the
110 event of any discrepancy between this version and the original IEEE and
111 The Open Group Standard, the original IEEE and The Open Group Standard
112 is the referee document. The original Standard can be obtained online
113 at http://www.opengroup.org/unix/online.html .
114IEEE/The Open Group 2003 FLOCKFILE(P)