1GETC_UNLOCKED(3P) POSIX Programmer's Manual GETC_UNLOCKED(3P)
2
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.
10
12 getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked -
13 stdio with explicit client locking
14
16 #include <stdio.h>
17 int getc_unlocked(FILE *stream);
19 int getchar_unlocked(void);
20 int putc_unlocked(int c, FILE *stream);
21 int putchar_unlocked(int c);
22
25 Versions of the functions getc(), getchar(), putc(), and putchar()
26 respectively named getc_unlocked(), getchar_unlocked(),
27 putc_unlocked(), and putchar_unlocked() shall be provided which are
28 functionally equivalent to the original versions, with the exception
29 that they are not required to be implemented in a thread-safe manner.
30 They may only safely be used within a scope protected by flockfile()
31 (or ftrylockfile()) and funlockfile(). These functions may safely be
32 used in a multi-threaded program if and only if they are called while
33 the invoking thread owns the ( FILE *) object, as is the case after a
34 successful call to the flockfile() or ftrylockfile() functions.
35
37 See getc(), getchar(), putc(), and putchar().
38
40 See getc(), getchar(), putc(), and putchar().
41 The following sections are informative.
43
45 None.
46
48 Since they may be implemented as macros, getc_unlocked() and
49 putc_unlocked() may treat incorrectly a stream argument with side
50 effects. In particular, getc_unlocked(*f++) and putc_unlocked(*f++) do
51 not necessarily work as expected. Therefore, use of these functions in
52 such situations should be preceded by the following statement as appro‐
53 priate:
54 #undef getc_unlocked
57 #undef putc_unlocked
58
60 Some I/O functions are typically implemented as macros for performance
61 reasons (for example, putc() and getc()). For safety, they need to be
62 synchronized, but it is often too expensive to synchronize on every
63 character. Nevertheless, it was felt that the safety concerns were more
64 important; consequently, the getc(), getchar(), putc(), and putchar()
65 functions are required to be thread-safe. However, unlocked versions
66 are also provided with names that clearly indicate the unsafe nature of
67 their operation but can be used to exploit their higher performance.
68 These unlocked versions can be safely used only within explicitly
69 locked program regions, using exported locking primitives. In particu‐
70 lar, a sequence such as:
71 flockfile(fileptr);
74 putc_unlocked('1', fileptr);
75 putc_unlocked('\n', fileptr);
76 fprintf(fileptr, "Line 2\n");
77 funlockfile(fileptr);
78 is permissible, and results in the text sequence:
80 1
83 Line 2
84 being printed without being interspersed with output from other
86 threads.
87 It would be wrong to have the standard names such as getc(), putc(),
89 and so on, map to the "faster, but unsafe" rather than the "slower, but
90 safe'' versions. In either case, you would still want to inspect all
91 uses of getc(), putc(), and so on, by hand when converting existing
92 code. Choosing the safe bindings as the default, at least, results in
93 correct code and maintains the "atomicity at the function" invariant.
94 To do otherwise would introduce gratuitous synchronization errors into
95 converted code. Other routines that modify the stdio ( FILE *) struc‐
96 tures or buffers are also safely synchronized.
97 Note that there is no need for functions of the form getc_locked(),
99 putc_locked(), and so on, since this is the functionality of getc(),
100 putc(), et al. It would be inappropriate to use a feature test macro to
101 switch a macro definition of getc() between getc_locked() and
102 getc_unlocked(), since the ISO C standard requires an actual function
103 to exist, a function whose behavior could not be changed by the feature
104 test macro. Also, providing both the xxx_locked() and xxx_unlocked()
105 forms leads to the confusion of whether the suffix describes the behav‐
106 ior of the function or the circumstances under which it should be used.
107 Three additional routines, flockfile(), ftrylockfile(), and funlock‐
109 file() (which may be macros), are provided to allow the user to delin‐
110 eate a sequence of I/O statements that are executed synchronously.
111 The ungetc() function is infrequently called relative to the other
113 functions/macros so no unlocked variation is needed.
114
116 None.
117
119 getc(), getchar(), putc(), putchar(), the Base Definitions volume of
120 IEEE Std 1003.1-2001, <stdio.h>
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123 Portions of this text are reprinted and reproduced in electronic form
124 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
125 -- Portable Operating System Interface (POSIX), The Open Group Base
126 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
127 Electrical and Electronics Engineers, Inc and The Open Group. In the
128 event of any discrepancy between this version and the original IEEE and
129 The Open Group Standard, the original IEEE and The Open Group Standard
130 is the referee document. The original Standard can be obtained online
131 at http://www.opengroup.org/unix/online.html .
132IEEE/The Open Group 2003 GETC_UNLOCKED(3P)