1GETC_UNLOCKED(3P) POSIX Programmer's Manual GETC_UNLOCKED(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.
10
13 getc_unlocked, getchar_unlocked, putc_unlocked, putchar_unlocked —
14 stdio with explicit client locking
15
17 #include <stdio.h>
18 int getc_unlocked(FILE *stream);
20 int getchar_unlocked(void);
21 int putc_unlocked(int c, FILE *stream);
22 int putchar_unlocked(int c);
23
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 If getc_unlocked() or putc_unlocked() are implemented as macros they
37 may evaluate stream more than once, so the stream argument should never
38 be an expression with side-effects.
39
41 See getc(), getchar(), putc(), and putchar().
42
44 See getc(), getchar(), putc(), and putchar().
45 The following sections are informative.
47
49 None.
50
52 Since they may be implemented as macros, getc_unlocked() and
53 putc_unlocked() may treat incorrectly a stream argument with side-
54 effects. In particular, getc_unlocked(*f++) and putc_unlocked(c,*f++)
55 do not necessarily work as expected. Therefore, use of these functions
56 in such situations should be preceded by the following statement as
57 appropriate:
58 #undef getc_unlocked
60 #undef putc_unlocked
61
63 Some I/O functions are typically implemented as macros for performance
64 reasons (for example, putc() and getc()). For safety, they need to be
65 synchronized, but it is often too expensive to synchronize on every
66 character. Nevertheless, it was felt that the safety concerns were more
67 important; consequently, the getc(), getchar(), putc(), and putchar()
68 functions are required to be thread-safe. However, unlocked versions
69 are also provided with names that clearly indicate the unsafe nature of
70 their operation but can be used to exploit their higher performance.
71 These unlocked versions can be safely used only within explicitly
72 locked program regions, using exported locking primitives. In particu‐
73 lar, a sequence such as:
74 flockfile(fileptr);
76 putc_unlocked('1', fileptr);
77 putc_unlocked('\n', fileptr);
78 fprintf(fileptr, "Line 2\n");
79 funlockfile(fileptr);
80 is permissible, and results in the text sequence:
82 1
84 Line 2
85 being printed without being interspersed with output from other
87 threads.
88 It would be wrong to have the standard names such as getc(), putc(),
90 and so on, map to the ``faster, but unsafe'' rather than the ``slower,
91 but safe'' versions. In either case, you would still want to inspect
92 all uses of getc(), putc(), and so on, by hand when converting existing
93 code. Choosing the safe bindings as the default, at least, results in
94 correct code and maintains the ``atomicity at the function'' invariant.
95 To do otherwise would introduce gratuitous synchronization errors into
96 converted code. Other routines that modify the stdio (FILE *) struc‐
97 tures or buffers are also safely synchronized.
98 Note that there is no need for functions of the form getc_locked(),
100 putc_locked(), and so on, since this is the functionality of getc(),
101 putc(), et al. It would be inappropriate to use a feature test macro
102 to switch a macro definition of getc() between getc_locked() and
103 getc_unlocked(), since the ISO C standard requires an actual function
104 to exist, a function whose behavior could not be changed by the feature
105 test macro. Also, providing both the xxx_locked() and xxx_unlocked()
106 forms leads to the confusion of whether the suffix describes the behav‐
107 ior of the function or the circumstances under which it should be used.
108 Three additional routines, flockfile(), ftrylockfile(), and funlock‐
110 file() (which may be macros), are provided to allow the user to delin‐
111 eate a sequence of I/O statements that are executed synchronously.
112 The ungetc() function is infrequently called relative to the other
114 functions/macros so no unlocked variation is needed.
115
117 None.
118
120 Section 2.5, Standard I/O Streams, flockfile(), getc(), getchar(),
121 putc(), putchar()
122 The Base Definitions volume of POSIX.1‐2008, <stdio.h>
124
126 Portions of this text are reprinted and reproduced in electronic form
127 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
128 -- Portable Operating System Interface (POSIX), The Open Group Base
129 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
130 cal and Electronics Engineers, Inc and The Open Group. (This is
131 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
132 event of any discrepancy between this version and the original IEEE and
133 The Open Group Standard, the original IEEE and The Open Group Standard
134 is the referee document. The original Standard can be obtained online
135 at http://www.unix.org/online.html .
136 Any typographical or formatting errors that appear in this page are
138 most likely to have been introduced during the conversion of the source
139 files to man page format. To report such errors, see https://www.ker‐
140 nel.org/doc/man-pages/reporting_bugs.html .
141IEEE/The Open Group 2013 GETC_UNLOCKED(3P)