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1 | .\" Copyright (C) 2001 Andries Brouwer <aeb@cwi.nl>. |
2 | .\" | |
93015253 | 3 | .\" %%%LICENSE_START(VERBATIM) |
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4 | .\" Permission is granted to make and distribute verbatim copies of this |
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8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
c13182ef | 12 | .\" |
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13 | .\" Since the Linux kernel and libraries are constantly changing, this |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
c13182ef | 20 | .\" |
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21 | .\" Formatted or processed versions of this manual, if unaccompanied by |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 23 | .\" %%%LICENSE_END |
fea681da | 24 | .\" |
f55a6d59 | 25 | .TH FLOCKFILE 3 2017-07-13 "" "Linux Programmer's Manual" |
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26 | .SH NAME |
27 | flockfile, ftrylockfile, funlockfile \- lock FILE for stdio | |
28 | .SH SYNOPSIS | |
29 | .nf | |
30 | .B #include <stdio.h> | |
68e4db0a | 31 | .PP |
fea681da | 32 | .BI "void flockfile(FILE *" filehandle ); |
fea681da | 33 | .BI "int ftrylockfile(FILE *" filehandle ); |
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34 | .BI "void funlockfile(FILE *" filehandle ); |
35 | .fi | |
68e4db0a | 36 | .PP |
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37 | .in -4n |
38 | Feature Test Macro Requirements for glibc (see | |
39 | .BR feature_test_macros (7)): | |
40 | .in | |
41 | .ad l | |
68e4db0a | 42 | .PP |
cfc2d88d | 43 | All functions shown above: |
0eda7eb5 | 44 | .RS 4 |
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45 | /* Since glibc 2.24: */ _POSIX_C_SOURCE\ >=\ 199309L |
46 | || /* Glibc versions <= 2.23: */ _POSIX_C_SOURCE | |
d59161f9 | 47 | || /* Glibc versions <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE |
0eda7eb5 | 48 | .RE |
cc4615cc | 49 | .ad b |
fea681da | 50 | .SH DESCRIPTION |
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51 | The stdio functions are thread-safe. |
52 | This is achieved by assigning | |
2f0af33b | 53 | to each |
097585ed | 54 | .I FILE |
c7094399 | 55 | object a lockcount and (if the lockcount is nonzero) |
fea681da | 56 | an owning thread. |
2f0af33b | 57 | For each library call, these functions wait until the |
097585ed | 58 | .I FILE |
2f0af33b | 59 | object |
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60 | is no longer locked by a different thread, then lock it, do the |
61 | requested I/O, and unlock the object again. | |
dd3568a1 | 62 | .PP |
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63 | (Note: this locking has nothing to do with the file locking done |
64 | by functions like | |
65 | .BR flock (2) | |
66 | and | |
67 | .BR lockf (3).) | |
dd3568a1 | 68 | .PP |
fea681da | 69 | All this is invisible to the C-programmer, but there may be two |
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70 | reasons to wish for more detailed control. |
71 | On the one hand, maybe | |
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72 | a series of I/O actions by one thread belongs together, and should |
73 | not be interrupted by the I/O of some other thread. | |
74 | On the other hand, maybe the locking overhead should be avoided | |
75 | for greater efficiency. | |
dd3568a1 | 76 | .PP |
2f0af33b | 77 | To this end, a thread can explicitly lock the |
097585ed | 78 | .I FILE |
2f0af33b | 79 | object, |
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80 | then do its series of I/O actions, then unlock. |
81 | This prevents | |
82 | other threads from coming in between. | |
83 | If the reason for doing | |
fea681da | 84 | this was to achieve greater efficiency, one does the I/O with |
24b74457 | 85 | the nonlocking versions of the stdio functions: with |
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86 | .BR getc_unlocked (3) |
87 | and | |
88 | .BR putc_unlocked (3) | |
89 | instead of | |
90 | .BR getc (3) | |
91 | and | |
92 | .BR putc (3). | |
dd3568a1 | 93 | .PP |
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94 | The |
95 | .BR flockfile () | |
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96 | function waits for |
97 | .I *filehandle | |
98 | to be | |
fea681da | 99 | no longer locked by a different thread, then makes the |
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100 | current thread owner of |
101 | .IR *filehandle , | |
102 | and increments | |
fea681da | 103 | the lockcount. |
dd3568a1 | 104 | .PP |
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105 | The |
106 | .BR funlockfile () | |
107 | function decrements the lock count. | |
dd3568a1 | 108 | .PP |
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109 | The |
110 | .BR ftrylockfile () | |
ff40dbb3 | 111 | function is a nonblocking version |
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112 | of |
113 | .BR flockfile (). | |
c13182ef | 114 | It does nothing in case some other thread |
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115 | owns |
116 | .IR *filehandle , | |
117 | and it obtains ownership and increments | |
fea681da | 118 | the lockcount otherwise. |
47297adb | 119 | .SH RETURN VALUE |
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120 | The |
121 | .BR ftrylockfile () | |
122 | function returns zero for success | |
c7094399 | 123 | (the lock was obtained), and nonzero for failure. |
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124 | .SH ERRORS |
125 | None. | |
3cdc8f57 | 126 | .SH ATTRIBUTES |
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127 | For an explanation of the terms used in this section, see |
128 | .BR attributes (7). | |
129 | .TS | |
130 | allbox; | |
131 | lbw29 lb lb | |
132 | l l l. | |
133 | Interface Attribute Value | |
134 | T{ | |
3cdc8f57 PH |
135 | .BR flockfile (), |
136 | .BR ftrylockfile (), | |
3cdc8f57 | 137 | .BR funlockfile () |
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138 | T} Thread safety MT-Safe |
139 | .TE | |
47297adb | 140 | .SH CONFORMING TO |
af8b5e04 | 141 | POSIX.1-2001, POSIX.1-2008. |
fea681da | 142 | .SH AVAILABILITY |
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143 | These functions are available when |
144 | .B _POSIX_THREAD_SAFE_FUNCTIONS | |
c13182ef | 145 | is defined. |
47297adb | 146 | .SH SEE ALSO |
fea681da | 147 | .BR unlocked_stdio (3) |