1PTHREAD_ATFORK(P) POSIX Programmer's Manual PTHREAD_ATFORK(P)
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6 pthread_atfork - register fork handlers
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9 #include <pthread.h>
10 int pthread_atfork(void (*prepare)(void), void (*parent)(void),
12 void (*child)(void));
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16 The pthread_atfork() function shall declare fork handlers to be called
17 before and after fork(), in the context of the thread that called
18 fork(). The prepare fork handler shall be called before fork() process‐
19 ing commences. The parent fork handle shall be called after fork() pro‐
20 cessing completes in the parent process. The child fork handler shall
21 be called after fork() processing completes in the child process. If
22 no handling is desired at one or more of these three points, the corre‐
23 sponding fork handler address(es) may be set to NULL.
24 The order of calls to pthread_atfork() is significant. The parent and
26 child fork handlers shall be called in the order in which they were
27 established by calls to pthread_atfork(). The prepare fork handlers
28 shall be called in the opposite order.
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31 Upon successful completion, pthread_atfork() shall return a value of
32 zero; otherwise, an error number shall be returned to indicate the
33 error.
34
36 The pthread_atfork() function shall fail if:
37 ENOMEM Insufficient table space exists to record the fork handler
39 addresses.
40 The pthread_atfork() function shall not return an error code of
43 [EINTR].
44 The following sections are informative.
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48 None.
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51 None.
52
54 There are at least two serious problems with the semantics of fork() in
55 a multi-threaded program. One problem has to do with state (for exam‐
56 ple, memory) covered by mutexes. Consider the case where one thread has
57 a mutex locked and the state covered by that mutex is inconsistent
58 while another thread calls fork(). In the child, the mutex is in the
59 locked state (locked by a nonexistent thread and thus can never be
60 unlocked). Having the child simply reinitialize the mutex is unsatis‐
61 factory since this approach does not resolve the question about how to
62 correct or otherwise deal with the inconsistent state in the child.
63 It is suggested that programs that use fork() call an exec function
65 very soon afterwards in the child process, thus resetting all states.
66 In the meantime, only a short list of async-signal-safe library rou‐
67 tines are promised to be available.
68 Unfortunately, this solution does not address the needs of multi-
70 threaded libraries. Application programs may not be aware that a multi-
71 threaded library is in use, and they feel free to call any number of
72 library routines between the fork() and exec calls, just as they always
73 have. Indeed, they may be extant single-threaded programs and cannot,
74 therefore, be expected to obey new restrictions imposed by the threads
75 library.
76 On the other hand, the multi-threaded library needs a way to protect
78 its internal state during fork() in case it is re-entered later in the
79 child process. The problem arises especially in multi-threaded I/O
80 libraries, which are almost sure to be invoked between the fork() and
81 exec calls to effect I/O redirection. The solution may require locking
82 mutex variables during fork(), or it may entail simply resetting the
83 state in the child after the fork() processing completes.
84 The pthread_atfork() function provides multi-threaded libraries with a
86 means to protect themselves from innocent application programs that
87 call fork(), and it provides multi-threaded application programs with a
88 standard mechanism for protecting themselves from fork() calls in a
89 library routine or the application itself.
90 The expected usage is that the prepare handler acquires all mutex locks
92 and the other two fork handlers release them.
93 For example, an application can supply a prepare routine that acquires
95 the necessary mutexes the library maintains and supply child and parent
96 routines that release those mutexes, thus ensuring that the child gets
97 a consistent snapshot of the state of the library (and that no mutexes
98 are left stranded). Alternatively, some libraries might be able to
99 supply just a child routine that reinitializes the mutexes in the
100 library and all associated states to some known value (for example,
101 what it was when the image was originally executed).
102 When fork() is called, only the calling thread is duplicated in the
104 child process. Synchronization variables remain in the same state in
105 the child as they were in the parent at the time fork() was called.
106 Thus, for example, mutex locks may be held by threads that no longer
107 exist in the child process, and any associated states may be inconsis‐
108 tent. The parent process may avoid this by explicit code that acquires
109 and releases locks critical to the child via pthread_atfork(). In
110 addition, any critical threads need to be recreated and reinitialized
111 to the proper state in the child (also via pthread_atfork()).
112 A higher-level package may acquire locks on its own data structures
114 before invoking lower-level packages. Under this scenario, the order
115 specified for fork handler calls allows a simple rule of initialization
116 for avoiding package deadlock: a package initializes all packages on
117 which it depends before it calls the pthread_atfork() function for
118 itself.
119
121 None.
122
124 atexit() , fork() , the Base Definitions volume of
125 IEEE Std 1003.1-2001, <sys/types.h>
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128 Portions of this text are reprinted and reproduced in electronic form
129 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
130 -- Portable Operating System Interface (POSIX), The Open Group Base
131 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
132 Electrical and Electronics Engineers, Inc and The Open Group. In the
133 event of any discrepancy between this version and the original IEEE and
134 The Open Group Standard, the original IEEE and The Open Group Standard
135 is the referee document. The original Standard can be obtained online
136 at http://www.opengroup.org/unix/online.html .
137IEEE/The Open Group 2003 PTHREAD_ATFORK(P)