pthread_atfork(3c)

1pthread_atfork(3C)       Standard C Library Functions       pthread_atfork(3C)
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NAME

6       pthread_atfork - register fork handlers
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SYNOPSIS

9       #include <sys/types.h>
10       #include <unistd.h>
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12       int pthread_atfork(void (*prepare) (void), void (*parent) (void),
13            void (*child) (void));
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DESCRIPTION

17       The pthread_atfork() function declares fork handlers to be called prior
18       to and following fork(2), within the thread  that  called  fork().  The
19       order of calls to pthread_atfork() is significant.
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22       Before  fork()  processing  begins, the prepare fork handler is called.
23       The prepare handler is not called if its address is NULL.
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26       The parent fork handler is called after fork() processing  finishes  in
27       the  parent  process, and the child fork handler is called after fork()
28       processing finishes in the child process. If the address of  parent  or
29       child is  NULL, then its handler is not called.
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32       The  prepare fork handler is called in  LIFO (last-in first-out) order,
33       whereas the parent and child fork handlers are called in  FIFO  (first-
34       in first-out) order. This calling order allows applications to preserve
35       locking order.
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RETURN VALUES

38       Upon successful completion, pthread_atfork() returns 0.  Otherwise,  an
39       error number is returned.
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ERRORS

42       The pthread_atfork() function will fail if:
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44       ENOMEM     Insufficient  table  space exists to record the fork handler
45                  addresses.
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USAGE

49       Solaris threads do not offer pthread_atfork() functionality  (there  is
50       no  thr_atfork() interface). However, a Solaris threads application can
51       call pthread_atfork() to ensure fork()-safety,  since  the  two  thread
52       APIs  are  interoperable. Seefork(2) for information relating to fork()
53       in a Solaris threads environment in Solaris  10  relative  to  previous
54       releases.
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EXAMPLES

57       Example 1 Make a library safe with respect to fork().
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60       All multithreaded applications that call fork() in a POSIX threads pro‐
61       gram and do more than simply call exec(2) in the child of the fork need
62       to ensure that the child is protected from deadlock.
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66       Since  the "fork-one" model results in duplicating only the thread that
67       called fork(), it is possible that at the  time  of  the  call  another
68       thread  in the parent owns a lock. This thread is not duplicated in the
69       child, so no thread will unlock  this  lock  in  the  child.   Deadlock
70       occurs if the single thread in the child needs this lock.
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74       The  problem  is more serious with locks in libraries.  Since a library
75       writer does not know if the application using the library calls fork(),
76       the  library must protect itself from such a deadlock scenario.  If the
77       application that links with this library calls fork() and does not call
78       exec() in the child, and if it needs a library lock that may be held by
79       some other thread in the parent that is inside the library at the  time
80       of the fork, the application deadlocks inside the library.
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84       The  following  describes  how  to make a library  safe with respect to
85       fork() by using  pthread_atfork().
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88           1.     Identify  all  locks  used  by  the  library  (for   example
89                  {L1,...Ln}). Identify also the locking order for these locks
90                  (for example {L1...Ln}, as well.)
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92           2.     Add a call to pthread_atfork(f1, f2, f3) in  the   library's
93                  .init section.  f1, f2, f3 are defined as follows:
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95                 f1()
96                 {
97                         /* ordered in lock order */
98                         pthread_mutex_lock(L1);
99                         pthread_mutex_lock(...);
100                         pthread_mutex_lock(Ln);
101                 }
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103                 f2()
104                 {
105                         pthread_mutex_unlock(L1);
106                         pthread_mutex_unlock(...);
107                         pthread_mutex_unlock(Ln);
108                 }
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110                 f3()
111                 {
112                         pthread_mutex_unlock(L1);
113                         pthread_mutex_unlock(...);
114                         pthread_mutex_unlock(Ln);
115                 }
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ATTRIBUTES

119       See attributes(5) for descriptions of the following attributes:
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124       ┌─────────────────────────────┬─────────────────────────────┐
125       │      ATTRIBUTE TYPE         │      ATTRIBUTE VALUE        │
126       ├─────────────────────────────┼─────────────────────────────┤
127       │Interface Stability          │Standard                     │
128       ├─────────────────────────────┼─────────────────────────────┤
129       │MT-Level                     │MT-Safe                      │
130       └─────────────────────────────┴─────────────────────────────┘
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