Programming with POSIX® Threads

45 * This routine will be called after executing the fork, within

46 * the child process.

47 */

48 void fork_child (void)

49 {

50 int status; 51

52 /*

53 * Update the file scope "self_pid" within the child process, and

54 * unlock the mutex.

55 */

56 self_pid = getpid ();

57 status = pthread_mutex_unlock (&mutex);

58 if (status != 0)

59 err_abort (status, "Unlock in child handler");

60 }

61

62 /*

63 * Thread start routine, which will fork a new child process.

64 */

65 void *thread_routine (void *arg)

66 {

67 pid_t child_pid;

68 int status;

69

70 child_pid = fork ( );

71 if (child_pid == (pid_t)-l)

72 errno_abort ("Fork");

73

74 /*

75 * Lock the mutex — without the atfork handlers, the mutex will

76 * remain locked in the child process and this lock attempt will

77 * hang (or fail with EDEADLK) in the child.

78 */

79 status = pthread_mutex_lock (&mutex);

80 if (status != 0)

81  err_abort (status, "Lock in child");

82 status = pthread_mutex_unlock (&mutex);

83 if (status != 0)

84  err_abort (status, "Unlock in child");

85 printf ("After fork: %d (%d)\n", child_pid, self_pid);

86 if (child_pid != 0) {

87 if ((pid_t)-l == waitpid (child_pid, (int*)0, 0))

88 errno_abort ("Wait for child");

89 }

90 return NULL;

91 }

92

93 int main (int argc, char *argv[])

94 {

95 pthread_t fork_thread;

96 int atfork_flag = 1;

97 int status;

98

99 if (argc > 1)

100 atfork_flag = atoi (argv[l]);

101 if (atfork_flag) {

102 status = pthread_atfork (

103 fork_prepare, fork_parent, fork_child);

104 if (status != 0)

105 err_abort (status, "Register fork handlers");

106 }

107 self_pid = getpid ();

108 status = pthread_mutex_lock (&mutex);

109 if (status != 0)

110 err_abort (status, "Lock mutex");

111 /*

112 * Create a thread while the mutex is locked. It will fork a

113 * process, which (without atfork handlers) will run with the

114 * mutex locked.

115 */

116 status = pthread_create (

117 ⋔_thread, NULL, thread_routine, NULL);

118 if (status != 0)

119 err_abort (status, "Create thread");

120 sleep (5);

121 status = pthread_mutex_unlock (&mutex);

122 if (status != 0)

123 err_abort (status, "Unlock mutex");

124 status = pthread_join (fork_thread, NULL);

125 if (status != 0)

126 err_abort (status, "Join thread");

127 return 0;

128 }

■ atfork.c