但是在多线程中处理信号的原则却完全不同，它的基本原则是：将对信号的异步处理，转换成同步处理，也就是说用一个线程专门的来“同步等待”信号的到来，而其它的线程可以完全不被该信号中断/打断(interrupt)。这样就在相当程度上简化了在多线程环境中对信号的处理。而且可以保证其它的线程不受信号的影响。这样我们对信号就可以完全预测，因为它不再是异步的，而是同步的（我们完全知道信号会在哪个线程中的哪个执行点到来而被处理！）。而同步的编程模式总是比异步的编程模式简单。其实多线程相比于多进程的其中一个优点就是：多线程可以将进程中异步的东西转换成同步的来处理。

1. sigwait - wait for a signal
2. #include <signal.h>
3. int sigwait(const sigset_t *set, int *sig);
4. Description
5. The sigwait() function suspends execution of the calling thread until the delivery of one
6. of the signals specified in the signal set set. The function accepts the signal (removes
7. it from the pending list of signals), and returns the signal number in sig.
8. The operation of sigwait() is the same as sigwaitinfo(2), except that:
9. * sigwait() only returns the signal number, rather than a siginfo_t structure describing
10. the signal.
11. * The return values of the two functions are different.
12. Return Value
13. On success, sigwait() returns 0. On error, it returns a positive error number.

1. pthread_sigmask - examine and change mask of blocked signals
2. #include <signal.h>
3. int pthread_sigmask(int how, const sigset_t *set, sigset_t *oldset);
4. Compile and link with -pthread.
5. DESCRIPTION
6. The pthread_sigmask() function is just like sigprocmask(2), with the difference that its use
7. in multithreaded programs is explicitly specified by POSIX.1-2001.
8. Other differences are noted in this page.
9. For a description of the arguments and operation of this function, see sigprocmask(2).
10. RETURN VALUE
11. On success, pthread_sigmask() returns 0; on error, it returns an error number.
12. NOTES
13. A new thread inherits a copy of its creator's signal mask.
14. (from man sigprocmask: )
15. The behavior of the call is dependent on the value of how, as follows.
16. SIG_BLOCK
17. The set of blocked signals is the union of the current set and the set argument.
18. SIG_UNBLOCK
19. The signals in set are removed from the current set of blocked signals. It is permissible
20. to attempt to unblock a signal which is not blocked.
21. SIG_SETMASK
22. The set of blocked signals is set to the argument set.
23. If oldset is non-NULL, the previous value of the signal mask is stored in oldset.
24. If set is NULL, then the signal mask is unchanged (i.e., how is ignored), but the current
25. value of the signal mask is nevertheless returned in oldset (if it is not NULL).

1. #include <signal.h>
2. int pthread_kill(pthread_t thread, int sig);
3. Compile and link with -pthread.
4. DESCRIPTION
5. The pthread_kill() function sends the signal sig to thread, another thread in the same
6. process as the caller. The signal is asynchronously directed to thread.
7. If sig is 0, then no signal is sent, but error checking is still performed; this can be
8. used to check for the existence of a thread ID.
9. RETURN VALUE
10. On success, pthread_kill() returns 0; on error, it returns an error number, and no signal
11. is sent.
12. ERRORS
13. ESRCH No thread with the ID thread could be found.
14. EINVAL An invalid signal was specified.

1. #include <pthread.h>
2. #include <stdio.h>
3. #include <stdlib.h>
4. #include <unistd.h>
5. #include <signal.h>
6. #include <errno.h>
7. /* Simple error handling functions */
8. #define handle_error_en(en, msg) \
9. do { errno = en; perror(msg); exit(EXIT_FAILURE); } while (0)
10. static void * 
    1. sig_thread(void *arg)
11. {
12. sigset_t *set = (sigset_t *) arg;
13. int s, sig;
14. for (;;) {
15. s = sigwait(set, &sig);
16. if (s != 0)
17. handle_error_en(s, "sigwait");
18. printf("Signal handling thread got signal %d\n", sig);
19. }
20. }
21. int 
    1. main(int argc, char *argv[])
22. {
23. pthread_t thread;
24. sigset_t set;
25. int s;
26. /*
27. Block SIGINT; other threads created by main() will inherit
28. a copy of the signal mask.
29. */
30. sigemptyset(&set);
31. sigaddset(&set, SIGQUIT);
32. sigaddset(&set, SIGUSR1);
33. s = pthread_sigmask(SIG_BLOCK, &set, NULL);
34. if (s != 0)
35. handle_error_en(s, "pthread_sigmask");
36. s = pthread_create(&thread, NULL, &sig_thread, (void *) &set);
37. if (s != 0)
38. handle_error_en(s, "pthread_create");
39. /*
40. Main thread carries on to create other threads and/or do
41. other work
42. */
43. pause(); /* Dummy pause so we can test program */
44. return 0;
45. }

1. digdeep@ubuntu:~/pthread/learnthread$ gcc -Wall -pthread -o pthread_sigmask pthread_sigmask.c
2. digdeep@ubuntu:~/pthread/learnthread$ ./pthread_sigmask &
3. [1] 4170
4. digdeep@ubuntu:~/pthread/learnthread$ kill -QUIT %1
5. digdeep@ubuntu:~/pthread/learnthread$ Signal handling thread got signal 3
6. digdeep@ubuntu:~/pthread/learnthread$ kill -USR1 %1
7. digdeep@ubuntu:~/pthread/learnthread$ Signal handling thread got signal 10
8. digdeep@ubuntu:~/pthread/learnthread$ kill -TERM %1
9. digdeep@ubuntu:~/pthread/learnthread$
10. [1]+ Terminated ./pthread_sigmask
11. digdeep@ubuntu:~/pthread/learnthread$