為什么SIGALRM信號(hào)捕捉不到了

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最近在學(xué)習(xí)linux信號(hào)時(shí)遇到一點(diǎn)問題，百思不得其解，其代碼如下：

1. #include<setjmp.h>                                                         
   
2.   2 #include "../include/apue.h"
   
3.   3
   
4.   4 static void sig_alrm(int);
   
5.   5 static jmp_buf env_alrm;
   
6.   6
   
7.   7 int main(void)
   
8.   8 {
   
9.   9     int n;
   
10. 10     char line[MAXLINE];
    
11. 11
    
12. 12     if(signal(SIGALRM, sig_alrm) == SIG_ERR)
    
13. 13     {
    
14. 14         err_sys("signal error");
    
15. 15     }
    
16. 16     if(setjmp(env_alrm) != 0)
    
17. 17         ;
    
18. 18     //  err_quit("set_jmp error");
    
19. 19     alarm(10);
    
20. 20     printf("read start\n");
    
21. 21     if((n = read(STDIN_FILENO, line,MAXLINE)) < 0)
    
22. 22         err_sys("read error");
    
23. alarm(0);
    
24. 24     write(STDOUT_FILENO, line, n);
    
25. 25     exit(0);
    
26. 26 }
    
27. 27
    
28. 28 static void sig_alrm(int signo)
    
29. 29 {
    
30. 30     signal(SIGALRM,sig_alrm);
    
31. 31     printf("signal start\n");;
    
32. 32     //write(STDOUT_FILENO, line, n);
    
33. 33     longjmp(env_alrm,1);
    
34. 34 }   

復(fù)制代碼

其運(yùn)行的結(jié)果如下：

1. [root@localhost signal]# gcc -o lread lread.c ../include/err.c
   
2. [root@localhost signal]# ./lread
   
3. read start
   
4. signal start
   
5. read start

復(fù)制代碼

會(huì)捕捉到第一個(gè)定時(shí)信號(hào)，但是為什么跳轉(zhuǎn)回去的時(shí)候?qū)�SIGALRM的信號(hào)就不處理了呢？望懂的童鞋指點(diǎn)一下，萬分感激

xiyoulaoyuanjia

打印下信號(hào)屏蔽子看看吧～～  

xiyoulaoyuanjia

1. man signal

復(fù)制代碼

In the original Unix systems, when a handler that was established using
       signal()  was  invoked  by the delivery of a signal, the disposition of
       the signal would be reset to SIG_DFL, and  the  system  did  not  block
       delivery  of  further  instances of the signal.  System V also provides
       these semantics for signal().  This was bad because the signal might be
       delivered  again before the handler had a chance to reestablish itself.
       Furthermore, rapid deliveries of the same signal could result in recur‐
       sive invocations of the handler.

The situation on Linux is as follows:

       * The kernel's signal() system call provides System V semantics.