[結貼]signal()注冊的信號處理函數(shù)里面，不需要再次注冊信號處理函數(shù)嗎?

sentto2

網(wǎng)上說，linux下面的signal函數(shù)，其實現(xiàn)機制是，在調用信號處理函數(shù)之前就會自動把信號處理函數(shù)重置，也就是信號處理函數(shù)只調用一遍。
如果需要多次都有效的話，信號處理函數(shù)里面要再次用signal()注冊自身。

但是我寫代碼測試了一下，發(fā)現(xiàn)不用在信號處理函數(shù)里面再去注冊自己啊，代碼如下:

1. 
   
2. #include<signal.h>
   
3. #include<stdio.h>
   
4. void f(int sig)
   
5. {
   
6.   printf("input=%d\n",sig);
   
7. }
   
8. int main()
   
9. {
   
10.   signal(SIGHUP,f);
    
11.   int i = getchar();
    
12.   puts("hello world");
    
13.   return 0;
    
14. }
    

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編譯運行g++ ss.cpp&&./a.out
然后在另一個終端里面去不停的發(fā)信號:

1. 
   
2. $ ps -ef|grep a.out
   
3. x        27498 27421  0 23:14 pts/3    00:00:00 ./a.out
   
4. x        27501 26574  0 23:15 pts/2    00:00:00 grep --color=auto a.out
   
5. [x@localhost ~]$ kill -1 27498
   
6. [x@localhost ~]$ kill -1 27498
   

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那么運行a.out的終端就會不停的輸出:

1. 
   
2. $ g++ ss.cpp&&./a.out
   
3. input=1
   
4. input=1
   

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這是為什么呢? 不是說signal()注冊的函數(shù)只是一次有效的嗎?

zsszss0000

signal注冊一次，就可以了啊

1. #include<signal.h>
   
2. #include<stdio.h>
   
3. void f(int sig)
   
4. {
   
5.     printf("input=%d\n",sig);
   
6. }
   
7. int main()
   
8. {
   
9.     signal(SIGINT,f);
   
10.     while(1)
    
11.     {   
    
12.         printf("a\n");
    
13.         sleep(3);
    
14.    
    
15.     }   
    
16.     return 0;
    
17. }

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你試試這個，每次發(fā)生信號，都會觸發(fā)這個函數(shù)，不需要再信號處理函數(shù)中重新注冊

sentto2

zsszss0000 發(fā)表于 2015-04-28 19:15
signal注冊一次，就可以了啊你試試這個，每次發(fā)生信號，都會觸發(fā)這個函數(shù)，不需要再信號處理函數(shù)中重新注冊

謝謝，我的問題就是這樣，你說的不是我想要的: 因為網(wǎng)上總說signal函數(shù)注冊的處理函數(shù)，里面必須再次注冊自己才能保證每次都被信號觸發(fā)。換句話說，signal()函數(shù)調用是不是會恢復默認的信號處理函數(shù)呢?

謝謝。

zsszss0000

signal的handler不會恢復默認的處理函數(shù)回復 3# sentto2


   

bfdhczw

參考man signal

1.        In  the  original UNIX systems, when a handler that was established using signal() was invoked by the delivery of a signal, the disposition
   
2.        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
   
3.        these  semantics  for  signal().   This  was bad because the signal might be delivered again before the handler had a chance to reestablish
   
4.        itself.  Furthermore, rapid deliveries of the same signal could result in recursive invocations of the handler.
   
5. 
   
6.        BSD improved on this situation by changing the semantics of signal handling (but, unfortunately, silently changed the semantics when estab‐
   
7.        lishing  a handler with signal()).  On BSD, when a signal handler is invoked, the signal disposition is not reset, and further instances of
   
8.        the signal are blocked from being delivered while the handler is executing.
   
9. 
   
10.        The situation on Linux is as follows:
    
11. 
    
12.        * The kernel's signal() system call provides System V semantics.
    
13. 
    
14.        * By default, in glibc 2 and later, the signal() wrapper function does not invoke the kernel system call.  Instead, it  calls  sigaction(2)
    
15.          using  flags  that supply BSD semantics.  This default behavior is provided as long as the _BSD_SOURCE feature test macro is defined.  By
    
16.          default, _BSD_SOURCE is defined; it is also implicitly defined if one defines _GNU_SOURCE, and can of course be explicitly defined.
    
17. 
    
18.          On glibc 2 and later, if the _BSD_SOURCE feature test macro is not defined, then signal() provides  System  V  semantics.   (The  default
    
19.          implicit definition of _BSD_SOURCE is not provided if one invokes gcc(1) in one of its standard modes (-std=xxx or -ansi) or defines var‐
    
20.          ious other feature test macros such as _POSIX_SOURCE, _XOPEN_SOURCE, or _SVID_SOURCE; see feature_test_macros(7).)
    
21. 
    
22.        * The signal() function in Linux libc4 and libc5 provide System V semantics.  If one on a libc5 system includes <bsd/signal.h>  instead  of
    
23.          <signal.h>, then signal() provides BSD semantics.
    

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sentto2

bfdhczw 發(fā)表于 2015-04-29 17:45
參考man signal

說的很好，默認編譯，signal()調用就是POSIX行為，等同于調用sigaction，會阻塞，沒有disposition.
如果編譯時加了額外的選項例如-std=c99，那么就是SysV的行為，調用內核的signal。

結貼!