Re: signal catching not work

Thanks for everyone's advice. I 've modified the program as below and 
come to these conclusions:

1. signal handler seems not having to register for multiple times, 
though the manual sounds like it has to.
2. return from signal handler that catches SIGSEGV is undefined and 
cannot go back and recover normal control flow. If we really want to go 
back, use long jump.
3. in practice, a signal may be raised at anytime during the execution 
of the program, so the signal handler must not call any lib routines 
except abort() and _Exit() to keep asynchronous-safe.
reference: 
https://www.securecoding.cert.org/confluence/display/seccode/SIG30-C.+Call+only+asynchronous-safe+functions+within+signal
+handlers
4. The 'long jump in sighandler example' failing to catch the second 
SIGSEGV, even if the handler is registered for the second time, seems 
still weird to me. I guess this is beyond the explaination of ANSI C.


#include <stdio.h>
#include <stdlib.h>
#include <signal.h>
#include <setjmp.h>

#define MYSIG SIGINT
#define y main

jmp_buf buf;

volatile sig_atomic_t i=0;
void sighandler(int a){
     longjmp(buf, ++i);
}

int x(){
     if(signal(MYSIG, sighandler)==SIG_ERR){
         perror("error signal");
         abort();
     }
     int *a=NULL;
     switch(setjmp(buf)){
         case 0:
         case 1:
             puts("First time caught.");
#if 1
             if(SIG_ERR==signal(MYSIG, sighandler))
                 return;
#endif
             raise(MYSIG);
             getchar();
             *a=1;
             break;
         default:
             puts("Second time caught.");
     }
}


volatile sig_atomic_t j=0;
void sighandler2(int a){
     ++j;
}

int y(){
     if(signal(MYSIG, sighandler2)==SIG_ERR){
         perror("error signal");
         abort();
     }
     int *a=NULL;
     while(1){
         switch(j){
             case 0:
                 raise(MYSIG);
                 //getchar(); // press ctrl+c here
                 //*a=10;
                 break;
             case 1:
                 puts("First time caught.");
#if 0
                 if(SIG_ERR==signal(MYSIG, sighandler2))
                     return;
#endif
                 raise(MYSIG);
                 //getchar();
                 //*a=10;
                 break;
             default:
                 puts("Second time caught.");
                 return;
         }
     }
}
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