Not understanding alarm() function-CodePudding

I am not understanding how the alarm() works. In this case the function waits for the 5 seconds, if there is not any event the signal is being sent and is returned NULL pointer. What confuse me, is that if I change the two lines between them below, the functions returns immediately.

 jmp_buf buff; 

    void handler(){ 
       longjmp(buff,1);
    }

      char* tfgets(char * str, int n ,FILE * stream){ 

  char *result; 

  if   (!(setjmp(buff))){ 

  if(signal(SIGALRM, handler) == SIG_ERR){ 
  char * str= "Signal cant be added";
  write(STDOUT_FILENO,str,strlen(str));
 }

 alarm(5);
 return fgets(str,n,stream); // this is the line to be changed      with the one below
                
 }else{
 return NULL;                //this one 
 }

}


 int main(int argc, char** argv, char**envp){ 
    char buf[100];
    char* input = tfgets(buf, 100, stdin);

     if (input == NULL) { 
        printf("not typed");
     }else { 
       printf("typed");
     }

}

CodePudding user response：

If I understand you correctly, you're wondering why fgets doesn't return immediately so that your tfgets function can return immediately?

That's because fgets will block until it has a line of input, it will simply not return.

Hence the alarm is needed as a timeout: If the user doesn't write a line of input within five seconds, the input is canceled.