關于signal 函數和  sigaction 函數的區別

注意 :
    <1> sigaction
    *一般用法:
          19
     20     phan.sa_handler = &sig_handler;
     21     sigemptyset(&phan.sa_mask);
     23     phan.sa_flags = 0;   或 //phan.sa_flags = SA_RESTART; 其效果和signal 一樣
     24     sigaction(SIGALRM, &phan, NULL);



     *注意
        一般不是重啟動的函數,除非把sa_flag=SA_RESTART. 也就是說當信號到來,中斷其阻塞的I/O操作,或其他阻塞的操作時,
他們會自動啟動這些操作,而不會中斷這些操作.而這些I/O操作會保持原來的阻塞狀態就好好象根本沒有信號到來一樣.
   
     <2>signal
       函數是重啟動的,當使用該函數安裝信號處理函數時,其阻塞的操作將會被中斷

      實際使用例子:

    

/*
* test for alarm
*/
#include "apue.h"
void sig_handler(int signo);
void sig_handler2(int signo);
void pr_mask(const char *str);
int
main(void)
{
int i;
struct sigaction phan;
char str[100] = {'\0'};
//phan.sa_handler = &sig_handler2;
phan.sa_handler = &sig_handler;
sigemptyset(&phan.sa_mask);
//phan.sa_flags = SA_RESTART;  //如果使用這一行,將會阻塞在read函數
phan.sa_flags = 0;
sigaction(SIGALRM, &phan, NULL);
//signal(SIGALRM, sig_handler2);
for(i=0;;i++){
alarm(3);
fprintf(stderr, "do something\n");
read(STDIN_FILENO, str, sizeof(str));
fprintf(stderr, "[%d] done\n", i);
//alarm(0);
//pause();
}
return(0);
}
void sig_handler(int signo)
{
struct sigaction phan;
switch(signo){
case SIGALRM: 
                        //如果下面的函數將會阻塞在read函數.因為它會讓阻塞的操作自動重啟動    
		        //signal(SIGALRM, sig_handler);
phan.sa_handler = &sig_handler;
sigemptyset(&phan.sa_mask);
phan.sa_flags = 0;
sigaction(SIGALRM, &phan, NULL);
pr_mask("sig_handler : ");  //打印出信號掩碼
fprintf(stderr, "time out\n");
break;
default:
fprintf(stderr, "[%d] ignores\n", signo);
}
}
void sig_handler2(int signo)
{
switch(signo){
case SIGALRM:
fprintf(stderr, "time out\n");
break;
default:
fprintf(stderr, "[%d] ignores\n", signo);
}
}
void
pr_mask(const char *str)
{
sigset_t    sigset;
int         errno_save;
errno_save = errno;     /* we can be called by signal handlers */
if (sigprocmask(0, NULL, &sigset) < 0){
fprintf(stderr, "sigprocmask error");
exit(1);
}
printf("%s", str);
if (sigismember(&sigset, SIGINT))   printf("SIGINT ");
if (sigismember(&sigset, SIGQUIT))  printf("SIGQUIT ");
if (sigismember(&sigset, SIGUSR1))  printf("SIGUSR1 ");
if (sigismember(&sigset, SIGALRM))  printf("SIGALRM ");
/* remaining signals can go here */
printf("\n");
errno = errno_save;
}