#include<stdio.h>
#include<signal.h>
#include <unistd.h>
void pdosig(int num ,struct __siginfo * _siginfo, void * ptr)
{
    static int count = 0;
    printf("I am parent ,pid is %d,the proc : %d send signal: %d to me count is: %d\n",getpid(),_siginfo->si_pid,num,count);
    count+=2;
    sleep(1);
    kill(_siginfo->si_pid,SIGUSR2);

}

void cdosig(int num,struct __siginfo * _siginfo, void * ptr)
{
    static int count = 1;
    printf("I am child ,pid is %d,the proc : %d send signal: %d to me count is: %d\n",getpid(),_siginfo->si_pid,num,count);
    count+=2;
    sleep(1);
    kill(_siginfo->si_pid,SIGUSR1);

}
int main()
{

    struct  sigaction sig;
    int pid = fork();
    if(pid >0)
    {
        printf("i am father %d\n",getpid());

        sig.sa_sigaction = pdosig;
        sigemptyset(&sig.sa_mask);
        sig.sa_flags=0;
        sigaction(SIGUSR1,&sig,NULL);

    }
    else
    {
        printf("i am child %d\n",getpid());
        sig.sa_sigaction = cdosig;
        sigemptyset(&sig.sa_mask);
        sig.sa_flags=0;
        sigaction(SIGUSR2,&sig,NULL);
        kill(getppid(),SIGUSR1);

    }
    while(1)
    {
        sleep(1);
    }
    return 0;
}
 

SIGUSR1和SIGUSR2是自定義訊號量，我們可以利用兩個訊號來實現父子程序同步輸出

int   sigaction(int sig, const struct sigaction *restrict act,   struct sigaction *restrict oact);函式來捕獲訊號，然後再利用KILL函式傳送訊號。從而實現同步。需要注意的是struct  sigaction中的成員__sigaction_u是一個聯合體，此處我們需要用第二個即sa_sigaction才行，因為此回掉函式提供的引數中一個結構體struct __siginfo，此結構體如下

typedef struct __siginfo {
   int    si_signo;     /* signal number */
   int    si_errno;     /* errno association */
   int    si_code;      /* signal code */
   pid_t  si_pid;          /* sending process */
   uid_t  si_uid;          /* sender's ruid */
   int    si_status;    /* exit value */
   void   *si_addr;     /* faulting instruction */
   union sigval si_value;    /* signal value */
   long   si_band;      /* band event for SIGPOLL */
   unsigned long  __pad[7];  /* Reserved for Future Use */
} siginfo_t;

其中pid_t包含傳送者的pid，因此我們可以根據傳送者的pid使父子程序建立聯絡，實現同步輸出