【说明】
kill，可以向包括本身在内的进程发送一个信号。
kill有一个变体叫 killall，可以给运行着某一命令的所有进程发送信号。
pause，将程序挂起直到有一个信号出现为止。
程序中信号的使用会带来一个特殊的问题：如果信号出现在系统调用的执行过程中会发生什么情况？
答案是不确定的。
因此，在程序中使用信号，需要注意一些系统调用会因为接收到一个信号而失败，而这种错误情况
可能是在添加信号处理函数之前没有考虑的。
在编写程序中处理信号部分的代码时必须非常小心，因为在使用信号的程序中会出现各种各样的
“竞态条件”。例如，如果想调用 pause 等待一个信号，可信号却出现在调用 pause 之前，就会使
程序无限期地等待一个不会发生的事件。
【示例】
#include
#include
#include
#include
void sig_alm(int sig)
{
pid_t p;
p = getpid();
printf("### Process(%d) receive a alarm signal.\r\n", p);
return;
}
int main(int argc, char *argv[])
{
pid_t pid,wpid,pid_self,ppid;
char message[64] = {0};
int n, val;
(void)signal(SIGALRM, sig_alm);
printf("[parent]start to fork child process.\r\n");
pid = fork();
switch(pid)
{
case -1:
perror("fork failed");
exit(1);
case 0:
pid_self = getpid();
sprintf(message, "[child]This is the child process(%d)", pid_self);
for(n=5; n>0; n--)
{
puts(message);
sleep(1);
}
ppid = getppid();
printf("[child] signal: child > parent\r\n");
kill(ppid, SIGALRM);
printf("[child]child process(%d) exit.\r\n", pid_self);
break;
default:
pid_self = getpid();
sprintf(message, "[parent]This is the parent process(%d)", pid_self);
for(n=3; n>0; n--) //for(n=8;n>0;n--)
{
puts(message);
sleep(1);
}
//pause();
printf("[parent] signal: parent > child\r\n");
kill(pid, SIGALRM);
wpid = wait(&val);
printf("[parent]wait_pid=%d, child_pid=%d \r\n", wpid, pid);
printf("[parent]parent process(%d) exit. \r\n", pid_self);
break;
}
exit(0);
}
【编译及执行】
(1) 原码执行效果
# gcc -o signal signal.c
# ./signal
[parent]start to fork child process.
[parent]This is the parent process(8964)
[child]This is the child process(8965)
[parent]This is the parent process(8964)
[child]This is the child process(8965)
[parent]This is the parent process(8964)
[child]This is the child process(8965)
[parent] signal: parent > child
### Process(8965) receive a alarm signal.
[child]This is the child process(8965)
[child]This is the child process(8965)
[child] signal: child > parent
### Process(8964) receive a alarm signal.
[child]child process(8965) exit.
[parent]wait_pid=8965, child_pid=8965
[parent]parent process(8964) exit.
(2) 若在父进程向子进程发送信号之前，先挂起一下(pause)，其执行的效果如下：
# ./signal
[parent]start to fork child process.
[parent]This is the parent process(9608)
[child]This is the child process(9609)
[parent]This is the parent process(9608)
[child]This is the child process(9609)
[parent]This is the parent process(9608)
[child]This is the child process(9609)
[child]This is the child process(9609)
[child]This is the child process(9609)
[child] signal: child > parent
### Process(9608) receive a alarm signal.
[parent] signal: parent > child
### Process(9609) receive a alarm signal.
[child]child process(9609) exit.
[parent]wait_pid=9609, child_pid=9609
[parent]parent process(9608) exit.
(3) 若将父进程中的for循环次数改为8，即达到在父进程pause之前，子进程已经将信号发送， 则程序执行的效果如下(父进程会无限期的等待下去)： # ./signal [parent]start to fork child process. [parent]This is the parent process(9623) [child]This is the child process(9624) [parent]This is the parent process(9623) [child]This is the child process(9624) [parent]This is the parent process(9623) [child]This is the child process(9624) [parent]This is the parent process(9623) [child]This is the child process(9624) [parent]This is the parent process(9623) [child]This is the child process(9624) [parent]This is the parent process(9623) [child] signal: child > parent ### Process(9623) receive a alarm signal. [parent]This is the parent process(9623) [child]child process(9624) exit. [parent]This is the parent process(9623)
