信号阻塞问题

  今天在看《深入理解计算机系统》第8章的内容时，对于图8-31的代码有一些疑惑.

  为什么会是阻塞第二个信号，自己在ubuntu14.04（内核为linux3.13.0-24）上敲了一遍代码

 

#include<stdio.h>
#include<stdlib.h>
#include<unistd.h>
#include<sys/types.h>
#include<sys/wait.h>
#include<signal.h>
#include<unistd.h>

void handler1(int sig){
	pid_t pid;
	pid = waitpid(-1 , NULL , 0);
	printf("Handler reaped child %d \n" , pid);
	sleep(2);
	return;
}

int main(){
	int i , n;

	if ( signal(SIGCHLD , handler1) == SIG_ERR ){
		printf("signal error\n");
	}

	for(i = 0; i < 3; ++i){
		if (fork() == 0){
			printf("Hello from child %d\n" , getpid());
			sleep(1);			
			exit(0);
		}
	}
	
	printf("Father love you!\n");
	while(1);
	exit(0);
}

 最后的运行结果为

yk@yk-desktop:~/Public$ gcc -o fork1 hello.c
yk@yk-desktop:~/Public$ ./fork1 
Father love you!
Hello from child 3225
Hello from child 3226
Hello from child 3227
Handler reaped child 3225 
Handler reaped child 3226 

证明了书上的结果是不正确的

我的理解为hadler在处理第一个信号时应该是将第二个信号放在待处理列表，当第三个信号到来时，由于不能同时将多个同类型信号放在待处理列表，从而忽略第三个信号。

而将handler函数优化之后产生了一个很奇妙的现象//改为while循环处理信号

void handler1(int sig){
	pid_t pid;
	while((pid = waitpid(-1 , NULL , 0)) > 0)
	printf("Handler reaped child %d \n" , pid);
	sleep(2);
	return;
}

由于父进程与子进程的处理顺序不一致，几次的运行结果都不同

yk@yk-desktop:~/Public$ gcc -o fork1 hello.c
yk@yk-desktop:~/Public$ ./fork1 
Father love you!
Hello from child 3530
Hello from child 3528
Hello from child 3529
Handler reaped child 3530 
Handler reaped child 3528 
Handler reaped child 3529 
^C
yk@yk-desktop:~/Public$ gcc -o fork1 hello.c
yk@yk-desktop:~/Public$ ./fork1 
Hello from child 3538
Father love you!
Hello from child 3539
Hello from child 3540
Handler reaped child 3538 
Handler reaped child 3539 
Handler reaped child 3540 
^C
yk@yk-desktop:~/Public$ gcc -o fork1 hello.c
yk@yk-desktop:~/Public$ ./fork1 
Hello from child 3551
Father love you!
Hello from child 3553
Hello from child 3552
Handler reaped child 3551 
Handler reaped child 3553 
Handler reaped child 3552

真有意思～