Linux--信号时序竞态

1. 有问题的mysleep函数，主要注释部分的解释

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

void sig_handle(int signo)
{
	
}

unsigned int mysleep(unsigned int nsec)
{
	//int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);
	struct sigaction newaction, oldaction;
	
	newaction.sa_handler = sig_handle;
	sigemptyset(&newaction.sa_mask);
	newaction.sa_flags = 0;
	
	if (sigaction(SIGALRM, &newaction, &oldaction) < 0)
	{
		printf("sigaction error\n");
		return -1;
	}
	
	//假如CPU太忙了，nsec之后内核还在处理别的进程的事，当处理完成后，内核执行这个进程的事，
	//由于这时候已经到时间了，按照内核的处理方式，先检查该进程有没有未处理的信号，结果发现
	//存在alarm信号没有处理，从而调用信号处理函数。这时候信号到达不了用户空间了。导致pause
	//无限制等待
	alarm(nsec);
	pause();
	
	unsigned int leaveSec = alarm(0);
	sigaction(SIGALRM, &oldaction, NULL);
	
	return leaveSec;
}

int main(int argc, char *argv[])
{
	while(1)
	{
		unsigned int leaveSec = mysleep(2);
		printf("2 seconds after, leave %u!\n", leaveSec);
	}
	return 0;
}

2. 改进后的mysleep函数，使用信号屏蔽和挂起的原子操作

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <signal.h>

void sig_handle(int signo)
{
	
}

unsigned int mysleep(unsigned int nsec)
{
	sigset_t newset, oldset, emptyset;
	sigemptyset(&newset);
	sigemptyset(&emptyset);
	
	sigaddset(&newset, SIGALRM);
	
	if (sigprocmask(SIG_BLOCK, &newset, &oldset) < 0)
	{
		printf("sigprocmask error\n");
		return -1;
	}
	
	struct sigaction newact, oldact;
	newact.sa_handler = sig_handle;
	sigemptyset(&newact.sa_mask);
	newact.sa_flags = 0;
	
	if (sigaction(SIGALRM, &newact, &oldact) < 0)
	{
		printf("sigaction error\n");
		return -1;
	}
	
	alarm(nsec);
	sigsuspend(&emptyset);
	
	unsigned int nleave = alarm(0);
	sigaction(SIGALRM, &oldact, NULL);
	
	sigprocmask(SIG_SETMASK, &oldset, NULL); 
	
	return nleave;
}

int main(int argc, char *argv[])
{
	while(1)
	{
		unsigned int leave = mysleep(1);
		
		printf("1s passed, time leave %u\n", leave);
	}
	return 0;
}