1. 有問題的mysleep函數，主要注釋部分的解釋

#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之后內核還在處理別的進程的事，當處理完成后，內核執行這個進程的事，	//由于這時候已經到時間了，按照內核的處理方式，先檢查該進程有沒有未處理的信號，結果發現	//存在alarm信號沒有處理，從而調用信號處理函數。這時候信號到達不了用戶空間了。導致pause	//無限制等待	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. 改進后的mysleep函數，使用信號屏蔽和掛起的原子操作
#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;}