以下代碼是父子進(jìn)程交替打印變量counter的值。由于fork之后子進(jìn)程會(huì)復(fù)制父進(jìn)程的堆棧，信號(hào)處理函數(shù)，信號(hào)屏蔽字，在下面的程序會(huì)根據(jù)counter的不同初值進(jìn)行分別。

#include <stdio.h>#include <signal.h>#include <unistd.h>#include <stdlib.h>#include <string.h>sigset_t newset, zeroset;static int counter = 1;static int sigFlag = 0;void sig_handler(int signo){	if (signo == SIGUSR1 || signo == SIGUSR2)	{		sigFlag = 1;	}}void tell_wait(){	sigemptyset(&newset);	sigemptyset(&zeroset);	sigaddset(&newset, SIGUSR1);	sigaddset(&newset, SIGUSR2);		struct sigaction action;	action.sa_handler = sig_handler;	sigemptyset(&action.sa_mask);	action.sa_flags = 0;		if (sigaction(SIGUSR1, &action, NULL) < 0)	{		PRintf("sigaction error/n");		exit(-1);	}		if (sigaction(SIGUSR2, &action, NULL) < 0)	{		printf("sigaction error/n");		exit(-1);	}		if (sigprocmask(SIG_BLOCK, &newset, NULL) < 0)	{		printf("sigprocmask error/n");		exit(-1);	}}void tell_parent(pid_t pid){	kill(pid, SIGUSR2);}void wait_parent(){	while(sigFlag == 0)	{		sigsuspend(&zeroset);	}		sigFlag = 0;		if (sigprocmask(SIG_BLOCK, &newset, NULL) < 0)	{		printf("set sigprocmask error/n");		exit(-1);	}}void tell_child(pid_t pid){	kill(pid, SIGUSR1);}void wait_child(){	while(sigFlag == 0)	{		sigsuspend(&zeroset);	}		sigFlag = 0;		if (sigprocmask(SIG_BLOCK, &newset, NULL) < 0)	{		printf("set sigprocmask error/n");		exit(-1);	}}int main(int argc, char **argv){	pid_t pid;		tell_wait();	if ((pid = fork()) < 0)	{		perror("fork");		return -1;	}	else if(pid == 0) //child	{		counter = 0;		while(1)		{			wait_parent();			counter+=2;			char line[128] = {0}; 			sprintf(line, "#========child: %d/n", counter);			write(STDOUT_FILENO, line, strlen(line)); 			//printf("#========child: %d/n", counter);			tell_parent(getppid());		}	}	else	{		counter = -1;		while(1)		{			counter+=2;			char line[128] = {0}; 			sprintf(line, "#===parent: %d/n", counter);			write(STDOUT_FILENO, line, strlen(line)); 			//printf("#===parent: %d/n", counter);			tell_child(pid);			wait_child();		}				waitpid(pid, NULL, 0);	}	return 0;}
需要注意的地方是注釋的printf部分，使用printf將輸出(./a.out >> test)重定向到文件的時(shí)候，不會(huì)出現(xiàn)這種交替打印的結(jié)果。原因是，重定向到的文件是全緩沖，printf是帶緩沖的，會(huì)緩沖到一定程度才能從內(nèi)存緩沖中輸出到文件。