计数器和信号量都在共享内存区中

#include <stdio.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <semaphore.h>
#include <stdlib.h>
#include <sys/mman.h>

const int maxline=4096;

struct shared{
	sem_t mutex;
	int count;
}shared;

int main(int argc, char **argv) {
	int fd, i, nloop;
	struct shared *ptr;
	char errbuff[maxline];

	if(argc!=3) {
		fprintf(stderr, "usage: ./a.out <pathname> <#nloops>\n");
		exit(-1);
	}

	nloop=atoi(argv[2]);

	fd=open(argv[1], O_RDWR|O_CREAT, 0644);
	if(write(fd, &shared, sizeof(struct shared)) != sizeof(struct shared)) {
		fprintf(stderr, "write error.\n");
		exit(-1);
	}

	if((ptr=mmap(NULL, sizeof(struct shared), PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0))==MAP_FAILED) {
		fprintf(stderr, "mmap error.\n");
		exit(-1);
	}
	close(fd);

	if(sem_init(&ptr->mutex, 1, 1)==-1) {
		strerror_r(errno, errbuff, maxline);
		fprintf(stderr, "sem_init error: %s\n", errbuff);
		exit(-1);
	}

	setbuf(stdout, NULL);

	if(fork()==0) {
		for(i=0;i<nloop;i++) {
			sem_wait(&ptr->mutex);
			printf("child: %d\n", ptr->count++);
			sem_post(&ptr->mutex);
		}
		exit(0);
	}

	for(i=0;i<nloop;i++) {
		sem_wait(&ptr->mutex);
		printf("parent: %d\n", ptr->count++);
		sem_post(&ptr->mutex);
	}
	exit(0);
}