Linux下在内存映射文件中给计数器持续加1

#include <unistd.h>#include <stdio.h>#include <fcntl.h>           /* For O_* constants */#include <sys/stat.h>        /* For mode constants */#include <semaphore.h>#include <stdlib.h>#include <sys/mman.h>#define SEM_NAME    "chansem"#define  FILE_MODE (S_IRUSR|S_IWUSR|S_IRGRP|S_IROTH)int main(int argc, char** argv){    int i, nloop;    int fd, zero = 0;    int *ptr;    sem_t   *mutex;    if(argc != 3)    {        printf("usage: incr2 <#loops>");    }    nloop = atoi(argv[2]);    fd = open(argv[1], O_RDWR | O_CREAT, FILE_MODE);    write(fd, &zero, sizeof(int));    ptr = mmap(NULL, sizeof(int), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);    close(fd);    mutex = sem_open(SEM_NAME, O_CREAT | O_EXCL, 1);    sem_unlink(SEM_NAME);    setbuf(stdout, NULL);    if(fork() == 0)    {        for(i = 0; i < nloop; i++)        {            sem_wait(mutex);            printf("child: %d\n", (*ptr)++);            sem_post(mutex);        }        exit(0);    }    for(i = 0; i < nloop; i++)    {        sem_wait(mutex);        printf("parent: %d\n", (*ptr)++);        sem_post(mutex);    }    exit(0);}