mmap基本用法

#include <sys/mman.h>
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <string.h>
 
int main(int argc,char **argv)
{
    int ret, inum = 0;
#if 0    // 该段用匿名映射区代替
    int fd = open("mytest.txt", O_CREAT|O_RDWR, 0644);
    
    if(fd < 0)
    {
    perror("open feild!");
    return -1;
    }
    
    ret = ftruncate(fd, 4);
    if(ret == -1)
    {
    perror("ftruncate feild!");
    return -1;
    }
    char *ptr = mmap(NULL, 4, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
    if(ptr == MAP_FAILED)
    {
    perror("mmap feild!");
    return -1;
    }
    close(fd);
#else
    char *ptr = mmap(NULL, 4, PROT_READ|PROT_WRITE, MAP_SHARED|MAP_ANONYMOUS, -1, 0);
    if(ptr == MAP_FAILED)
    {
    perror("mmap feild!");
    return -1;
    }
#endif
    *(int *)ptr = 1234;
    printf("ptr : %d, inum : %d\n", *(int *)ptr, inum);
    
    pid_t pid = fork();
    if(pid == 0)    // son 
    {
    *(int *)ptr = 5678;
    inum = 6666;
    printf("ptr : %d, inum : %d\n", *(int *)ptr, inum);
    }
 
    if(pid > 0)    // father
    {
    wait(NULL);    
    printf("ptr : %d, inum : %d\n", *(int *)ptr, inum);
 
    ret = munmap(ptr, 4);
    if(ret == -1)
    {
        perror("munmap feild!");
        return -1;
    }
    }
 
    return 0;
}