mmap代替通用IO读取文件数据（curious）

提供一份测试demo：

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/types.h>
#include <fcntl.h>
#include <sys/file.h>
#include <sys/mman.h>
#include <sys/stat.h>

int open_file(char *path)
{
    int fd;
    fd = open(path,O_RDONLY);
    if (-1 == fd){
        printf("error open\n");
        return -1;
    }
    return fd;
}

int get_file_size(int fd)
{
    struct stat st;
    int ret;
    ret = fstat(fd,&st);
    if (-1 == ret){
        printf("fstat failed\n");
        return -1;
    }
    return st.st_size;
}

typedef struct gpios{
    char value;
}GPIOS;

int get_value_mmap_files(int fd, int len, char *value)
{
    GPIOS *gpio;
    gpio = mmap(NULL,len,PROT_READ,MAP_SHARED,fd,0);
    if (NULL == gpio || gpio == (void*) - 1){
        printf("error mmap\n");
        return -1;
    }
    *value = gpio->value;
    munmap(gpio,len);
    close(fd);
    return 0;
}

int main(int argc, char **argv)
{
    //open file
    //get size of file
    //use mmap
    //close 
    
    char path[32] = "/tmp/wat";
    int fd;
    int len;
    int ret;
    char value;

    fd = open_file(path);
    if(-1 == fd)
        return -1;
    len = get_file_size(fd);
    if (-1 == len)
        return -1;

    ret = get_value_mmap_files(fd,len,&value);
    if (-1 == ret)
        return -1;
    printf("The gpio level is %c\n",value);
    return 0;
}