利用mmap函数实现内存映射的文件拷贝

////本程序的功能是把一个文件映射到
//内存然后再把内存中的东西利用write函数给输出到标准输出
//最后利用munmap解除内存映射
#include<stdio.h>
#include<sys/mman.h>
#include<fcntl.h>
#include<unistd.h>
int main(int argc,char *argv[])
{
    if(argc!=4)
    {
         write(STDOUT_FILENO,"hello\n",6);
         printf("usage:%s  \n",argv[0]);
         return 1;
    }
    char *filename=argv[1];//第二个参数表示文件名
    printf("the file name to be mapped is :%s\n",filename);
    int fd=open(filename,O_RDONLY);
    int offset=atoi(argv[2]);//第三个参数标识要映射时的开始偏移量
    printf("start offset of file to be mapped is :%d\n",offset);
    printf("page size is %ld\n",sysconf(_SC_PAGE_SIZE));
    int realOffset=offset&~(sysconf(_SC_PAGE_SIZE)-1);
    printf("real start offset of file to be mapped is %d\n",realOffset);
    int length=atoi(argv[3]);//要映射的文件的长度
    printf("the length to be map is :%d\n",length);
    int reallen=length+offset-realOffset;//实际映射的长度
    printf("the real length to be map is %d\n",reallen);
    char *addr=mmap(NULL,reallen,PROT_READ,MAP_PRIVATE,fd,realOffset);//有关该函数我们可以使用 //man命令去查看
    close(fd);
    write(STDOUT_FILENO,addr,reallen);/*输出到标准输出*/
    munmap(addr,reallen);//解除内存映射
    printf("\n");

    return 0;

}