mmap offset偏移量的简单使用

mmap原型为：

SYNOPSIS
#include <sys/mman.h>
void *mmap(void *addr, size_t length, int prot, int flags,int fd, off_t offset);
int munmap(void *addr, size_t length);

offset 必须为PAGE_SIZE的整数倍
LINUX下面获取PAGE_SIZE的命令

root@bt:~/c# getconf PAGE_SIZE
4096

root@bt:~/c# cat mmap.c
#include 
#include 
#include 
#include 
#include 
#include 
#include 
int main(int argc, char **argv)
{
    char buff[1024];
    char *mmaped;
    int fd;
    int offset, realOffset, length, realLen;
    if(argc != 4) {
        printf("Usage:%s   n", argv[0]);
        exit(1);
    }
    if((fd=open(argv[1], O_RDWR)) < 0) {
        perror("open");
        exit(1);
    }
    offset = atoi(argv[2]);
    length = atoi(argv[3]);
    printf("System Page_Size:[%ld]n", sysconf(_SC_PAGE_SIZE));
    realOffset = offset & ~(sysconf(_SC_PAGE_SIZE) - 1);  //必须为PAGE_SIZE倍数
    printf("realOffset:[%d]n", realOffset);
    realLen = length + offset - realOffset;
    printf("realLen:[%d]n", realLen);
    if((mmaped = mmap(NULL, realLen, PROT_READ|PROT_WRITE, MAP_SHARED, fd, realOffset)) == (void *)-1) {
        perror("mmap");
        exit(1);
    }
    close(fd);
    memset(buff,0x00,sizeof(buff));
    memcpy(buff, mmaped, realLen);
    printf("buff:[%s]n", buff);
    munmap(mmaped, realLen);
    return 0;
}
root@bt:~/c# gcc mmap.c
root@bt:~/c# ./a.out data.txt 4096  7
System Page_Size:[4096]
realOffset:[4096]
realLen:[7]
buff:[567890
]
root@bt:~/c# ./a.out data.txt 1  7
System Page_Size:[4096]
realOffset:[0]
realLen:[8]
buff:[12345678]
root@bt:~/c#