代码：

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

#define FILELENGTH 80
int main(void)
{
	int fd=-1;
	char buf[]="quick brown fox jumps over the lazy dog";
	char *ptr=NULL;
	fd=open("mmap.txt",O_RDWR|O_CREAT|O_TRUNC,S_IRWXU);
	if(-1==fd)
	{
		return -1;
	}
	lseek(fd,FILELENGTH-1,SEEK_SET);
	write(fd,buf,1);
	ptr=(char*)mmap(NULL,FILELENGTH,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);
	if((char*)-1==ptr)
	{
		printf("mnap failure\n");
		close(fd);
		return -1;
	}
	memcpy(ptr+16,buf,strlen(buf));
	munmap(ptr,FILELENGTH);
	close(fd);
	return 0;
}

编译运行