mmap函数用法及示例程序

#include <unistd.h>

#include <stdio.h>

#include <sys/mman.h>

#include <fcntl.h>

#include <stdlib.h>

//定义存放记录的结构体

typedef struct

{

int index; //编号

char text[10]; //内容

} RECORD;

#define SIZE (50)

#define EDIT_INDEX (10)

int main(void)

{

RECORD record, *p_mapped_memory_addr;

int i, fd;

FILE *fp;

//创建文件并写入测试数据

fp = fopen("records.dat", "w+");

for (i = 0; i < SIZE; i++)

{

record.index = i;

sprintf(record.text, "No.%d", i);

fwrite(&record, sizeof(record), 1, fp);//因为字节序对齐，在32位机上，sizeof(record)＝16，并不是14。

}

fclose(fp);

printf("Ok, write %d records to the file: records.dat .\n", SIZE);

//将第一30条记录编号修改为300，并相应地修改其内容。

//采用传统方式

fp = fopen("records.dat", "r+");

fseek(fp, EDIT_INDEX * sizeof(record), SEEK_SET);

fread(&record, sizeof(record), 1, fp);

record.index = EDIT_INDEX*10;

sprintf(record.text, "No.%d", record.index);

fseek(fp, EDIT_INDEX * sizeof(record), SEEK_SET);

fwrite(&record, sizeof(record), 1, fp);

fclose(fp);

printf("Ok, edit the file of records.dat using traditional method.\n");

/////////////////////////////////////////

//同样的修改，这次使用内存映射方式。

//将记录映射到内存中

fd = open("records.dat", O_RDWR);

p_mapped_memory_addr = (RECORD *)mmap(0, SIZE * sizeof(record), PROT_READ | PROT_WRITE, MAP_SHARED,fd, 0);

//修改数据

p_mapped_memory_addr[EDIT_INDEX].index = EDIT_INDEX*10;

sprintf(p_mapped_memory_addr[EDIT_INDEX].text, "No.%d",

p_mapped_memory_addr[EDIT_INDEX].index);

/* Synchronize the region starting at ADDR and extending LEN bytes with the

file it maps. Filesystem operations on a file being mapped are

unpredictable before this is done. Flags are from the MS_* set.

This function is a cancellation point and therefore not marked with

__THROW. extern int msync (void *__addr, size_t __len, int __flags);

*/

//将修改写回映射文件中(采用异步写方式)

msync((void *)p_mapped_memory_addr, SIZE * sizeof(record), MS_ASYNC);

/* Deallocate any mapping for the region starting at ADDR and extending LEN

bytes. Returns 0 if successful, -1 for errors (and sets errno).

extern int munmap (void *__addr, size_t __len) __THROW;

*/

//释放内存段

munmap((void *)p_mapped_memory_addr, SIZE * sizeof(record));

printf("Ok, edit the file of records.dat using mmap method.\n");

//关闭文件

close(fd);

return 0;

}

1 #include<stdio.h>
2 #include<stdlib.h>
3 #include<string.h>
4 #include<error.h>
5 #include<fcntl.h>
6 #include<sys/mman.h>
7 #include<unistd.h>
8 int main(int argc,char *argv[]){
9     int fd,len;
10     char *ptr;
11     if(argc<2){
12         printf("please enter a file\n");
13         return 0;
14     }
15     if((fd=open(argv[1],O_RDWR))<0){
16         perror("open file error");
17         return -1;
18     }
19     len=lseek(fd,0,SEEK_END);
20     ptr=mmap(NULL,len,PROT_READ|PROT_WRITE,MAP_SHARED,fd,0);//读写得和open函数的标志相一致，否则会报错
21     if(ptr==MAP_FAILED){
22         perror("mmap error");
23         close(fd);
24         return -1;
25     }
26     close(fd);//关闭文件也ok
27     printf("length is %d\n",strlen(ptr));
28     printf("the %s content is:\n%s\n",argv[1],ptr);
29     ptr[0]='c';//修改其中的一个内容
30     printf("the %s content is:\n%s\n",argv[1],ptr);
31     munmap(ptr,len);//将改变的文件写入内存
32     return 0;
33 }