mmap函数的用法

#include	<unistd.h>
#include	<stdio.h>
#include	<sys/mman.h>
#include	<fcntl.h>
#include	<stdlib.h>

typedef struct 
{
	int integer;
	char string[24];
}RECORD;

#define NRECORDS (100)

int main()
{
	RECORD record, *mapped;
	int i, f;
	FILE *fp;

	fp = fopen("records.dat", "w+");
	for (i = 0; i < NRECORDS; i++)
	{
		memset(&record, 0, sizeof(record));/* 若不清空，内容未知 */
		record.integer = i;
		sprintf(record.string, "RECORD-%d", i);
		fwrite(&record,sizeof(record), 1, fp);
	}
	fclose(fp);

	/* 修改43条记录 */
	fp = fopen("records.dat", "r+");
	fseek(fp, 43*sizeof(record), SEEK_SET);
	fread(&record, sizeof(record), 1, fp);

	record.integer = 143;
	sprintf(record.string, "RECORD-%d", record.integer);
	fseek(fp, 43*sizeof(record), SEEK_SET);
	fwrite(&record, sizeof(record), 1, fp);
	fclose(fp);

	/* 修改43条记录 */
	f = open("records.dat", O_RDWR);
	mapped = (RECORD *)mmap(0, NRECORDS*sizeof(record), 
		PROT_READ|PROT_WRITE, MAP_SHARED, f, 0);

	mapped[43].integer = 243;
	sprintf(mapped[43].string, "RECORD-%d", mapped[43].integer);
	msync((void *)mapped,NRECORDS*sizeof(record), MS_ASYNC);
	munmap((void *)mapped,NRECORDS*sizeof(record));
	close(f);

	return 0;
}