示例：用二进制I/O进行随机访问

#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define ARSIZE 1000

int main(int argc, char* argv[])
{
	double numbers[ARSIZE];
	double value;
	const char * file = "numbers.dat";
	int i;
	long pos;
	FILE *iofile;

	//创建一组double类型的值
	for (i = 0; i < ARSIZE; i++)
		numbers[i] = 100.0 * i + 1.0 / (i + 1);
	//尝试打开文件
	//注意：对于UNIX和Linux文本模式和二进制模式完全相同
	if ((iofile = fopen(file, "wb")) == NULL)
	{
		fprintf(stderr, "Could not open %s for output.\n", file);
		exit(EXIT_FAILURE);
	}
	//以二进制格式把数组写入文件
	fwrite(numbers, sizeof(double), ARSIZE, iofile);
	fclose(iofile);
	if ((iofile = fopen(file, "rb")) == NULL)
	{
		fprintf(stderr, "Could not open %s for random access.\n", file);
		exit(EXIT_FAILURE);
	}
	//从文件中读取选定的内容
	printf("Enter an index in the range 0-%d.\n", ARSIZE - 1);
	while (scanf("%d", &i) == 1 && i >= 0 && i < ARSIZE)
	{
		pos = (long)i * sizeof(double); //计算偏移量
		fseek(iofile, pos, SEEK_SET); //定位到此处
		fread(&value, sizeof(double), 1, iofile);
		printf("The value there is %f.\n", value);
		printf("Next index (out of range to quit):\n");
	}
	//完成
	fclose(iofile);
	puts("Bye!");

	system("pause");
	return 0;
}

运行测试