fopen fwrite fread 问题小记

测试代码（测试环境ubuntu + gcc）：

#include <stdio.h>
#include <string.h>

struct que_st1 {
	int a;
	int b;
	int c;
	char buf[100];
}str1;

struct que_st2 {
	int a;
	int b;
	int c;
	char buf[1000];
}str2;

// 测试1，无存储文件时，直接读取，运行输出
//fopen: No such file or directory
//len=0
//str2=0 0 0, 0
//Segmentation fault (core dumped)

int main()
{
	FILE *fp = NULL;
	size_t len;
	
	str1.a = 100;
	str1.b = 200;
	str1.c = 300;
	str1.buf[99] = 99;
	
//	fp = fopen("/home/wy/ctyq/ctyq_mini/save.ty", "w+");
//	len = fwrite(&str1, sizeof(str1), 1, fp);
//	printf("len=%d
", len);
//	fclose(fp);
	
	fp = fopen("/home/wy/ctyq/ctyq_mini/save.ty", "r+");
	if (fp == NULL)
	{
		perror("fopen");
	}
	printf("len=%d
", len);
	printf("str2=%d %d %d, %d
", str2.a, str2.b, str2.c, str2.buf[99]);
	len = fread(&str2, sizeof(str2), 1, fp);
	printf("len=%d
", len);
	printf("str2=%d %d %d, %d
", str2.a, str2.b, str2.c, str2.buf[99]);
	fclose(fp);
	
	system("rm -rf save.ty");
}

// 测试2，无存储文件时，直接读取，运行输出
//len=1
//len=0
//str2=100 200 300, 99

int main()
{
	FILE *fp = NULL;
	size_t len;
	
	str1.a = 100;
	str1.b = 200;
	str1.c = 300;
	str1.buf[99] = 99;
	
	fp = fopen("/home/wy/ctyq/ctyq_mini/save.ty", "w+");
	len = fwrite(&str1, sizeof(str1), 1, fp);
	printf("len=%d
", len);
	fclose(fp);
	
	fp = fopen("/home/wy/ctyq/ctyq_mini/save.ty", "r+");
	if (fp == NULL)
	{
		perror("fopen");
	}
	len = fread(&str2, sizeof(str2), 1, fp);
	printf("len=%d
", len);
	printf("str2=%d %d %d, %d
", str2.a, str2.b, str2.c, str2.buf[99]);
	fclose(fp);
	
	system("rm -rf save.ty");
}

// 测试3，先写文件，再用另一种格式读取，运行输出
//lenw=112
//lenr=112
//str2=100 200 300, 99

int main()
{
	FILE *fp = NULL;
	size_t len;
	
	str1.a = 100;
	str1.b = 200;
	str1.c = 300;
	str1.buf[99] = 99;
	
	fp = fopen("/home/wy/ctyq/ctyq_mini/save.ty", "w+");
	len = fwrite(&str1, 1, sizeof(str1), fp);
	printf("lenw=%d
", len);
	fclose(fp);
	
	fp = fopen("/home/wy/ctyq/ctyq_mini/save.ty", "r+");
	len = fread(&str2, 1, sizeof(str2), fp);
	printf("lenr=%d
", len);
	printf("str2=%d %d %d, %d
", str2.a, str2.b, str2.c, str2.buf[99]);
	fclose(fp);
}

根据以上代码及运行结果，需要注意事项：

1、fopen操作的返回值一定要判断，以免因为无文件或不能创建文件导致 fwrite 或 fread操作发生段错误。

2、fread读取单元超过文件大小时，返回值为0，这种情况下要注意，实际上接收单元str2，已经被赋值，要小心使用以免因此异常。

3、补充说明2，注意下面这两种写法，文件实际体积小于读取体积时，写法2会返回0，但实际上str2已经赋值一部分。

fread(&str2, 1, sizeof(str2), fp);

fread(&str2, sizeof(str2), 1, fp);