Question

I need to write data to a binary file using C s I/O functions. The following code causes a runtime exception :


#include "stdio.h"

int main(int argc,char* argv[]) {
    FILE *fp = fopen("path_to_file.bin","wb");
    if(fp == NULL) {
        printf("error creating file");
        return -1;
    }
    int val = 4;
    fwrite((const void*)val,sizeof(int),1,fp);
    fclose(fp);
    return 0;
}

The code dies at the fwrite. Can you spot what I m doing wrong? Apparently, I m, trying to access data at 0x0000004 or something like that.

谢谢！

Answer 1

 fwrite((const void*)val,sizeof(int),1,fp);

应该是：

 fwrite((const void*) & val,sizeof(int),1,fp);

顺便说一句，如果您不使用强制转换，您将得到一个合理的错误消息。强制转换往往被C（和C ++）程序员使用得比他们应该的更多 - 一个好的经验法则是“如果需要强制转换，则可能是错误的”。

Answer 2

我认为尼尔的答案可以改进。我知道它已经被接受了，所以这只是为了展示一些对比（这就是为什么我没有直接编辑他的答案）。

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

Two improvements:

No pointer casting, since it s not necessary in C and can hide errors.
Use sizeof directly on the object, since that is what you re passing a pointer to. Makes a lot of sense to me, and is safer than repeating yourself and using the type name.

Answer 3

Adding to Neil s answer: this works when you are reading and writing the file on the same platform. Things can become weird if you are reading/writing across platforms with different endianness.

Answer 4

#include "stdio.h"

int main(int argc,char* argv[]) {
    FILE *fp = fopen("path_to_file.bin","wb");
    if(fp == NULL) {
        printf("error creating file");
        return -1;
    }
    int val = 4;
    fwrite((const void*)val,sizeof(int),1,fp);

You should supply an address not integer itself.

此外，您不应该以这种方式使用整数：

It may differ in endianess on different computers (as mentioned)
It may differ in size. On really old computers it may be 1 or 2 bytes. On most modern it will be 4 but it may be 8 as well (some 64-bit computers). On some strange architectures it may be even 36 bits. int32_t val = 4; fwrite((const void *)val, 4, 1, fp) should solve the problem.

You may think that your software will never need to be ported. Well - many designers (software and hardware) made similar assumptions. Sometimes it is too costly to not make them - but in this case it is just a matter of few additional checks.

    fclose(fp);
    return 0;
}

Answer 5

我也遇到了这种问题。所以这是我的解决方案。

fwrite（val，sizeof（val [0]，sizeof（val）/sizeof（val [0]），fp）;）

Answer 6

显然，我正在尝试访问0x0000004或类似的数据。

int val = 4

这就是问题所在。fwrite 的设计是用来处理字符串的，因此它的第一个输入参数是指针，即字符串在内存中的位置。你传递的是 val 的值 (4)，而不是它的地址给 fwrite；然而，内存地址 0x00000004 并不是有效的程序内存，因此会报错。

解决此问题，请更改此项：

fwrite((const void*)val,sizeof(int),1,fp);

Into this:

fwrite((const void*)&val, sizeof(int), 1, fp);

"&" 运算符指示了 val 的内存地址。这将是一个有效的内存地址。

友情链接