Problem writing int to binary file in C

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.

Thanks !

Solution 1

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

should be:

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

BTW, if you don't use the cast, you will get a sensible error message. Casts tend to be used by C (and C++) programmers far more often than they should be - a good rule of thumb is "if it needs a cast, it's probably wrong".

Solution 2

I think Neil's answer can be improved upon. I realize it's already accepted, so this is just to show some contrast (which is why I didn't just edit his).

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.

Solution 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.

Comments

[dirkgently]

+1 Good points, both. Moral of the story: prefer as little casts and use sizeof judiciously.

[MarkD]

Can you please explain why the cast isn't necessary? I'm not a C programmer ... and I would like to understand as much as possible of it's "magic".

[Ferruccio]

fwrite() expects a void* as its first argument. Any pointer type can be used where a void* is expected without casting. void* effectively says "I need a pointer, I don't care what type it points to".

[quinmars]

Not only endianness, also the size may vary.

[Juan Techera]

C is soo not multi platform, in the file writing/reading you need to make a implementation for each platform most times, different end of line markers and things like that become a nightmare when dealing with file writing/reading across platforms (kindda off topic anyway :P)

[Greg]

+ another 1 for that rule. i have it on a sticky note on my monitor.