Memory leak? Strange characters writing to file

Question

Good Morning. This kinda ties in to a question I posted a few days ago, this is sort of a smaller example that all ties in. When I write the contents of array b to a text file it prints a bunch of strange characters. I'm new to C, and I'm also a student so I'm trying to learn a brand new language under a time constraint.

int main(int argc, char *argv[])
{

    char a[3][10] = {"Apple", "Giraffe", "Boat"};
    char b[4][10];
    
    
    
    
    for(int i = 0; i < sizeof(a)/sizeof(a[0]); i++)
    {
        char tmp[15];
        sprintf(tmp, a[i]);
        
        
        sprintf(b[i], tmp);
        
    }
    
    

    FILE *f = fopen(argv[1], "wb");
    fwrite(b, 15, sizeof(b), f);
    fclose(f);

}

And this is a piece of the output that was written to the text file

Apple     Giraffe 2€Boat q€           ¶¤6€   Apple     Giraffe   Boat        H  

Was it that I'm doing wrong that's causing this output? Should I be using memset? Malloc? Null terminators? Please help, thank you.

Answer 1

fwrite's signature is:

size_t fwrite(const void *restrict ptr, size_t size, size_t nmemb,
              FILE *restrict stream);

• size is the size of each item to write
• nmemb is how many items you'd like to write

So, your call

fwrite(b, 15, sizeof(b), f);

will write sizeof(b) (4*10 => 40) items of size 15. That's 600 bytes and b is only 40 bytes. Since that's out of bounds for b, your program will have undefined behavior.

---

Also note that you haven't initialized b so it can contain just about anything. Either zero initialize it:

char b[4][10] = {0};

or set every character to a character you like:

char b[4][10];
memset(b, '*', sizeof b);    // #include <string.h>

then write sizeof b items of size 1:

fwrite(b, 1, sizeof b, f);

or write 1 item of size sizeof b:

fwrite(b, sizeof b, 1, f);