C-style String Homework

Question

My C++ class is going over C-style strings and working with pointers. I'm to write a function that has three parameters: a char * s1, a const char * s2, and a size_t max, which represents that maximum size of the s1 buffer. I am to append the characters in s2 to the end of s1. The directions advise me to make sure there is only one '\0' at the end of the combined characters and I am not to go beyond the end of the buffer I'm asked to copy to. The function will return a pointer to the first character in s1.

I cannot use any functions in the standard library. What I can use are pointers, pointer arithmetic or array notation.

I've started, but not sure where to go.

const char * myFunction (char * s1, const char * s2, size_t max)
{
    char * begin = s1;
    while (*s1) s1++;
    while ((s1 < begin + max - 1) && (*s2 != '\0')) {
      *s1++ = *s2++;     
    }

    return s1;
}

Not sure what to do after reaching the end of s1. How would I put s2 to the end of s1?

Answer 1

I cannot use any functions in the standard library.

The only required one would be strlen(), I think. Roll your own:

#define min(a,b) ((a) < (b)) ? (a) : (b)

size_t str_length(const char* s)
{
    size_t len = 0;
    for( ; *s; ++s, ++len);
    
    return len;
}

const char * append (char * s1, const char * s2, size_t max)
{
    if(!s1 || !s2) //If either of strings is NULL, nothing to be done
        return s1;
        
    size_t s1_length = str_length(s1);
    
    if(s1_length < max - 1) //If there is a room in s1...
    {
        size_t s2_length = str_length(s2); //Get length of s2
        size_t append_size = min(s2_length, max - s1_length - 1); //Make sure we won't copy more than buffer can hold
        
        if(append_size > 0)
        {
            memcpy(s1 + s1_length, s2, append_size); //This will overwrite null terminator in 's1'
            s1[s1_length + append_size] = 0;
        }
    }

    return s1;
}

Test:

int main(void)
{
    char dest[64] = "This is a test";
    
    append(dest, " of strings appending", 64);
    printf("%s (length: %d)\n", dest, str_length(dest));
    
    append(dest, " of appending too long string, that will probably not fit inside destination buffer", 64);
    printf("%s (length: %d)\n", dest, str_length(dest));
    
    return 0;
}

Output:

This is a test of strings appending (length: 35)

This is a test of strings appending of appending too long strin (length: 63)

Here is working sample.