Question

I was interviewed recently and asked to write mystrcat(*s1, *s2, *s3) where s1 and s2 are source string and the concatenated results are given by s3. I was told, don’t worry about memory allocation of s3 and assume s1 and s2 are not null / invalid strings. So I wrote the following lame (crude) program. I was told there is something wrong with s3 or something can go wrong with s3. Could you please tell what it is/ could that be?

void mystrcat(char *s1, char *s2, char *s3)
{
    if (! (s1 || s2 || s3)) return; // one/more pointers are invalid

    // copy string s1 into s3
    while(*s1) {
      *s3 = *s1;
      s1++;
      s3++;
    }

    // concatenate string s2 into s3
    while(*s2) {
      *s3 = *s2;
      s2++;
      s3++;
    }

    *s3 = '\0';
}

Could you please tell me what is wrong here? What would be more professional way of doing it?

Answer 1

Here would be my comments

• Both s1 and s2 should be typed to const char* since you have no intention of modifying them.
• You did not verify s3 had enough allocated space to contain s1 and s2’s combined length
• You are failing silently when the user passed in bad data (NULL values). This is not good because there is no way for a caller to differentiate between a successful and a failing call
• Your method of verifying s1,s2 and s3 are not null is incorrect (Igor has it right)

Questions I would have liked you to ask or self answered during the interview

• How should I express failure to copy the string?
• Do you want an exact clone of strcat or one that is more modern and does better argument validation?
• Can I use other standard str functions to complete the answer?