What happens to memory after '\0' in a C string?

Question

Surprisingly simple/stupid/basic question, but I have no idea: Suppose I want to return the user of my function a C-string, whose length I do not know at the beginning of th

Answer 1

You can certainly preallocate to an upperbound, and use all or something less. Just make sure you actually use all or something less.

Making two passes is also fine.

You asked the right questions about the tradeoffs.

How do you decide?

Use two passes, initially, because:

1. you'll know you aren't wasting memory.
2. you're going to profile to find out where
   you need to optimize for speed anyway.
3. upperbounds are hard to get right before
   you've written and tested and modified and
   used and updated the code in response to new
   requirements for a while.
4. simplest thing that could possibly work.

You might tighten up the code a little, too. Shorter is usually better. And the more the code takes advantage of known truths, the more comfortable I am that it does what it says.

char* copyWithoutDuplicateChains(const char* str)
    {
    if (str == NULL) return NULL;

    const char* s = str;
    char prev = *s;               // [prev][s+1]...
    unsigned int outlen = 1;      // first character counted

    // Determine length necessary by mimicking processing

    while (*s)
        { while (*++s == prev);  // skip duplicates
          ++outlen;              // new character encountered
          prev = *s;             // restart chain
        }

    // Construct output

    char* outstr = (char*)malloc(outlen);
    s = str;
    *outstr++ = *s;               // first character copied
    while (*s)
        { while (*++s == prev);   // skip duplicates
          *outstr++ = *s;         // copy new character
        }

    // done

    return outstr;
    }