Implementing printf to a string by calling vsnprintf twice

I'm experimenting with writing a dynamic string library in C, and I decided to write an implementation of sprintf.

In my code, I call vsnprintf with a buffer of 0 bytes, just so I can get the number of bytes it requires. I was wondering if there is anything wrong with this code and if there a better way to do this. It feels wrong to me, but I can't seem to think of a better way to do so yet have the same effect.

string string_printf(const char *fmt, ...) {
    va_list in, copy;
    string a = NULL;
    int size;

    /* start the main input */
    va_start(in, fmt);
    /* make a copy of the input */
    va_copy(copy, in);

    /* run vsnprintf just to see how much bytes needed */
    size = vsnprintf(a, 0, fmt, copy) + 1;
    va_end(copy);

    /* allocate the amount now */
    a = string_new_size(size);
    string_to_sr(a)->len = size - 1;

    /* re-run */
    vsnprintf(a, size, fmt, in);
    va_end(in);

    /* done */
    return a;
}

Looks reasonable to me.

The efficiency of this code really depends on your expected workload. For example, if you're expecting to use this code with a lot of strings of length 50 to 60, and you don't mind wasting a bit of memory in the case that the strings are shorter, then you could probably avoid a vsnprintf in the common case by writing

string string_printf(const char *fmt, ...) {
    va_list in, copy;
    const int INITIAL_SIZE = 60;
    string a = string_new_size(INITIAL_SIZE);

    /* start the main input */
    va_start(in, fmt);
    /* make a copy of the input */
    va_copy(copy, in);

    /* run vsnprintf just to see how much bytes needed */
    int length = vsnprintf(a, INITIAL_SIZE, fmt, copy);
    va_end(copy);

    /* allocate the amount now */
    if (length >= INITIAL_SIZE) {
        /* re-run */
        a = string_new_size(length+1);
        int new_length = vsnprintf(a, length+1, fmt, in);
        assert(new_length len = length;
    va_end(in);

    /* done */
    return a;
}

Notice that I've also changed your size = ... + 1, size-1 to length = ..., length, on the principle that adding and subtracting 1 all the time is less efficient than not adding and subtracting 1 all the time; and on the principle that everybody knows what "string length" means (it's strlen), whereas "string size" is a bit ambiguous (does it include the null terminator or not?).

Somewhat off-topic: I'm leery of your use of string a; to declare a pointer. To me, a bare identifier always means "value-semantic object", as in FILE or char; and if the object is supposed to have pointer semantics, you add a star, as in FILE or char. Hiding the star inside a typedef strikes me as a bad idea. Although I admit that if you're coming from Java or Objective-C or Python or any other non-C, non-C++ language where

a = b;

traditionally means "don't copy the value, just reseat the pointer a to point to the same heap object as b", then your use of string a; a->something(); will seem perfectly natural. To me it seems confusing; I'd rather write string *a; a->something();.

StackExchange Code Review Q#156504, answer score: 2