Question

What is the clean way to do that in C?

wchar_t* ltostr(long value) {
    int size = string_size_of_long(value);
    wchar_t *wchar_copy = malloc(value * sizeof(wchar_t));
    swprintf(wchar_copy, size, L"%li", self);
    return wchar_copy;
}

The solutions I came up so far are all rather ugly, especially allocate_properly_size_whar_t uses double float base math.

Answer 1

The maximum number of digits is given by ceil(log10(LONG_MAX)). You can precompute this value for the most common ranges of long using the preprocessor:

#include <limits.h>

#if LONG_MAX < 1u << 31
#define LONG_MAX_DIGITS 10
#elif LONG_MAX < 1u << 63
#define LONG_MAX_DIGITS 19
#elif LONG_MAX < 1u << 127
#define LONG_MAX_DIGITS 39
#else
#error "unsupported LONG_MAX"
#endif

Now, you can use

wchar_t buffer[LONG_MAX_DIGITS + 2];
int len = swprintf(buffer, sizeof buffer / sizeof *buffer, L"%li", -42l);

to get a stack-allocated wide-character string. For a heap-allocated string, use wcsdup() if available or a combination of malloc() and memcpy() otherwise.