Question

When using standard char* strings, the snprintf and vsnprintf functions will return the length of the output string, even if that string was truncated due to overflow.* It seems like the ISO C committee didn’t like this functionality when they added swprintf and vswprintf, which return -1 on overflow.

Does anyone know of a function that will provide this length? I don’t know the size of the potential strings. I might be asking too much, but.. I’d rather not:

• allocate a huge static temp buffer
• iteratively allocate and free memory until i’ve found a size that fits
• add an additional library dependency
• write my own format string parser

*I realize MSVC doesn’t do this, and instead provides the scprintf and vscprintf functions, but I’m looking for other compilers, mainly GCC.

Answer 1

My best suggestion to you would be not to use wchar_t strings at all, especially if you’re not writing Windows-oriented code. In case that’s not an option, here are some other ideas:

1. If your format string does not contain non-ASCII characters itself, what about first calling vsnprintf with the same set of arguments to get the length in bytes, then use that as a safe upper bound for the length in wchar_t characters (if there are few or non-ASCII characters, the bound will be tight).

2. If you’re okay with introducing a dependency on a POSIX function (which is likely to be added to C1x), use open_wmemstream and fwprintf.

3. Just iterate allocating larger buffers, but do it smart: increase the size geometrically at each step, e.g. 127, 255, 511, 1023, 2047, … I like this pattern better than whole powers of 2 because it’s easy to avoid dangerous case where allocation might succeed for SIZE_MAX/2+1 but then wrap to 0 at the next iteration.