A more realistic version of the former is that the call is opportunistically given a buffer and returns an error with the required buffer size if the buffer is too small. If the buffer is big enough you only need a single call, otherwise you need two calls with a redundant processing, which is better then the original version---this even works when the caller can't preallocate a buffer. This approach is pretty common in the Windows API.

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ADDED: I thought charcircuit was talking about always calling two functions in a row, missing the original comment that charcircuit was replying to. My bad, so I'd like to update this comment as follows...

I think the former approach would indeed work if the first call is always given a pointer to the space that can be used to record what has been done so far. Like, `snprintf(ptr, n, &recover, "format", ...)`, and `recover` can be relatively small, like 16 bytes. I would record original `ptr` and `format` arguments to be safe, and largest offsets to `ptr` and `format` that are known to be written and synchronized to each other. Of course this is just a workaround for C's inability to construct and return a sum type.