This can typically be avoided though. With your sprintf example you could have one function that does the first half that calculates the size and then a second function that does the second half that actually copies into the destination buffer. Alternatively if you want to use realloc you could have a function like snprintf which allows resuming from where it ran out of space.
The second idea is neat though... it could return an extra argument which is a pointer to the first part of the format string that wasn't printed
The caller can allocate memory to be shared between the 2 function calls to avoid needing duplicate work
With the approach of having the caller allocate you have carefully design the API to make it possible.
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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.