Generally speaking, you don't do this. You figure out the size you need and allocate it dynamically. If you don't know the size, you use a dynamically growing

Generally speaking, you don't do this. You figure out the size you need and allocate it dynamically. If you don't know the size, you use a dynamically growing array type (that does the same thing as C++'s std::vector). E.g. use asprintf (or implement your own using vsnprintf) as necessary. What you don't do is emulate the behavior of janky C programs that allocate a fixed size array. That's bad. (Sometimes you have arrays where the size doesn't affect the correctness of your program. An example is if you're processing a file in chunks (e.g. to compute ROT-13), then chunk size will affect performance, but not correctness. You pick the size appropriate for your use case.)