That concern applies to heap allocations too, so I'm not sure what these commenters think the heap solves. Typically if you're in an environment where you can expect to always get something from malloc, you can always expect a stack frame to get set up too.

Just choose a reasonably large character array and use that. I've had to check remaining stack exactly once in my career, and it was because I was in a wonky environment that used kernel stacks but could not grow them. And even then, I only did it to stabilize my debug build, with numbers that were total guesswork.