That is a good point. I don't know if write has a limit on 'count' (I can't find anything saying that it does), but if it's simply "whatever you can fit in size_t" then you certainly want to be careful about putting that on your stack. Failed alloca's don't really give you any indication, though you might sigsegv later.

What would you suggest instead then? I'm not C programmer.

Avoid the allocation entirely. Use writev for example (or just use multiple 'write's, if you don't care about it being atomic).

Thx. Improved version in master: https://github.com/sickill/stderred/blob/master/stderred.c