[mh7]

No it is not reserved, it's just posix's personal style guides. There's just as much of a name clash possibility when not using _t because lots of other libraries and platforms uses some other convention.

Only ISO C can reserve things, no one else, and ISO C does not reserve the _t suffix.

[heywhatupboys]

I think it is pedantic to say that POSIX's guidance is not the de-facto C standard, or at least that the union of rules between ISO and POSIX