[Bug target/24012] [5/6/7 regression] #define _POSIX_C_SOURCE breaks #include <iostream>

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=24012

--- Comment #21 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Jonathan Wakely from comment #19)
> (In reply to Gerald Pfeifer from comment #4)
> > Historically, on some systems one had to define _POSIX_C_SOURCE to get at
> > fileno(), which has been used by code generated by flex() and probably
> > others.
> 
> Yes, but you don't have to define it to 1. If you define _POSIX_C_SOURCE to
> 199506L or set _XOPEN_SOURCE to 500 then I think the functions we use are
> available, as well as fileno.

Correction: _POSIX_C_SOURCE=200112L or _XOPEN_SOURCE=600 should declare them.