Hi,

>> is cygwin's __STRICT_ANSI__ and stdio.h behavior not so compatible to glibc's?
>
> Cygwin is using newlib, newlib is BSD based.  We introduced the
> compatibility checking macros from FreeBSD lately.

i roughly checked FreeBSD include/stdio.h and sys/sys/cdefs.h.
https://github.com/freebsd/freebsd/blob/master/include/stdio.h
https://github.com/freebsd/freebsd/blob/master/sys/sys/cdefs.h

it looks very different to newlib's.
FreeBSD has visibility for popen()/pclose() if __POSIX_VISIBLE >= 199209,
it looks no checking about __STRICT_ANSI__ in their cdefs.h.
only one thing i worried about is _ANSI_SOURCE in their cdefs.h,
(b/c i don't understand where _ANSI_SOURCE comes from...)
but it looks _POSIX_C_SOURCE wins anyway.
for ease to see, i'd attach simplified their cdefs.h for their
visibility handling.

anyway, IIUC, newlib's behavior in regard this point looks not
equivalent to FreeBSD's...

Peace,