Problems with evolving feature test macros?

[Paul Eggert]

Christoph Hellwig wrote:
> if I want to write a portable application having libc help me using only APIs provided in a well defined standard is useful

In theory that argument makes sense, but in practice almost nobody 
writes strictly conforming applications because it's too much trouble 
and these applications would have to do without too many useful features 
and speedups.  Instead, portable applications tend to take an 
Autoconf-style approach, and accommodate themselves to whatever the 
platform has to offer.

For more about why strictly-conforming apps are rare, please see:

Walli SR. The myth of application source-code conformance. StandardView 
1996;4(2);94–9. http://dx.doi.org/10.1145/234999.235002