Here we document details of how the preprocessor's implementation affects its user-visible behavior. You should try to avoid undue reliance on behavior described here, as it is possible that it will change subtly in future implementations.
Also documented here are obsolete features and changes from previous versions of CPP.
© Free Software Foundation
Licensed under the GNU Free Documentation License, Version 1.3.
https://gcc.gnu.org/onlinedocs/gcc-6.3.0/cpp/Implementation-Details.html