faq.md

Frequently asked questions

This is a collection of some explanations discovered during making the build system.

Preprocessor macros in configuration header file

The configuration header (main/php_config.h on *nix systems and main/config.w32.h on Windows) is generated during the configuration step based on the tests for particular system. The style might be on the first glance very inconsistent due to history reasons.

For example,

some macros have two states like undefined/defined, some have undefined/defined to 1, and some have defined to 0 or 1. In modern code the preprocessor macros are in theory considered a bad practice due to making the C code unreadable. However, in practice they are unavoidable to ensure the portability of the code on a variety of systems.

The undefined/defined to 1 style is there mostly due to the Autotools historic reasons where in the early C days it was easier to write C like this:

#if HAVE_SOMETHING
    /* ... */
#endif

When HAVE_SOMETHING is not defined, compiler automatically resolves it to value 0. However, in today's C code the -Wundef warnings appear in such code if HAVE_SOMETHING is undefined, so the code should be written as:

#ifdef HAVE_SOMETHING
    /* ... */
#endif

The undefined/defined to 1 is in these configuration headers only a relic from the past. New code can use the undefined/defined states or defined to 0 or 1.