Hi folks.

The Platform.h configuration file family has been useful for WebKit for a long 
time. We created it to try to organize configuration options in WebKit so the 
would not be spread out through the whole project.

One way to look at these, particularly the ENABLE options, is as a set of 
configuration options that let each consumer of the WebKit source code create a 
unique flavor of WebKit with the particular features they want enabled turned 
on and others turned off. Another is to think of this as a mechanism for 
keeping decisions made by the WebKit contributors organized and easy to 

If these truly are WebKit consumer options, then it’s important they be set as 
part of the build process. The macros can be defined with a build and 
configuration system outside WebKit, and the Platform.h headers should 
interpret those values correctly.

On the other hand, if we are just trying to keep our decisions straight, then 
it would be best if the logic for what is on and what is off by in the header 
files, written with preprocessor macro logic, and not spread across multiple 
make files and scripts.

Thus I think the pattern on macOS, for example, of setting these in .xcconfig 
files doesn’t make a lot of sense. I think the .xcconfig files should compute 
the things that need to be determined about the build environment, but the 
configuration decisions should be in files like PlatformHaveCocoa.h, for 

I think the guideline should be like this:

All code to compute configuration should be in the Platform.h files, not in 
makefiles, with only the following exceptions:

1) Options like ENABLE(XXX) that some ports wish to offer as options to people 
building WebKit can have overridden values in makefiles. But even these options 
should have sensible defaults in the Platform.h headers that express the 
current status of the port from the point of view of the port’s maintainers. 
Ideally we’d find a way to not repeat these default settings twice.

2) In some cases, the build machinery needs to contribute to things like 
feature detection. So on some platforms, things like HAVE(READLINE) would be 
set correctly by the build system.

Options intended to be set by the environment would carefully be wrapped in 

But other options, which simply express relationships between configuration 
elements, are designed to be set by Platform.h and not overridden by the 
environment, and so they would not be wrapped in #ifndef. Many HAVE options and 
most USE options would fall into this category.

What do you all think?

— Darin
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