Conditional Compilation

Overview

Every translation unit in WebKit starts by including “config.h”. This file defines a set of C++ preprocessor macros used to enable or disable code based on the target operating system, platform, and whether a given feature is enabled or disabled.

For example, the following #if condition says that the code inside of it is only compiled if SERVICE_WORKER feature is enabled:

#if ENABLE(SERVICE_WORKER)
...
#endif

Similarly, the following #if condition will enable the in-between code only on macOS:

#if PLATFORM(MAC)
...
#endif

For code which should be enabled in iOS, watchOS, tvOS, and Mac Catalyst we use PLATFORM(IOS_FAMILY). For each specific variant of iOS family, we also have PLATFORM(IOS), PLATFORM(WATCHOS), PLATFORM(APPLETV), and PLATFORM(MACCATALYST).

The following #if condition will enable the in-between code only if CoreGraphics is used:

#if USE(CG)
...
#endif

Finally, if a certain piece of code should only be enabled in an operating system newer than some version, we use __IPHONE_OS_VERSION_MIN_REQUIRED or __MAC_OS_X_VERSION_MIN_REQUIRED. For example, the following #if enables the in-between code only on macOS 10.14 (macOS Mojave) or above:

#if PLATFORM(MAC) && __MAC_OS_X_VERSION_MIN_REQUIRED >= 101400
...
#endif