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Precompiled Headers and Prefix Files

Precompiled headers are binary files that represent the compiler's intermediate form of the headers required to compile a source file. Generally, all source files in a given target use a large common subset of system and project headers, so by precompiling these headers into intermediate form once and reusing it for many source files, the compiler can build source files more quickly.

CodeWarrior users must manage precompiled headers manually. In CodeWarrior, you can use a precompiled header interchangeably with a normal header file. It can be included in an #include directive or used as a prefix header in a target's Target Settings. You can create a precompiled header file using the Precompile menu item, or by including a .pch file in your project (you use a #pragma directive in that file to define the name of the precompiled header itself, which usually has a .mch suffix). In many larger projects, developers have separate targets (or even whole projects) that just build common precompiled headers that are shared among multiple projects.

In Xcode, precompiled headers are generated automatically and invisibly by the development environment. By setting the Prefix Header build setting of a target to your .pch file (or any other header file that contains #include statements for your framework headers and common target headers), Xcode will generate the precompiled header file and use it when compiling every source file in that target. (The Precompile Prefix Header build setting must also be enabled.)

The precompiled header is regenerated whenever any files it depends on are changed, so you don't need to manually build or maintain the precompiled header.

You can see the Prefix Header and Precompile Prefix Header build settings in the Build pane in Figure 1-3. For more information on using precompiled headers in Xcode, see Optimizing the Edit-Build-Debug Cycle in Xcode User Guide.

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