How Does GCC Handle Precompiled Headers and When Do They Need Rebuilding?

GCC and Precompiled Headers

Precompiled headers (PCHs) can be useful for speeding up compilation times in large projects. However, understanding how PCHs work in GCC can be challenging.

Triggering PCH Rebuild

In GCC, a PCH is rebuilt when:

• A header that is included in the PCH is modified.
• A preprocessing directive that affects headers in the PCH is #defined or #undefed.
• A file that includes the PCH is modified.

Coding Style Considerations and Restrictions

To ensure that PCHs work correctly, GCC requires a strict coding style. Specifically:

• Each source file must include only a single PCH as its first non-comment line.
• No #defines or #includes should appear before the PCH is included.
• The same preprocessing options (e.g., -D, -I) must be used for all source files that include the PCH.

Practical Implementation

In a project with multiple libraries, detecting when a PCH needs to be rebuilt can be tricky. Custom build scripts may be necessary to monitor header modifications and trigger PCH rebuilds accordingly.

Current Limitations

Current versions of GCC (including version 4.7) require a single, common header for PCHs to work effectively. This may not be suitable for all projects.

Future Developments

Work is ongoing to improve PCH support in GCC through the PreParsed Header (PPH) branch. However, this feature is not yet available in the mainline GCC release.

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