When Should I Rebuild My GCC Precompiled Headers?

GCC and Precompiled Headers

Precompiled headers aim to optimize C compilation by serializing the preprocessed state of common headers. However, their use in real-world scenarios raises concerns about triggering rebuilds.

When to Rebuild Precompiled Headers

GCC requires a single common header that is included first in all source files. Any subsequent #define or header #include directives will not be precompiled. Therefore, rebuilds are necessary if:

• A #define in a .cpp file alters the preprocessor's interpretation of a header included in the precompiled header.
• An additional header is included in a .cpp file that defines a preprocessor directive affecting a precompiled header.
• Header inclusions recurse, introducing multiple layers of preprocessor dependencies.

Coding Style for Precompiled Headers

To avoid these rebuild issues, GCC recommends a restrictive coding style:

• Include a single common header that incorporates all necessary system and library headers.
• Place this include statement at the beginning of every source file.
• Avoid #define statements in .cpp files as they cannot be precompiled.

However, this style may not always align with real-world coding practices.

GCC Limitations and Complexity

GCC's handling of precompiled headers requires extensive manual configuration in the Makefile. There are no readily available templates that fully address the potential pitfalls. For example, when multiple libraries are built using precompiled headers, complex shell scripts are required to detect and rebuild modified headers.

Alternative Approaches

Due to the complexities of GCC's precompiled header implementation, it may be preferable to explore alternative approaches such as:

• Single Include Headers: Define a common header with #include statements for all required headers. This header is #included in all source files.
• Pre-Parsed Headers (abandoned in GCC): This experimental feature attempted to handle header dependencies more effectively.
• Module Mechanism (future C standard): The C 20 standard may introduce a module mechanism that addresses some of these issues.

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