Extern Inline vs. Inline and Static Inline: When Should You Use Each?

Understanding the Role of extern inline

Inline functions are a valuable tool for optimizing code performance by allowing the compiler to directly insert the code of a function into the calling code, eliminating function call overhead. However, inline functions still produce linkable object code, potentially allowing other modules to reference them.

extern inline vs. inline and static inline

In pre-C99 C, the inline keyword was implemented as an extension by various compilers. GNU's implementation introduced three constructs: inline, static inline, and extern inline. While inline functions might be inlined at the compiler's discretion, they always produced linkable object code.

In contrast, extern inline functions did not produce linkable object code when inlined. This means that they cannot be referenced by other modules, ensuring that they are inlined consistently throughout the codebase.

Static inline functions, on the other hand, did not produce externally visible object code. This makes them suitable for internal use within a single module without the need for coordination across multiple modules.

Ensuring Function Inlining

In your scenario, where you want to ensure that a function is inlined and require the use of FILE and LINE macros, extern inline would not suffice. It would still allow the compiler to choose to call an out-of-line version if available.

To ensure inlining, you can use a combination of the following techniques:

• Static inline: This limits the scope of the function to a single compilation unit.
• Link-time optimization (LTO): LTO performs whole-program optimization after all compilation units have been merged. This can often force the inlining of functions that might otherwise not be optimized.

Differences Across Compilers and Versions

The behavior of extern inline varies across different compiler vendors and versions. Some compilers may not support it, while others may follow different conventions. It is best to refer to the documentation of the specific compiler you are using.

In C , inline functions must be fully defined in every translation unit where they are used. Therefore, extern inline and static inline do not have the same significance as in C.

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