What's the Difference Between `inline`, `extern inline`, and `static inline` in C and C ?

What does extern inline do?

Unlike inline and static inline, extern inline is used in a specific context:

In pre-C99 compilers, particularly GCC, inline did not have defined semantics. However, GCC implemented inline as a hint to the compiler to inline the function if possible. Despite this suggestion, an out-of-line version of the function was always emitted and remained externally visible.

To address this limitation, GCC introduced extern inline and static inline.

GNU89:

• inline: May be inlined, but an out-of-line version is always emitted and externally visible.
• extern inline: Does not generate an out-of-line version, but may call one from another compilation unit.
• static inline: Does not generate an externally visible out-of-line version, but may generate a file-static one.

C99 (or GNU99):

• inline: Similar to GNU89 extern inline, no externally visible function is emitted.
• extern inline: Similar to GNU89 inline, an externally visible code is emitted, allowing only one translation unit to use it.
• static inline: Similar to GNU89 static inline, no external symbol or call to one is emitted.

C :

• Inline functions must be defined identically in all compilation units.
• There is no standard definition for extern inline or static inline, but some compilers provide these constructs based on the GNU89 model.

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