Editing Inline function (section)

=== gnu89 ===
gnu89 semantics of <code>inline</code> and <code>extern inline</code> are essentially the exact opposite of those in C99,<ref>{{Cite web|url=http://blahg.josefsipek.net/?p=529|title = Josef "Jeff" Sipek » GNU inline vs. C99 inline}}</ref> with the exception that gnu89 permits redefinition of an <code>extern inline</code> function as an unqualified function, while C99 <code>inline</code> does not.<ref name="gcc-5-porting">{{Cite web|url=https://gcc.gnu.org/gcc-5/porting_to.html|title = Porting to GCC 5 - GNU Project}}</ref> Thus, gnu89 <code>extern inline</code> without redefinition is like C99 <code>inline</code>, and gnu89 <code>inline</code> is like C99 <code>extern inline</code>; in other words, in gnu89, a function defined <code>inline</code> will always and a function defined <code>extern inline</code> will never emit an externally visible function. The rationale for this is that it matches variables, for which storage will never be reserved if defined as <code>extern</code> and always if defined without. The rationale for C99, in contrast, is that it would be [[principle of least astonishment|astonishing]] if using <code>inline</code> would have a side-effect&mdash;to always emit a non-inlined version of the function&mdash;that is contrary to what its name suggests.

The remarks for C99 about the need to provide exactly one externally visible function instance for inlined functions and about the resulting problem with unreachable code apply mutatis mutandis to gnu89 as well.

gcc up to and including version 4.2 used gnu89 <code>inline</code> semantics even when <code>-std=c99</code> was explicitly specified.<ref>{{Cite web|url=https://gcc.gnu.org/ml/gcc-patches/2007-02/msg00119.html|title = Ian Lance Taylor - Clean up extern inline}}</ref> With version 5,<ref name="gcc-5-porting"/> gcc switched from gnu89 to the gnu11 dialect, effectively enabling C99 <code>inline</code> semantics by default. To use gnu89 semantics instead, they have to be enabled explicitly, either with <code>-std=gnu89</code> or, to only affect inlining, <code>-fgnu89-inline</code>, or by adding the <code>gnu_inline</code> attribute to all <code>inline</code> declarations. To ensure C99 semantics, either <code>-std=c99</code>, <code>-std=c11</code>, <code>-std=gnu99</code> or <code>-std=gnu11</code> (without <code>-fgnu89-inline</code>) can be used.<ref name="gcc-inline"/>