The GNU C spec for inline functions, before GCC version 5, defined
extern inline on a function definition to mean to inline calls
to it but not generate code for the function that could be
called at run time. By contrast, inline without extern
specified to generate run-time code for the function. In effect, ISO
incompatibly flipped the meanings of these two cases. We changed GCC
in version 5 to adopt the ISO specification.
Many programs still use these cases with the previous GNU C meanings.
You can specify use of those meanings with the option
-fgnu89-inline. You can also specify this for a single
function with __attribute__ ((gnu_inline)). Here’s an example:
inline __attribute__ ((gnu_inline))
int
inc (int *a)
{
(*a)++;
}