The inline keyword is not part of the original ANSI C standard
(C89) so the library does not export any inline function definitions
by default. Inline functions were introduced officially in the newer
C99 standard but most C89 compilers have also included inline as
an extension for a long time.
To allow the use of inline functions, the library provides optional
inline versions of performance-critical routines by conditional
compilation in the exported header files. The inline versions of these
functions can be included by defining the macro HAVE_INLINE
when compiling an application,
$ gcc -Wall -c -DHAVE_INLINE example.c
If you use autoconf this macro can be defined automatically. If
you do not define the macro HAVE_INLINE then the slower
non-inlined versions of the functions will be used instead.
By default, the actual form of the inline keyword is extern
inline, which is a gcc extension that eliminates unnecessary
function definitions. If the form extern inline causes
problems with other compilers a stricter autoconf test can be used,
see Autoconf Macros.
When compiling with gcc in C99 mode (gcc -std=c99) the
header files automatically switch to C99-compatible inline function
declarations instead of extern inline. With other C99
compilers, define the macro GSL_C99_INLINE to use these
declarations.