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By declaring a function, inline, you can direct GNU CC to integrate that function’s code into the code for its callers. This makes execution faster by eliminating the function-call overhead; in addition, if any of the actual argument values are constant, their known values may permit simplifications at compile time so that not all of the inline function’s code needs to be included.
The effect on code size is less predictable; object code may be larger or smaller with function inlining, depending on the particular case. Inlining of functions is an optimization and it really “works” only in optimizing compilation. If you don’t use ‘-O’, no function is really inline.
To declare a function inline, use the inline keyword in its declaration, like the following example shows.
inline int inc (int *a) { (*a)++; }(If you are writing a header file to be included in ANSI C programs, write __inline__ instead of inline. See Alternate keywords.)
You can also make all “simple enough” functions inline with the option, ‘-finline-functions’. Note that certain usage in a function definition can make it unsuitable for inline substitution.
When an inline
function is not static,
then the compiler must assume that there may be calls from other source
files; since a global symbol can be defined only once in any program, the
function must not be defined in the other source files, so the calls therein
cannot be integrated. Therefore, a non-static
inline function
is always compiled on its own in the usual fashion.
If you specify both inline
and extern
in the function definition, then the definition is used only for inlining.
In no case is the function compiled on its own, not even if you refer to
its address explicitly. Such an address becomes an external reference,
as if you had only declared the function, and had not defined it.
This combination of inline
and extern
has almost the effect of a macro. The way to use it is to put a function
definition in a header file with these keywords, and put another copy of
the definition (lacking inline
and extern)
in a library file. The definition in the header file will cause most calls
to the function to be inlined. If any uses of the function remain, they
will refer to the single copy in the library.
GNU C does not inline
any functions when not optimizing. It is not clear whether it is better
to inline
or not, in this case, but we found that a correct implementation when not
optimizing was difficult. So we did the easy thing, and turned it off.
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