An inline function is one for which the compiler copies the code from the function definition directly into the code of the calling function rather than creating a separate set of instructions in memory. Instead of transferring control to and from the function code segment, a modified copy of the function body may be substituted directly for the function call. In this way, the performance overhead of a function call is avoided. Using the inline specifier is only a suggestion to the compiler that an inline expansion can be performed; the compiler is free to ignore the suggestion.
Any function, with the exception of main,
can be declared or defined as inline with the inline function
specifier. Static local variables are not allowed to be defined within
the body of an inline function.
C++ functions implemented inside of
a class declaration are automatically defined inline. Regular C++
functions and member functions declared outside of a class declaration,
with the exception of main, can be declared or defined
as inline with the inline function specifier. Static
locals and string literals defined within the body of an inline function
are treated as the same object across translation units; see Linkage of inline functions for details.
inline int add(int i, int j) { return i + j; }
The
use of the inline specifier does not change the meaning
of the function. However, the inline expansion of a function may not
preserve the order of evaluation of the actual arguments.The most efficient way to code an inline function is to place the inline function definition in a header file, and then include the header in any file containing a call to the function which you would like to inline.
The inline keyword
is recognized under compilation with xlc or c99, or with the -qlanglvl=stdc99 or -qlanglvl=extc99 options
or -qkeyword=inline.
The __inline__ keyword is recognized at all language
levels; however, see Linkage of inline functions below
for the semantics of this keyword.
The inline and __inline__ keywords
are recognized at all language levels.
In
C, inline functions are treated by default as having static linkage;
that is, they are only visible within a single translation unit. Therefore, in the following example, even though function foo is
defined in exactly the same way, foo in file a.c and foo in
file b.c are treated as separate functions: two function
bodies are generated, and assigned two different addresses in memory: // a.c
#include <stdio.h>
inline int foo(){
return 3;
}
void g() {
printf("foo called from g: return value = %d, address = %p\n", foo(), &foo);
}
// b.c
#include <stdio.h>
inline int foo(){
return 3;
}
void g();
int main() {
printf("foo called from main: return value = %d, address = %p\n", foo(), &foo);
g();
}
The output from the compiled program is: foo called from main: return value = 3, address = 0x10000580 foo called from g: return value = 3, address = 0x10000500
// a.c
#include <stdio.h>
inline int foo(){
return 6;
}
void g() {
printf("foo called from g: return value = %d\n", foo());
}
// b.c
#include <stdio.h>
int foo(){
return 3;
}
void g();
int main() {
printf("foo called from main: return value = %d\n", foo());
g();
}
Similarly, if you define a function as extern
inline, or redeclare an inline function
as extern, the function simply becomes a regular,
external function and is not inlined.
If
you specify the __inline__ keyword, with the trailing
underscores, the compiler uses the GNU C semantics for inline functions.
In contrast to the C99 semantics, a function defined as __inline__ provides
an external definition only; a function defined as static
__inline__ provides an inline definition with internal linkage
(as in C99); and a function defined as extern __inline__,
when compiled with optimization enabled, allows the co-existence of
an inline and external definition of the same function. For more information
on the GNU C implementation of inline functions, see the GCC information,
available at http://gcc.gnu.org/onlinedocs/.
You must define an inline function in exactly the same way
in each translation unit in which the function is used or called.
Furthermore, if a function is defined as inline,
but never used or called within the same translation unit, it is discarded
by the compiler (unless you compile with
the -qkeepinlines option).
foo called from main: return value = 3, address = 0x10000580 foo called from g: return value = 3, address = 0x10000580Redefining an inline function with the same name but with a different function body is illegal; however, the compiler does not flag this as an error, but simply generates a function body for the version defined in the first file entered on the compilation command line, and discards the others. Therefore, the following example, in which inline function foo is defined differently in two different files, may not produce the expected results:
// a.C
#include <stdio.h>
inline int foo(){
return 6;
}
void g() {
printf("foo called from g: return value = %d, address = %p\n", foo(), &foo);
}
// b.C
#include <stdio.h>
inline int foo(){
return 3;
}
void g();
int main() {
printf("foo called from main: return value = %d, address = %p\n", foo(), &foo);
g();
}
When compiled with the command xlc++
a.C b.C ,
the output is: foo called from main: return value = 6, address = 0x10001640 foo called from g: return value = 6, address = 0x10001640The call to foo from main does not use the inline definition provided in b.C, but rather calls foo as a regular external function defined in a.C. It is your responsibility to ensure that inline function definitions with the same name match exactly across translation units, to avoid unexpected results.
// a.C
#include <stdio.h>
inline int foo(){
static int x = 23;
printf("address of x = %p\n", &x);
x++;
return x;
}
void g() {
printf("foo called from g: return value = %d\n", foo());
}
// b.C
#include <stdio.h>
inline int foo()
{
static int x=23;
printf("address of x = %p\n", &x);
x++;
return x;
}
void g();
int main() {
printf("foo called from main: return value = %d\n", foo());
g();
}
The output of this program shows that x in
both definitions of foo is indeed the same object: address of x = 0x10011d5c foo called from main: return value = 24 address of x = 0x10011d5c foo called from g: return value = 25
If
you want to ensure that each instance of function defined as inline
is treated as a separate function, you can use the static specifier
in the function definition in each translation unit, or compile with
the -qstaticinline option.
Note, however, that static inline functions are removed from name
lookup during template instantiation, and are not found. 