The inline function specifier

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.

C only 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 only

C++ only 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. C++ only

The following code fragment shows an inline function definition:

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.

Note:

To enable the inline function specifier in C, you must compile with c99 or the LANGLVL(STDC99) or LANGLVL(EXTC99) options. C only

Linkage of inline functions

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

Since inline functions are treated as having internal linkage, an inline function definition can co-exist with a regular, external definition of a function with the same name in another translation unit. However, when you call the function from the file containing the inline definition, the compiler may choose either the inline version defined in the same file or the external version defined in another file for the call; your program should not rely on the inline version being called. In the following example, the call to foo from function g could return either 6 or 3:

// 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.