inline function specifier

Syntax


inline function_declaration		(since C99)

Explanation

The intent of the inline specifier is to serve as a hint for the compiler to perform optimizations, such as function inlining, which require the definition of a function to be visible at the call site. The compilers can (and usually do) ignore presence or absence of the inline specifier for the purpose of optimization.

If the compiler performs function inlining, it replaces a call of that function with its body, avoiding the overhead of a function call (placing data on stack and retrieving the result), which may result in a larger executable as the code for the function has to be repeated multiple times. The result is similar to function-like macros, except that identifiers and macros used in the function refer to the definitions visible at the point of definition, not at the point of call.

Regardless of whether inlining takes place, the following semantics of inline functions are guaranteed:

Any function with internal linkage may be declared static inline with no other restrictions.

A non-static inline function cannot define a non-const function-local static and cannot refer to a file-scope static.

static int x;
inline void f(void)
{
    static int n = 1; // error: non-const static in a non-static inline function
    int k = x; // error: non-static inline function accesses a static variable
}

If a non-static function is declared inline, then it must be defined in the same translation unit. The inline definition that does not use extern is not externally visible and does not prevent other translation units from defining the same function. This makes the inline keyword an alternative to static for defining functions inside header files, which may be included in multiple translation units of the same program.

If a function is declared inline in some translation units, it does not need to be declared inline everywhere: at most one translation unit may also provide a regular, non-inline non-static function, or a function declared extern inline. This one translation unit is said to provide the external definition. One external definition must exist in the program if the name of the function with external linkage is used in an expression, see one definition rule.

If the external definition exists in the program, the address of the function is always the address of the external function, but when this address is used to make a function call, it's unspecified whether the inline definition (if present in the translation unit) or the external definition is called. The static objects defined within an inline definition are distinct from the static objects defined within the external definition:

inline const char *saddr(void) // the inline definition for use in this file
{
    static const char name[] = "saddr";
    return name;
}
int compare_name(void)
{
    return saddr() == saddr(); // unspecified behavior, one call could be external
}
extern const char *saddr(void); // an external definition is generated, too

A valid C program must not depend on whether the inline version or the external version of a function is called.

Keywords

inline

Notes

The inline keyword was adopted from C++, but in C++, if a function is declared inline, it must be declared inline in every translation unit, and also every definition of an inline function must be exactly the same (in C, the definitions may be different, as long as the behavior of the program does not depend on the differences). On the other hand, C++ allows non-const function-local statics and all function-local statics from different definitions of an inline function are the same in C++ but distinct in C.

Example

Run this code

// file test.h
#ifndef TEST_H_INCLUDED
#define TEST_H_INCLUDED
inline int sum (int a, int b)
{
    return a+b;
}
#endif
 
// file sum.c
#include "test.h"
extern inline int sum (int a, int b); // provides external definition
 
// file test1.c
#include <stdio.h>
#include "test.h"
extern int f(void);
 
int main(void)
{
    printf("%d\n", sum(1, 2) + f());
}
 
// file test2.c
#include "test.h"
 
int f(void)
{
    return sum(2, 3);
}

Output:

References

C11 standard (ISO/IEC 9899:2011):

6.7.4 Function specifiers (p: 125-127)

C99 standard (ISO/IEC 9899:1999):

6.7.4 Function specifiers (p: 112-113)

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