Declares aninline function.
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| inlinefunction_declaration | (since C99) | ||||||||
The intent of theinline specifier is to serve as a hint for the compiler to perform optimizations, such asfunction inlining, which usually require the definition of a function to be visible at the call site. The compilers can (and usually do) ignore presence or absence of theinline 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 tofunction-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 declaredstatic 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.
staticint x; inlinevoid f(void){staticint n=1;// error: non-const static in a non-static inline functionint k= x;// error: non-static inline function accesses a static variable}
If a non-static function is declaredinline, then it must be defined in the same translation unit. The inline definition that does not useextern is not externally visible and does not prevent other translation units from defining the same function. This makes theinline keyword an alternative tostatic for defining functions inside header files, which may be included in multiple translation units of the same program.
If a function is declaredinline in some translation units, it does not need to be declaredinline everywhere: at most one translation unit may also provide a regular, non-inline non-static function, or a function declaredextern inline. This one translation unit is said to provide theexternal definition. In order to avoid undefined behavior, one external definition must exist in the program if the name of the function with external linkage is used in an expression, seeone definition rule.
The address of an inline function with external linkage is always the address of the external definition, but when this address is used to make a function call, it's unspecified whether theinline definition (if present in the translation unit) or theexternal definition is called. The static objects defined within an inline definition are distinct from the static objects defined within the external definition:
inlineconstchar*saddr(void)// the inline definition for use in this file{staticconstchar name[]="saddr";return name;} int compare_name(void){return saddr()== saddr();// unspecified behavior, one call could be external} externconstchar*saddr(void);// an external definition is generated, too
A C program should not depend on whether the inline version or the external version of a function is called, otherwise the behavior is unspecified.
Theinline keyword was adopted from C++, but in C++, if a function is declaredinline, it must be declaredinline in every translation unit, and also every definition of an inline function must be exactly the same (in C, the definitions may be different, and depending on the differences only results in unspecified behavior). 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.
Header file "test.h"
#ifndef TEST_H_INCLUDED#define TEST_H_INCLUDED inlineint sum(int a,int b){return a+ b;} #endif
Source file "sum.c"
#include "test.h" externinlineint sum(int a,int b);// provides external definition
Source file "test1.c"
#include <stdio.h>#include "test.h" externint f(void); int main(void){printf("%d\n", sum(1,2)+ f());}
Source file "test2.c"
#include "test.h" int f(void){return sum(3,4);}
Output
10C++ documentation for inline specifier |
