What s the difference between static and static inline function

Question

IMO both make the function to have a scope of the translation unit only   What s the difference between  static  and  static inline  function   Why should inline be put in a header file  not in  c file

User · Answer

In C  static means the function or variable you define can be only used in this file i e  the compile unit   So  static inline means the inline function which can be used in this file only   EDIT   The compile unit should be The Translation Unit

User · Answer

One difference that s not at the language level but the popular implementation level  certain versions of gcc will remove unreferenced static inline functions from output by default  but will keep plain static functions even if unreferenced  I m not sure which versions this applies to  but from a practical standpoint it means it may be a good idea to always use inline for static functions in headers

User · Answer

From my experience with GCC I know that static and static inline differs in a way how compiler issue warnings about unused functions  More precisely when you declare static function and it isn t used in current translation unit then compiler produce warning about unused function  but you can inhibit that warning with changing it to static inline   Thus I tend to think that static should be used in translation units and benefit from extra check compiler does to find unused functions  And static inline should be used in header files to provide functions that can be in-lined  due to absence of external linkage  without issuing warnings   Unfortunately I cannot find any evidence for that logic  Even from GCC documentation I wasn t able to conclude that inline inhibits unused function warnings  I d appreciate if someone will share links to description of that

User · Answer

inline instructs the compiler to attempt to embed the function content into the calling code instead of executing an actual call   For small functions that are called frequently that can make a big performance difference   However  this is only a  hint   and the compiler may ignore it  and most compilers will try to  inline  even when the keyword is not used  as part of the optimizations  where its possible   for example   static int Inc int i   return i 1           some code int i          some more code for  i 0  i lt 999999  i   Inc i      do something here       This tight loop will perform a function call on each iteration  and the function content is actually significantly less than the code the compiler needs to put to perform the call  inline will essentially instruct the compiler to convert the code above into an equivalent of    int i         for  i 0  i lt 999999  i   i 1       do something here        Skipping the actual function call and return  Obviously this is an example to show the point  not a real piece of code   static refers to the scope  In C it means that the function variable can only be used within the same translation unit

User · Answer

By default  an inline definition is only valid in the current translation unit   If the storage class is extern  the identifier has external linkage and the inline definition also provides the external definition   If the storage class is static  the identifier has internal linkage and the inline definition is invisible in other translation units   If the storage class is unspecified  the inline definition is only visible in the current translation unit  but the identifier still has external linkage and an external definition must be provided in a different translation unit  The compiler is free to use either the inline or the external definition if the function is called within the current translation unit   As the compiler is free to inline  and to not inline  any function whose definition is visible in the current translation unit  and  thanks to link-time optimizations  even in different translation units  though the C standard doesn t really account for that   for most practical purposes  there s no difference between static and static inline function definitions   The inline specifier  like the register storage class  is only a compiler hint  and the compiler is free to completely ignore it  Standards-compliant non-optimizing compilers only have to honor their side-effects  and optimizing compilers will do these optimizations with or without explicit hints   inline and register are not useless  though  as they instruct the compiler to throw errors when the programmer writes code that would make the optimizations impossible  An external inline definition can t reference identifiers with internal linkage  as these would be unavailable in a different translation unit  or define modifiable local variables with static storage duration  as these wouldn t share state accross translation units   and you can t take addresses of register-qualified variables   Personally  I use the convention to mark static function definitions within headers also inline  as the main reason for putting function definitions in header files is to make them inlinable   In general  I only use static inline function and static const object definitions in addition to extern declarations within headers   I ve never written an inline function with a storage class different from static

[c] What's the difference between "static" and "static inline" function?

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