What is a static function in C

Question

The question was about plain c functions  not c   static methods  as clarified in comments   I understand what a static variable is  but what is a static function   And why is it that if I declare a function  let s say void print matrix  in let s say a c  WITHOUT a h  and include  a c  -  I get  print matrix        already defined in a obj   BUT if I declare it as static void print matrix then it compiles   UPDATE Just to clear things up - I know that including  c is bad  as many of you pointed out  I just do it to temporarily clear space in main c until I have a better idea of how to group all those functions into proper  h and  c files  Just a temporary  quick solution

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static functions are functions that are only visible to other functions in the same file  more precisely the same translation unit    EDIT  For those who thought  that the author of the questions meant a  class method   As the question is tagged C he means a plain old C function  For  C   Java      class methods  static means that this method can be called on the class itself  no instance of that class necessary

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First  It s generally a bad idea to include a  cpp file in another file - it leads to problems like this  -  The normal way is to create separate compilation units  and add a header file for the included file   Secondly   C   has some confusing terminology here - I didn t know about it until pointed out in comments   a  static functions - inherited from C  and what you are talking about here  Outside any class  A static function means that it isn t visible outside the current compilation unit - so in your case a obj has a copy and your other code has an independent copy   Bloating the final executable with multiple copies of the code    b  static member function - what Object Orientation terms a static method  Lives inside a class  You call this with the class rather than through an object instance    These two different static function definitions are completely different  Be careful - here be dragons

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Minor nit  static functions are visible to a translation unit  which for most practical cases is the file the function is defined in  The error you are getting is commonly referred to as violation of the One Definition Rule    The standard probably says something like       Every program shall contain exactly one definition of every noninline   function or object that is used in that program  no diagnostic   required      That is the C way of looking at static functions  This is deprecated in C   however   In C    additionally  you can declare member functions static  These are mostly metafunctions i e  they do not describe modify a particular object s behavior state but act on the whole class itself  Also  this means that you do not need to create an object to call a static member function  Further  this also means  you only get access to static member variables from within such a function   I d add to Parrot s example the Singleton pattern which is based on this sort of a static member function to get use a single object throughout the lifetime of a program

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There is a big difference between static functions in C and static member functions in C     In C  a static function is not visible outside of its translation unit  which is the object file it is compiled into   In other words  making a function static limits its scope   You can think of a static function as being  private  to its   c file  although that is not strictly correct    In C     static  can also apply to member functions and data members of classes   A static data member is also called a  class variable   while a non-static data member is an  instance variable   This is Smalltalk terminology   This means that there is only one copy of a static data member shared by all objects of a class  while each object has its own copy of a non-static data member   So a static data member is essentially a global variable  that is a member of a class   Non-static member functions can access all data members of the class  static and non-static   Static member functions can only operate on the static data members   One way to think about this is that in C   static data members and static member functions do not belong to any object  but to the entire class

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quot What is a    static    function in C  quot   Let s start at the beginning  It  s all based upon a thing called  quot linkage quot     quot An identifier declared in different scopes or in the same scope more than once can be made to refer to the same object or function by a process called linkage  29 There are three kinds of linkage  external  internal  and none  quot    Source  C18  6 2 2 1     quot In the set of translation units and libraries that constitutes an entire program  each declaration of a particular identifier with external linkage denotes the same object or function  Within one translation unit  each declaration of an identifier with internal linkage denotes the same object or function  Each declaration of an identifier with no linkage denotes a unique entity  quot    Source  C18  6 2 2 2   If a function is defined without a storage-class specifier  the function has external linkage by default    quot If the declaration of an identifier for a function has no storage-class specifier  its linkage is determined exactly as if it were declared with the storage-class specifier extern  quot    Source  C18  6 2 2 5  That means that - if your program is contained of several translation units source files   c or  cpp  - the function is visible in all translation units source files your program has  This can be a problem in some cases  What if you want to use f e  two different function  definitions   but with the same function name in two different contexts  actually the file-context   In C and C    the static storage-class qualifier applied to a function at file scope  not a static member function of a class in C   or a function within another block  now comes to help and signifies that the respective function is only visible inside of the translation unit source file it was defined in and not in the other TLUs files    quot If the declaration of a file scope identifier for an object or a function contains the storage-class specifier static  the identifier has internal linkage  30  quot      A function declaration can contain the storage-class specifier static only if it is at file scope  see 6 7 1     Source  C18  6 2 2 3   Thus  A static function only makes sense  iff   Your program is contained of several translation units source files   c or  cpp    and  You want to limit the scope of a function to the file  in which the specific function is defined   If not both of these requirements match  you don t need to wrap your head around about qualifying a function as static   Side Notes   As already mentioned  A static function has absolutely no difference at all between C and C    as this is a feature C   inherited from C   It does not matter that in the C   community  there is a heartbreaking debate about the depreciation of qualifying functions as static in comparison to the use of unnamed namespaces instead  first initialized by a misplaced paragraph in the C  03 standard  declaring the use of static functions as deprecated which soon was revised by the committee itself and removed in C  11  This was subject to various SO questions  Unnamed anonymous namespaces vs  static functions Superiority of unnamed namespace over static  Why an unnamed namespace is a  quot superior quot  alternative to static  Deprecation of the static keyword    no more  In fact  it is not deprecated per C   standard yet  Thus  the use of static functions is still legit  Even if unnamed namespaces have advantages  the discussion about using or not using static functions in C   is subject to one  s one mind  opinion-based  and with that not suitable for this website

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The answer to static function depends on the language   1  In languages without OOPS like C  it means that the function is accessible only within the file where its defined   2 In languages with OOPS like C     it means that the function can be called directly on the class without creating an instance of it

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The following is about plain C functions - in a C   class the modifier  static  has another meaning   If you have just one file  this modifier makes absolutely no difference  The difference comes in bigger projects with multiple files   In C  every  module   a combination of sample c and sample h  is compiled independently and afterwards every of those compiled object files  sample o  are linked together to an executable file by the linker    Let s say you have several files that you include in your main file and two of them have a function that is only used internally for convenience called add int a  b  - the compiler would easily create object files for those two modules  but the linker will throw an error  because it finds two functions with the same name and it does not know which one it should use  even if there s nothing to link  because they aren t used somewhere else but in it s own file    This is why you make this function  which is only used internal  a static function  In this case the compiler does not create the typical  you can link this thing -flag for the linker  so that the linker does not see this function and will not generate an error

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A static function is one that can be called on the class itself  as opposed to an instance of the class   For example a non-static would be   Person  tom   new Person    tom- gt setName  Tom      This method works on an instance of the class  not the class itself   However you can have a static method that can work without having an instance   This is sometimes used in the Factory pattern   Person  tom   Person  createNewPerson

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Minimal runnable multi-file scope example  Here I illustrate how static affects the scope of function definitions across multiple files   a c   include  lt stdio h gt      Undefined behavior  already defined in main     Binutils 2 24 gives an error and refuses to link     https   stackoverflow com questions 27667277 why-does-borland-compile-with-multiple-definitions-of-same-object-in-different-c       void f     puts  a f            OK  only declared  not defined  Will use the one in main     void f void       OK  only visible to this file     static void sf     puts  a sf       void a         f        sf        main c   include  lt stdio h gt   void a void            void f     puts  main f       static void sf     puts  main sf       void m         f        sf       int main         m        a        return 0      GitHub upstream   Compile and run   gcc -c a c -o a o gcc -c main c -o main o gcc -o main main o a o   main   Output   main f main sf main f a sf   Interpretation   there are two separate functions sf  one for each file there is a single shared function f   As usual  the smaller the scope  the better  so always declare functions static if you can   In C programming  files are often used to represent  classes   and static functions represent  private  methods of the class   A common C pattern is to pass a this struct around as the first  method  argument  which is basically what C   does under the hood   What standards say about it  C99 N1256 draft 6 7 1  Storage-class specifiers  says that static is a  storage-class specifier    6 2 2 3  Linkages of identifiers  says static implies internal linkage      If the declaration of a file scope identifier for an object or a function contains the storage-class specifier static  the identifier has internal linkage    and 6 2 2 2 says that internal linkage behaves like in our example      In the set of translation units and libraries that constitutes an entire program  each declaration of a particular identifier with external linkage denotes the same object or function  Within one translation unit  each declaration of an identifier with internal linkage denotes the same object or function     where  translation unit  is a source file after preprocessing   How GCC implements it for ELF  Linux    With the STB LOCAL binding   If we compile   int f     return 0    static int sf     return 0      and disassemble the symbol table with   readelf -s main o   the output contains   Num     Value          Size Type    Bind   Vis      Ndx Name   5  000000000000000b    11 FUNC    LOCAL  DEFAULT    1 sf   9  0000000000000000    11 FUNC    GLOBAL DEFAULT    1 f   so the binding is the only significant difference between them  Value is just their offset into the  bss section  so we expect it to differ   STB LOCAL is documented on the ELF spec at http   www sco com developers gabi 2003-12-17 ch4 symtab html      STB LOCAL Local symbols are not visible outside the object file containing their definition  Local symbols of the same name may exist in multiple files without interfering with each other   which makes it a perfect choice to represent static   Functions without static are STB GLOBAL  and the spec says      When the link editor combines several relocatable object files  it does not allow multiple definitions of STB GLOBAL symbols with the same name     which is coherent with the link errors on multiple non static definitions   If we crank up the optimization with -O3  the sf symbol is removed entirely from the symbol table  it cannot be used from outside anyways  TODO why keep static functions on the symbol table at all when there is no optimization  Can they be used for anything   See also   Same for variables  https   stackoverflow com a 14339047 895245 extern is the opposite of static  and functions are already extern by default  How do I use extern to share variables between source files    C   anonymous namespaces  In C    you might want to use anonymous namespaces instead of static  which achieves a similar effect  but further hides type definitions  Unnamed anonymous namespaces vs  static functions

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There are two uses for the keyword static when it comes to functions in C     The first is to mark the function as having internal linkage so it cannot be referenced in other translation units  This usage is deprecated in C    Unnamed namespaces are preferred for this usage      inside some  cpp file   static void foo          old  C  way of having internal linkage     C   way  namespace      void this function has internal linkage                              The second usage is in the context of a class  If a class has a static member function  that means the function is a member of the class  and has the usual access to other members   but it doesn t need to be invoked through a particular object  In other words  inside that function  there is no  this  pointer

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static function definitions will mark this symbol as internal  So it will not be visible for linking from outside  but only to functions in the same compilation unit  usually the same file

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Since static function is only visible in this file  Actually  compiler can do some optimization for you if you declare  static  to some function    Here is a simple example   main c   include  lt stdio h gt   static void test          ghost       This is an unexist function     int main         int ret   0    ifdef TEST  else     test     endif     return  ret        And compile with   gcc -o main main c   You will see it failed  Because you even not implement ghost   function   But what if we use following command   gcc -DTEST -O2 -o main main c   It success  and this program can be execute normally   Why  There are 3 key points    -O2   Compiler optimization level at least 2  -DTEST   Define TEST  so test   will not be called  Defined  static  to test      Only if these 3 conditions are all true  you can pass compilation  Because of this  static  declaration  compiler can confirm that test   will NEVER be called in other file  Your compiler can remove test   when compiling  Since we don t need test    it does not matter whether ghost   is defined or implemented

[c] What is a "static" function in C?

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