static function in C

Question

What is the point of making a function static in C

User · Answer

The static keyword in C is used in a compiled file   c as opposed to  h  so that the function exists only in that file   Normally  when you create a function  the compiler generates cruft the linker can use to  well  link a function call to that function  If you use the static keyword  other functions within the same file can call this function  because it can be done without resorting to the linker   while the linker has no information letting other files access the function

User · Answer

Making a function static hides it from other translation units  which helps provide encapsulation   helper file c  int f1 int             prototype    static int f2 int      prototype     int f1 int foo        return f2 foo      ok  f2 is in the same translation unit                            basically same  c file  as f1               int f2 int foo        return 42   foo      main c   int f1 int      prototype    int f2 int      prototype     int main void        f1 10      ok  f1 is visible to the linker        f2 12      nope  f2 is not visible to the linker        return 0

User · Answer

Looking at the posts above I would like to give a more clarified answer  Suppose our main c file looks like this   include  quot header h quot   int main void        FunctionInHeader       Now consider three cases   Case 1  Our header h file looks like this    include  lt stdio h gt    static void FunctionInHeader      void FunctionInHeader          printf  quot Calling function inside header n quot        Then the following command on linux   gcc main c -o main  will succeed  That s because after the main c file includes the header h  the static function definition will be in the same main c file  more precisely  in the same translation unit  to where it s called  If one runs   main  the output will be Calling function inside header  which is what that static function should print   Case 2  Our header header h looks like this    static void FunctionInHeader          and we also have one more file header c  which looks like this     include  lt stdio h gt     include  quot header h quot     void FunctionInHeader           printf  quot Calling function inside header n quot         Then the following command   gcc main c header c -o main  will give an error  In this case main c includes only the declaration of the static function  but the definition is left in another translation unit and the static keyword prevents the code defining a function to be linked  Case 3  Similar to case 2  except that now our header header h file is    void FunctionInHeader       keyword static removed  Then the same command as in case 2 will succeed  and further executing   main will give the expected result  Here the FunctionInHeader definition is in another translation unit  but the code defining it can be linked    Thus  to conclude  static keyword prevents the code defining a function to be linked  when that function is defined in another translation unit than where it is called

User · Answer

pmg is spot on about encapsulation  beyond hiding the function from other translation units  or rather  because of it   making functions static can also confer performance benefits in the presence of compiler optimizations   Because a static function cannot be called from anywhere outside of the current translation unit  unless the code takes a pointer to its address   the compiler controls all the call points into it   This means that it is free to use a non-standard ABI  inline it entirely  or perform any number of other optimizations that might not be possible for a function with external linkage

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C programmers use the static attribute to hide variable and function declarations inside modules  much as you would use public and private declarations in Java and C    C source files play the role of modules  Any global variable or function declared with the static attribute is private to that module  Similarly  any global variable or function declared without the static attribute is public and can be accessed by any other module  It is good programming practice to protect your variables and functions with the static attribute wherever possible

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When there is a need to restrict access to some functions  we ll use the static keyword while defining and declaring a function                  file ab c     static void function1 void        puts  function1 called        And store the following code in another file ab1 c     file ab1 c      int main void       function1        getchar       return 0           in this code  we ll get a  Undefined reference to function1  Because function 1 is declared static in file ab c and can t be used in ab1 c

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pmg s answer is very convincing  If you would like to know how static declarations work at object level then this below info could be interesting to you  I reused the same program written by pmg and compiler it into a  so shared object  file  Following contents are after dumping the  so file into something human readable  0000000000000675 f1  address of f1 function  000000000000068c f2  address of f2 staticc  function  note the difference in the function address   it means something   For a function that s declared with different address   it can very well signify that f2 lives very far away or in a different segment of the object file   Linkers use something called PLT Procedure linkage table  and GOT Global offsets table  to understand symbols that they have access to link to    For now think that GOT and PLT magically bind all the addresses and a dynamic section holds information of all these functions that are visible by linker   After dumping the dynamic section of the  so file we get a bunch of entries but only interested in f1 and f2 function   The dynamic section holds entry only for f1 function at address 0000000000000675 and not for f2    Num     Value          Size Type    Bind   Vis      Ndx Name   9  0000000000000675    23 FUNC    GLOBAL DEFAULT   11 f1   And thats it    From this its clear that the linker will be unsuccessful in finding the f2 function since its not in the dynamic section of the  so file

[c] static function in C

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Examples related to static