How to correctly use the extern keyword in C

Question

My question is about when a function should be referenced with the extern keyword in C   I am failing to see when this should be used in practice   As I am writing a program all of the functions that I use are made available through the header files I have included   So why would it be useful to extern to get access to something that was not exposed in the header file   I could be thinking about how extern works incorrectly  and if so please correct me   Edit  Should you extern something when it is the default declaration without the keyword in a header file

User · Answer

It has already been stated that the extern keyword is redundant for functions   As for variables shared across compilation units  you should declare them in a header file with the extern keyword  then define them in a single source file  without the extern keyword  The single source file should be the one sharing the header file s name  for best practice

User · Answer

A very good article that I came about the extern keyword  along with the examples  http   www geeksforgeeks org understanding-extern-keyword-in-c   Though I do not agree that using extern in function declarations is redundant  This is supposed to be a compiler setting  So I recommend using the extern in the function declarations when it is needed

User · Answer

extern tells the compiler that this data is defined somewhere and will be connected with the linker   With the help of the responses here and talking to a few friends here is the practical example of a use of extern   Example 1 - to show a pitfall   File stdio h   int errno     other stuff          myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      If myCFile1 o and myCFile2 o are linked  each of the c files have separate copies of errno   This is a problem as the same errno is supposed to be available in all linked files   Example 2 - The fix   File stdio h   extern int errno     other stuff          File stdio c  int errno      myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      Now if both myCFile1 o and MyCFile2 o are linked by the linker they will both point to the same errno   Thus  solving the implementation with extern

User · Answer

It has already been stated that the extern keyword is redundant for functions   As for variables shared across compilation units  you should declare them in a header file with the extern keyword  then define them in a single source file  without the extern keyword  The single source file should be the one sharing the header file s name  for best practice

User · Answer

If each file in your program is first compiled to an object file  then the object files are linked together  you need extern  It tells the compiler  This function exists  but the code for it is somewhere else  Don t panic

User · Answer

In C   extern  is implied for function prototypes  as a prototype declares a function which is defined somewhere else  In other words  a function prototype has external linkage by default  using  extern  is fine  but is redundant    If static linkage is required  the function must be declared as  static  both in its prototype and function header  and these should normally both be in the same  c file

User · Answer

In C   extern  is implied for function prototypes  as a prototype declares a function which is defined somewhere else  In other words  a function prototype has external linkage by default  using  extern  is fine  but is redundant    If static linkage is required  the function must be declared as  static  both in its prototype and function header  and these should normally both be in the same  c file

User · Answer

If each file in your program is first compiled to an object file  then the object files are linked together  you need extern  It tells the compiler  This function exists  but the code for it is somewhere else  Don t panic

User · Answer

Functions actually defined in other source files should only be declared in headers  In this case  you should use extern when declaring the prototype in a header   Most of the time  your functions will be one of the following  more like a best practice     static  normal functions that aren t visible outside that  c file  static inline  inlines from  c or  h files  extern  declaration in headers of the next kind  see below    no keyword whatsoever   normal functions meant to be accessed using extern declarations

User · Answer

It has already been stated that the extern keyword is redundant for functions   As for variables shared across compilation units  you should declare them in a header file with the extern keyword  then define them in a single source file  without the extern keyword  The single source file should be the one sharing the header file s name  for best practice

User · Answer

extern tells the compiler that this data is defined somewhere and will be connected with the linker   With the help of the responses here and talking to a few friends here is the practical example of a use of extern   Example 1 - to show a pitfall   File stdio h   int errno     other stuff          myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      If myCFile1 o and myCFile2 o are linked  each of the c files have separate copies of errno   This is a problem as the same errno is supposed to be available in all linked files   Example 2 - The fix   File stdio h   extern int errno     other stuff          File stdio c  int errno      myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      Now if both myCFile1 o and MyCFile2 o are linked by the linker they will both point to the same errno   Thus  solving the implementation with extern

User · Answer

All declarations of functions and variables in header files should be extern   Exceptions to this rule are inline functions defined in the header and variables which - although defined in the header - will have to be local to the translation unit  the source file the header gets included into   these should be static   In source files  extern shouldn t be used for functions and variables defined in the file  Just prefix local definitions with static and do nothing for shared definitions - they ll be external symbols by default   The only reason to use extern at all in a source file is to declare functions and variables which are defined in other source files and for which no header file is provided     Declaring function prototypes extern is actually unnecessary  Some people dislike it because it will just waste space and function declarations already have a tendency to overflow line limits  Others like it because this way  functions and variables can be treated the same way

User · Answer

When you have that function defined on a different dll or lib  so that the compiler defers to the linker to find it  Typical case is when you are calling functions from the OS API

User · Answer

extern tells the compiler that this data is defined somewhere and will be connected with the linker   With the help of the responses here and talking to a few friends here is the practical example of a use of extern   Example 1 - to show a pitfall   File stdio h   int errno     other stuff          myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      If myCFile1 o and myCFile2 o are linked  each of the c files have separate copies of errno   This is a problem as the same errno is supposed to be available in all linked files   Example 2 - The fix   File stdio h   extern int errno     other stuff          File stdio c  int errno      myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      Now if both myCFile1 o and MyCFile2 o are linked by the linker they will both point to the same errno   Thus  solving the implementation with extern

User · Answer

When you have that function defined on a different dll or lib  so that the compiler defers to the linker to find it  Typical case is when you are calling functions from the OS API

User · Answer

If each file in your program is first compiled to an object file  then the object files are linked together  you need extern  It tells the compiler  This function exists  but the code for it is somewhere else  Don t panic

User · Answer

All declarations of functions and variables in header files should be extern   Exceptions to this rule are inline functions defined in the header and variables which - although defined in the header - will have to be local to the translation unit  the source file the header gets included into   these should be static   In source files  extern shouldn t be used for functions and variables defined in the file  Just prefix local definitions with static and do nothing for shared definitions - they ll be external symbols by default   The only reason to use extern at all in a source file is to declare functions and variables which are defined in other source files and for which no header file is provided     Declaring function prototypes extern is actually unnecessary  Some people dislike it because it will just waste space and function declarations already have a tendency to overflow line limits  Others like it because this way  functions and variables can be treated the same way

User · Answer

A very good article that I came about the extern keyword  along with the examples  http   www geeksforgeeks org understanding-extern-keyword-in-c   Though I do not agree that using extern in function declarations is redundant  This is supposed to be a compiler setting  So I recommend using the extern in the function declarations when it is needed

User · Answer

It has already been stated that the extern keyword is redundant for functions   As for variables shared across compilation units  you should declare them in a header file with the extern keyword  then define them in a single source file  without the extern keyword  The single source file should be the one sharing the header file s name  for best practice

User · Answer

All declarations of functions and variables in header files should be extern   Exceptions to this rule are inline functions defined in the header and variables which - although defined in the header - will have to be local to the translation unit  the source file the header gets included into   these should be static   In source files  extern shouldn t be used for functions and variables defined in the file  Just prefix local definitions with static and do nothing for shared definitions - they ll be external symbols by default   The only reason to use extern at all in a source file is to declare functions and variables which are defined in other source files and for which no header file is provided     Declaring function prototypes extern is actually unnecessary  Some people dislike it because it will just waste space and function declarations already have a tendency to overflow line limits  Others like it because this way  functions and variables can be treated the same way

User · Answer

In C   extern  is implied for function prototypes  as a prototype declares a function which is defined somewhere else  In other words  a function prototype has external linkage by default  using  extern  is fine  but is redundant    If static linkage is required  the function must be declared as  static  both in its prototype and function header  and these should normally both be in the same  c file

User · Answer

When you have that function defined on a different dll or lib  so that the compiler defers to the linker to find it  Typical case is when you are calling functions from the OS API

User · Answer

In C   extern  is implied for function prototypes  as a prototype declares a function which is defined somewhere else  In other words  a function prototype has external linkage by default  using  extern  is fine  but is redundant    If static linkage is required  the function must be declared as  static  both in its prototype and function header  and these should normally both be in the same  c file

User · Answer

extern  changes the linkage  With the keyword  the function   variable is assumed to be available somewhere else and the resolving is deferred to the linker   There s a difference between  extern  on functions and on variables  on variables it doesn t instantiate the variable itself  i e  doesn t allocate any memory  This needs to be done somewhere else  Thus it s important if you want to import the variable from somewhere else  For functions  this only tells the compiler that linkage is extern  As this is the default  you use the keyword  static  to indicate that a function is not bound using extern linkage  you don t need to use it explicitly

User · Answer

If each file in your program is first compiled to an object file  then the object files are linked together  you need extern  It tells the compiler  This function exists  but the code for it is somewhere else  Don t panic

User · Answer

Functions actually defined in other source files should only be declared in headers  In this case  you should use extern when declaring the prototype in a header   Most of the time  your functions will be one of the following  more like a best practice     static  normal functions that aren t visible outside that  c file  static inline  inlines from  c or  h files  extern  declaration in headers of the next kind  see below    no keyword whatsoever   normal functions meant to be accessed using extern declarations

User · Answer

Many years later  I discover this question  After reading every answer and comment  I thought I could clarify a few details    This could be useful for people who get here through Google search   The question is specifically about using  extern  functions  so I will ignore the use of  extern  with global variables    Let s define 3 function prototypes     --------------------------------------   Filename   my project H  extern int function 1 void   static int function 2 void          int function 3 void     The header file can be used by the main source code as follows     --------------------------------------   Filename   my project C   include  my project H   void main void       int v1   function 1        int v2   function 2        int v3   function 3       int function 2 void  return 1234    In order to compile and link  we must define  function 2  in the same source code file where we call that function  The two other functions could be defined in different source code   C  or they may be located in any binary file   OBJ    LIB    DLL   for which we may not have the source code   Let s include again the header  my project H  in a different    C  file to understand better the difference  In the same project  we add the following file     --------------------------------------   Filename   my big project splitted C   include  my project H   void old main test void       int v1   function 1        int v2   function 2        int v3   function 3       int function 2 void  return 5678   int function 1 void  return 12  int function 3 void  return 34    Important features to notice    When a function is defined as  static  in a header file  the compiler linker must find an instance of a function with that name in each module which uses that include file  A function which is part of the C library can be replaced in only one module by redefining a prototype with  static  only in that module  For example  replace any call to  malloc  and  free  to add memory leak detection feature  The specifier  extern  is not really needed for functions  When  static  is not found  a function is always assumed to be  extern   However   extern  is not the default for variables  Normally  any header file that defines variables to be visible across many modules needs to use  extern   The only exception would be if a header file is guaranteed to be included from one and only one module   Many project manager would then require that such variable be placed at the beginning of the module  not inside any header file  Some large projects  such as the video game emulator  Mame  even require that such variable appears only above the first function using them

User · Answer

extern  changes the linkage  With the keyword  the function   variable is assumed to be available somewhere else and the resolving is deferred to the linker   There s a difference between  extern  on functions and on variables  on variables it doesn t instantiate the variable itself  i e  doesn t allocate any memory  This needs to be done somewhere else  Thus it s important if you want to import the variable from somewhere else  For functions  this only tells the compiler that linkage is extern  As this is the default  you use the keyword  static  to indicate that a function is not bound using extern linkage  you don t need to use it explicitly

User · Answer

extern tells the compiler that this data is defined somewhere and will be connected with the linker   With the help of the responses here and talking to a few friends here is the practical example of a use of extern   Example 1 - to show a pitfall   File stdio h   int errno     other stuff          myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      If myCFile1 o and myCFile2 o are linked  each of the c files have separate copies of errno   This is a problem as the same errno is supposed to be available in all linked files   Example 2 - The fix   File stdio h   extern int errno     other stuff          File stdio c  int errno      myCFile1 c   include  lt stdio h gt   Code        myCFile2 c   include  lt stdio h gt   Code      Now if both myCFile1 o and MyCFile2 o are linked by the linker they will both point to the same errno   Thus  solving the implementation with extern

User · Answer

When you have that function defined on a different dll or lib  so that the compiler defers to the linker to find it  Typical case is when you are calling functions from the OS API

User · Answer

extern  changes the linkage  With the keyword  the function   variable is assumed to be available somewhere else and the resolving is deferred to the linker   There s a difference between  extern  on functions and on variables  on variables it doesn t instantiate the variable itself  i e  doesn t allocate any memory  This needs to be done somewhere else  Thus it s important if you want to import the variable from somewhere else  For functions  this only tells the compiler that linkage is extern  As this is the default  you use the keyword  static  to indicate that a function is not bound using extern linkage  you don t need to use it explicitly

User · Answer

Functions actually defined in other source files should only be declared in headers  In this case  you should use extern when declaring the prototype in a header   Most of the time  your functions will be one of the following  more like a best practice     static  normal functions that aren t visible outside that  c file  static inline  inlines from  c or  h files  extern  declaration in headers of the next kind  see below    no keyword whatsoever   normal functions meant to be accessed using extern declarations

User · Answer

Functions actually defined in other source files should only be declared in headers  In this case  you should use extern when declaring the prototype in a header   Most of the time  your functions will be one of the following  more like a best practice     static  normal functions that aren t visible outside that  c file  static inline  inlines from  c or  h files  extern  declaration in headers of the next kind  see below    no keyword whatsoever   normal functions meant to be accessed using extern declarations

User · Answer

extern  changes the linkage  With the keyword  the function   variable is assumed to be available somewhere else and the resolving is deferred to the linker   There s a difference between  extern  on functions and on variables  on variables it doesn t instantiate the variable itself  i e  doesn t allocate any memory  This needs to be done somewhere else  Thus it s important if you want to import the variable from somewhere else  For functions  this only tells the compiler that linkage is extern  As this is the default  you use the keyword  static  to indicate that a function is not bound using extern linkage  you don t need to use it explicitly

User · Answer

All declarations of functions and variables in header files should be extern   Exceptions to this rule are inline functions defined in the header and variables which - although defined in the header - will have to be local to the translation unit  the source file the header gets included into   these should be static   In source files  extern shouldn t be used for functions and variables defined in the file  Just prefix local definitions with static and do nothing for shared definitions - they ll be external symbols by default   The only reason to use extern at all in a source file is to declare functions and variables which are defined in other source files and for which no header file is provided     Declaring function prototypes extern is actually unnecessary  Some people dislike it because it will just waste space and function declarations already have a tendency to overflow line limits  Others like it because this way  functions and variables can be treated the same way

User · Answer

Many years later  I discover this question  After reading every answer and comment  I thought I could clarify a few details    This could be useful for people who get here through Google search   The question is specifically about using  extern  functions  so I will ignore the use of  extern  with global variables    Let s define 3 function prototypes     --------------------------------------   Filename   my project H  extern int function 1 void   static int function 2 void          int function 3 void     The header file can be used by the main source code as follows     --------------------------------------   Filename   my project C   include  my project H   void main void       int v1   function 1        int v2   function 2        int v3   function 3       int function 2 void  return 1234    In order to compile and link  we must define  function 2  in the same source code file where we call that function  The two other functions could be defined in different source code   C  or they may be located in any binary file   OBJ    LIB    DLL   for which we may not have the source code   Let s include again the header  my project H  in a different    C  file to understand better the difference  In the same project  we add the following file     --------------------------------------   Filename   my big project splitted C   include  my project H   void old main test void       int v1   function 1        int v2   function 2        int v3   function 3       int function 2 void  return 5678   int function 1 void  return 12  int function 3 void  return 34    Important features to notice    When a function is defined as  static  in a header file  the compiler linker must find an instance of a function with that name in each module which uses that include file  A function which is part of the C library can be replaced in only one module by redefining a prototype with  static  only in that module  For example  replace any call to  malloc  and  free  to add memory leak detection feature  The specifier  extern  is not really needed for functions  When  static  is not found  a function is always assumed to be  extern   However   extern  is not the default for variables  Normally  any header file that defines variables to be visible across many modules needs to use  extern   The only exception would be if a header file is guaranteed to be included from one and only one module   Many project manager would then require that such variable be placed at the beginning of the module  not inside any header file  Some large projects  such as the video game emulator  Mame  even require that such variable appears only above the first function using them

[c] How to correctly use the extern keyword in C

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