What is the effect of extern C in C

Question

What exactly does putting extern  C  into C   code do   For example   extern  C       void foo

User · Answer

Without conflicting with other good answers, I will add a bit of my example.

What exactly C++ Compiler does: it mangles the names in the compilation process, hence we require telling the compiler to treat C implementation specially.

When we are making C++ classes and adding extern "C", we're telling our C++ compiler that we are using C calling convention.

Reason (we are calling C implementation from C++): either we want to call C function from C++ or calling C++ function from C (C++ classes ... etc do not work in C).

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This answer is for the impatient  have deadlines to meet to  only a part simple explanation is below    in C    you can have same name in class via overloading  for example  since they are all same name can t be exported as-is from dll  etc   solution to these problems is they are converted to different strings  called symbols   symbols accounts the name of function  also the arguments  so each of these functions even with same name  can be uniquely identified  also called  name mangling  in C  you don t have overloading  the function name is unique  so  a separate string for identifying the a function name uniquely is not required  so symbol is function name itself    So in C    with name mangling uniquely identities each function in C  even without name mangling uniquely identities each function    To change the behaviour of C    that is  to specify that name mangling should not happen for a particular function  you can use extern  C  before the function name  for whatever reason  like exporting a function with a specific name from a dll  for use by its clients   Read other answers  for more detailed more correct answers

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In every C   program  all non-static functions are represented in the binary file as symbols  These symbols are special text strings that uniquely identify a function in the program   In C  the symbol name is the same as the function name  This is possible because in C no two non-static functions can have the same name   Because C   allows overloading and has many features that C does not     like classes  member functions  exception specifications - it is not possible to simply use the function name as the symbol name  To solve that  C   uses so-called name mangling  which transforms the function name and all the necessary information  like the number and size of the arguments  into some weird-looking string processed only by the compiler and linker   So if you specify a function to be extern C  the compiler doesn t performs name mangling with it and it can be directly  accessed using its symbol name as the function name   This comes handy while using dlsym   and dlopen   for calling such functions

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It informs the C   compiler to look up the names of those functions in a C-style when linking  because the names of functions compiled in C and C   are different during the linking stage

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Just wanted to add a bit of info  since I haven t seen it posted yet   You ll very often see code in C headers like so    ifdef   cplusplus extern  C     endif     all of your legacy C code here   ifdef   cplusplus    endif   What this accomplishes is that it allows you to use that C header file with your C   code  because the macro    cplusplus  will be defined  But you can also still use it with your legacy C code  where the macro is NOT defined  so it won t see the uniquely C   construct   Although  I have also seen C   code such as   extern  C     include  legacy C header h      which I imagine accomplishes much the same thing   Not sure which way is better  but I have seen both

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I used  extern  C   before for dll dynamic link library  files to make etc  main   function  exportable  so it can be used later in another executable from dll  Maybe an example of where I used to use it can be useful   DLL   include  lt string h gt   include  lt windows h gt   using namespace std    define DLL extern  C    declspec dllexport    I defined DLL for dllexport function DLL main          MessageBox NULL  Hi from DLL   DLL  MB OK       EXE   include  lt string h gt   include  lt windows h gt   using namespace std   typedef LPVOID  WINAPI Function      make a placeholder for function from dll Function mainDLLFunc   make a variable for function placeholder  int main         char winDir MAX PATH    will hold path of above dll     GetCurrentDirectory sizeof winDir  winDir    dll is in same dir as exe     strcat winDir    exmple dll     concentrate dll name with path     HINSTANCE DLL   LoadLibrary winDir    load example dll     if DLL  NULL                FreeLibrary  HMODULE DLL    if load fails exit         return 0            mainDLLFunc  Function GetProcAddress  HMODULE DLL   main          defined variable is used to assign a function from dll       GetProcAddress is used to locate function with pre defined extern name  DLL        and matcing function name     if mainDLLFunc  NULL                FreeLibrary  HMODULE DLL    if it fails exit         return 0            mainDLLFunc     run exported function      FreeLibrary  HMODULE DLL

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When mixing C and C    i e   a  calling C function from C    and b  calling C   function from C   the C   name mangling causes linking problems  Technically speaking  this issue happens only when the callee functions have been already compiled into binary  most likely  a   a library file  using the corresponding compiler   So we need to use extern  C  to disable the name mangling in C

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A function void f   compiled by a C compiler and a function with the same name void f   compiled by a C   compiler are not the same function  If you wrote that function in C  and then you tried to call it from C    then the linker would look for the C   function and not find the C function    extern  C  tells the C   compiler that you have a function which was compiled by the C compiler  Once you tell it that it was compiled by the C compiler  the C   compiler will know how to call it correctly    It also allows the C   compiler to compile a C   function in such a way that the C compiler can call it  That function would officially be a C function  but since it is compiled by the C   compiler  it can use all the C   features and has all the C   keywords

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Decompile a g   generated binary to see what is going on main cpp void f      void g     extern  quot C quot        void ef          void eg          Prevent g and eg from being optimized away     void h     g    eg       Compile and disassemble the generated ELF output  g   -c -std c  11 -Wall -Wextra -pedantic -o main o main cpp readelf -s main o  The output contains       8  0000000000000000     7 FUNC    GLOBAL DEFAULT    1  Z1fv      9  0000000000000007     7 FUNC    GLOBAL DEFAULT    1 ef     10  000000000000000e    17 FUNC    GLOBAL DEFAULT    1  Z1hv     11  0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND  GLOBAL OFFSET TABLE      12  0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND  Z1gv     13  0000000000000000     0 NOTYPE  GLOBAL DEFAULT  UND eg  Interpretation We see that   ef and eg were stored in symbols with the same name as in the code  the other symbols were mangled  Let s unmangle them    c  filt  Z1fv f     c  filt  Z1hv h     c  filt  Z1gv g      Conclusion  both of the following symbol types were not mangled   defined declared but undefined  Ndx   UND   to be provided at link or run time from another object file  So you will need extern  quot C quot  both when calling   C from C    tell g   to expect unmangled symbols produced by gcc C   from C  tell g   to generate unmangled symbols for gcc to use  Things that do not work in extern C It becomes obvious that any C   feature that requires name mangling will not work inside extern C  extern  quot C quot           Overloading         error  declaration of C function    void f int     conflicts with     void f        void f int i           Templates         error  template with C linkage     template  lt class C gt  void f C i         Minimal runnable C from C   example For the sake of completeness and for the newbs out there  see also  How to use C source files in a C   project  Calling C from C   is pretty easy  each C function only has one possible non-mangled symbol  so no extra work is required  main cpp  include  lt cassert gt    include  quot c h quot   int main         assert f      1      c h  ifndef C H  define C H     This ifdef allows the header to be used from both C and C       because C does not know what this extern  quot C quot  thing is      ifdef   cplusplus extern  quot C quot     endif int f     ifdef   cplusplus    endif   endif  c c  include  quot c h quot   int f void    return 1     Run  g   -c -o main o -std c  98 main cpp gcc -c -o c o -std c89 c c g   -o main out main o c o   main out  Without extern  quot C quot  the link fails with  main cpp 6  undefined reference to  f     because g   expects to find a mangled f  which gcc did not produce  Example on GitHub  Minimal runnable C   from C example Calling C   from C is a bit harder  we have to manually create non-mangled versions of each function we want to expose  Here we illustrate how to expose C   function overloads to C  main c  include  lt assert h gt    include  quot cpp h quot   int main void        assert f int 1     2       assert f float 1 0     3       return 0     cpp h  ifndef CPP H  define CPP H   ifdef   cplusplus    C cannot see these overloaded prototypes  or else it would get confused  int f int i   int f float i   extern  quot C quot     endif int f int int i   int f float float i    ifdef   cplusplus    endif   endif  cpp cpp  include  quot cpp h quot   int f int i        return i   1     int f float i        return i   2     int f int int i        return f i      int f float float i        return f i      Run  gcc -c -o main o -std c89 -Wextra main c g   -c -o cpp o -std c  98 cpp cpp g   -o main out main o cpp o   main out  Without extern  quot C quot  it fails with  main c 6  undefined reference to  f int  main c 7  undefined reference to  f float   because g   generated mangled symbols which gcc cannot find  Example on GitHub  Where is the extern  quot c quot  when I include C headers from C     C   versions of C headers like cstdio might be relying on  pragma GCC system header which https   gcc gnu org onlinedocs cpp System-Headers html mentions   quot On some targets  such as RS 6000 AIX  GCC implicitly surrounds all system headers with an  extern  quot C quot   block when compiling as C    quot   but I didn t fully confirm it  POSIX headers like  usr include unistd h are covered at  Do I need an extern  quot C quot  block to include standard POSIX C headers  via   BEGIN DECLS  reproduced on Ubuntu 20 04    BEGIN DECLS is included via  include  lt features h gt    Tested in Ubuntu 18 04

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extern  quot C quot  makes a function-name in C   have C linkage  compiler does not mangle the name  so that client C code can link to  use  your function using a C compatible header file that contains just the declaration of your function  Your function definition is contained in a binary format  that was compiled by your C   compiler  that the client C linker will then link to using the C name  Since C   has overloading of function names and C does not  the C   compiler cannot just use the function name as a unique id to link to  so it mangles the name by adding information about the arguments   A C compiler does not need to mangle the name since you can not overload function names in C   When you state that a function has extern  quot C quot  linkage in C    the C   compiler does not add argument parameter type information to the name used for linkage  Just so you know  you can specify extern  quot C quot  linkage to each individual declaration definition explicitly or use a block to group a sequence of declarations definitions to have a certain linkage  extern  quot C quot  void foo int   extern  quot C quot       void g char      int i     If you care about the technicalities  they are listed in section 7 5 of the C  03 standard  here is a brief summary  with emphasis on extern  quot C quot     extern  quot C quot  is a linkage-specification Every compiler is required to provide  quot C quot  linkage A linkage specification shall occur only in namespace scope All function types  function names and variable names have a language linkage  See Richard s Comment  Only function names and variable names with external linkage have a language linkage Two function types with distinct language linkages are distinct types even if otherwise identical Linkage specs nest  inner one determines the final linkage extern  quot C quot  is ignored for class members At most one function with a particular name can have  quot C quot  linkage  regardless of namespace  extern  quot C quot  forces a function to have external linkage  cannot make it static    See Richard s comment     static inside extern  quot C quot  is valid  an entity so declared has internal linkage  and so does not have a language linkage Linkage from C   to objects defined in other languages and to objects defined in C   from other languages is implementation-defined and language-dependent  Only where the object layout strategies of two language implementations are similar enough can such linkage be achieved

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Not any C-header can be made compatible with C   by merely wrapping in extern  C   When identifiers in a C-header conflict with C   keywords the C   compiler will complain about this   For example  I have seen the following code fail in a g      extern  C    struct method       int virtual         Kinda makes sense  but is something to keep in mind when porting C-code to C

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extern  quot C quot  is meant to be recognized by a C   compiler and to notify the compiler that the noted function is  or will be  compiled in C style  so that while linking  it links to the correct version of the function from C

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C   mangles function names to create an object-oriented language from a procedural language Most programming languages aren t built on-top of existing programming languages  C   is built on-top of C  and furthermore it s an object-oriented programming language built from a procedural programming language  and for that reason there are C   expressions like extern  quot C quot  which provide backwards compatibility with C  Let s look at the following example   include  lt stdio h gt      Two functions are defined with the same name    but have different parameters  void printMe int a      printf  quot int   i n quot   a      void printMe char a      printf  quot char   c n quot   a      int main       printMe  quot a quot      printMe 1     return 0     A C compiler will not compile the above example  because the same function printMe is defined twice  even though they have different parameters int a vs char a    gcc -o printMe printMe c  amp  amp    printMe  1 error  PrintMe is defined more than once   A C   compiler will compile the above example  It does not care that printMe is defined twice   g   -o printMe printMe c  amp  amp    printMe   This is because a C   compiler implicitly renames  mangles  functions based on their parameters  In C  this feature was not supported  However  when C   was built over C  the language was designed to be object-oriented  and needed to support the ability to create different classes with methods  functions  of the same name  and to override methods  method overriding  based on different parameters  extern  quot C quot  says  quot don t mangle C function names quot  However  imagine we have a legacy C file named  quot parent c quot  that includes function names from other legacy C files   quot parent h quot    quot child h quot   etc  If the legacy  quot parent c quot  file is run through a C   compiler  then the function names will be mangled  and they will no longer match the function names specified in  quot parent h quot    quot child h quot   etc - so the function names in those external files would also need to be mangled  Mangling function names across a complex C program  those with lots of dependencies  can lead to broken code  so it might be convenient to provide a keyword which can tell the C   compiler not to mangle a function name  The extern  quot C quot  keyword tells a C   compiler not to mangle  rename  C function names  For example  extern  quot C quot  void printMe int a

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extern  C  is a linkage specification which is used to call C functions in the Cpp source files  We can call C functions  write Variables   amp  include headers  Function is declared in extern entity  amp  it is defined outside  Syntax is  Type 1   extern  language  function-prototype   Type 2   extern  language         function-prototype      eg    include lt iostream gt  using namespace std   extern  C          include lt stdio h gt        Include C Header      int n                   Declare a Variable      void func int int       Declare a function  function prototype     int main         func int a  int b        Calling function           return 0        Function definition       void func int m  int n

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It changes the linkage of a function in such a way that the function is callable from C  In practice that means that the function name is not mangled

[c++] What is the effect of extern "C" in C++?

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