How to call C function from C

Question

I know this   Calling C function from C     If my application was in C   and I had to call functions from a library written in C  Then I would have used     main cpp  extern  C  void C library function int x  int y    prototype C library function 2 4     directly using it    This wouldn t mangle the name C library function and linker would find the same name in its input   lib files and problem is solved   Calling C   function from C     But here I m extending a large application which is written in C and I need to use a library which is written in C    Name mangling of C   is causing trouble here  Linker is complaining about the unresolved symbols  Well I cannot use C   compiler over my C project because thats breaking lot of other stuff  What is the way out   By the way I m using MSVC

User · Answer

export your C   functions as extern  C   aka C style symbols   or use the  def file format to define undecorated export symbols for the C   linker when it creates the C   library  then the C linker should have no troubles reading it

User · Answer

include  lt iostream gt                     C   code                   struct A     int i    int j     A    i 1  j 2  std  cout  lt  lt   class A created n      void dump    std  cout  lt  lt   class A dumped     lt  lt  i  lt  lt       lt  lt  j  lt  lt  std  endl      A    std  cout  lt  lt   class A destroyed n        extern  C         this is the C code interface to the class A   static void  createA  void             create a handle to the A class     return  void    new A         static void dumpA  void  thisPtr             call A- gt dump        if  thisPtr    NULL     I m an anal retentive programmer             A  classPtr   static cast lt A   gt  thisPtr         classPtr- gt dump                 static void  deleteA  void  thisPtr             destroy the A class     if  thisPtr    NULL              delete  static cast lt A   gt  thisPtr                                                         this can be compiled as C code                                         int main  int argc  char   argv      void  handle   createA       dumpA  handle     deleteA  handle      return 0

User · Answer

You can prefix the function declaration with extern    C    keyword  e g   extern    C    int Mycppfunction          Code goes here  return 0      For more examples you can search more on Google about    extern    keyword  You need to do few more things  but it s not difficult you ll get lots of examples from Google

User · Answer

You will have to write a wrapper for C in C   if you want to do this  C   is backwards compatible  but C is not forwards compatible

User · Answer

I would do it in the following way    If working with MSVC  ignore the GCC compilation commands   Suppose that I have a C   class named AAA  defined in files aaa h  aaa cpp  and that the class AAA has a method named sayHi const char  name   that I want to enable for C code   The C   code of class AAA - Pure C    I don t modify it   aaa h   ifndef AAA H  define AAA H  class AAA       public          AAA            void sayHi const char  name        endif   aaa cpp   include  lt iostream gt    include  aaa h   AAA  AAA        void AAA  sayHi const char  name        std  cout  lt  lt   Hi    lt  lt  name  lt  lt  std  endl      Compiling this class as regularly done for C    This code  does not know  that it is going to be used by C code  Using the command   g   -fpic -shared aaa cpp -o libaaa so     Now  also in C    creating a C connector  Defining it in files aaa c connector h  aaa c connector cpp  This connector is going to define a C function  named AAA sayHi cosnt char  name   that will use an instance of AAA and will call its method   aaa c connector h   ifndef AAA C CONNECTOR H   define AAA C CONNECTOR H    ifdef   cplusplus extern  C     endif  void AAA sayHi const char  name     ifdef   cplusplus    endif    endif   aaa c connector cpp   include  lt cstdlib gt    include  aaa c connector h   include  aaa h    ifdef   cplusplus extern  C     endif     Inside this  extern C  block  I can implement functions in C    which will externally       appear as C functions  which means that the function IDs will be their names  unlike      the regular C   behavior  which allows defining multiple functions with the same name       overloading  and hence uses function signature hashing to enforce unique IDs     static AAA  AAA instance   NULL   void lazyAAA         if  AAA instance    NULL            AAA instance   new AAA             void AAA sayHi const char  name        lazyAAA        AAA instance- gt sayHi name       ifdef   cplusplus    endif   Compiling it  again  using a regular C   compilation command   g   -fpic -shared aaa c connector cpp -L  -laaa -o libaaa c connector so     Now I have a shared library  libaaa c connector so   that implements the C function AAA sayHi const char  name   I can now create a C main file and compile it all together   main c   include  aaa c connector h   int main         AAA sayHi  David        AAA sayHi  James         return 0      Compiling it using a C compilation command   gcc main c -L  -laaa c connector -o c aaa   I will need to set LD LIBRARY PATH to contain  PWD  and if I run the executable   c aaa  I will get the output I expect   Hi David Hi James   EDIT   On some linux distributions  -laaa and -lstdc   may also be required for the last compilation command  Thanks to  AlaaM  for the attention

User · Answer

You need to create a C API for exposing the functionality of your C   code  Basically  you will need to write C   code that is declared extern  C  and that has a pure C API  not using classes  for example  that wraps the C   library  Then you use the pure C wrapper library that you ve created   Your C API can optionally follow an object-oriented style  even though C is not object-oriented  Ex         h file           ifdef   cplusplus   define EXTERNC extern  C    else   define EXTERNC   endif   typedef void  mylibrary mytype t    EXTERNC mylibrary mytype t mylibrary mytype init     EXTERNC void mylibrary mytype destroy mylibrary mytype t mytype    EXTERNC void mylibrary mytype doit mylibrary mytype t self  int param      undef EXTERNC                 cpp file  mylibrary mytype t mylibrary mytype init        return new MyType       void mylibrary mytype destroy mylibrary mytype t untyped ptr        MyType  typed ptr   static cast lt MyType  gt  untyped ptr       delete typed ptr       void mylibrary mytype doit mylibrary mytype t untyped self  int param        MyType  typed self   static cast lt MyType  gt  untyped self       typed self- gt doIt param

User · Answer

Assuming the C   API is C-compatible  no classes  templates  etc    you can wrap it in extern  C           just as you did when going the other way   If you want to expose objects and other cute C   stuff  you ll have to write a wrapper API

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