Josh Lam writes
>>I need help on how to call C++ functions from C.
>>
>>I am currently working on a Sun Unix environment.  I have some C++
>>functions already available.  How can I write a C program to call these C++
>>functions (I will be compiling the C program with a C compiler).
>>
>>Would really appreciate some help!

calling C++ functions from C program is relatively easy but some precautions
have to be taken.

I will considere here an example under Sun environment.
the aim is to call C++ member functions foo() of a class A
from a main C program...

Assume the C++ file (named cxxfile.cc) contains following instructions:

 class A
 {
  ...

  int foo(int x);

  ...
 };

 ...

 A data;   /* declaration of an object A */

 ...

 extern "C" int cxxfoo(int x)
 {
  data.foo(x);
 }

Then the C program (main.c) has to contain:

 ...

 extern int cxxfoo(int);

 ...

 main()
 {
  _main();
  ...

  cxxfoo(36);

  ...

 }

Now follow next steps:

1/ Generate a C source file from your C++ file

  CC -Fc cxxfile.cc >cxxfile.c

2/ compile all C source file in order to produce object files

  cc -c cxxfile.c -o cxxfile.o
  cc -c main.c -o main.o

3/ build a C source file that allow call of static constructors/destructors

  nm cxxfile.o  | munch > cxxconstruct.c

  -nm gives symbol table of each object file
  -munch build a C source file containing initialization of ctors and
   dtors variables (called by _main() )

4/ compile the generated file

  cc -c cxxconstruct.c -o cxxconstruct.o

5/ link all objects files with C++ library (e.g. in Sun4 environment)

  cc -o main main.o cxxfile.o cxxconstruct.o -L /usr/CC/sun4 -lC


It works correctly and I think it will resolve your problem.


Good Luck !

Rachid HIMMI (emai rh@absstbg.uucp)

Hi Rachid

Thanks for your reply!

In your solution, you advised that I should use the following:

    nm cxxfile.o | munch > cxxconstruct.c

What is 'munch'?  When I ran the above, I received a message:

   munch:  Command not found.

My first guess is that munch is a script that I do not have.  If it is,
would it be possible to send me a copy of it?

Also, can you please comment a little about _main()?

Thanks again!

Josh Lam

hi Josh (sorry for my lateness!!)

"munch" is an executable file provided (I suppose in standard) by SUN with
the C++ environment.
On my machines it is located in /usr/CC/sun4 (or /usr/CC/sun3).

If this executable is not available on your machine, you can easily build it
if you know the rules of C++ mangling symbols.

For example the syntaxe for static C++ constructors is

   __sti___foo_()

and the corresponding C file generated by munch is

----------C source file generated by munch----------
typedef int (*PFV)();

int __sti___foo_();

extern PFV _ctors[];

PFV _ctors[] = {
 __sti___foo_;
  0
  };

--------end of file--------------------------------

"_main.c" is a very simple function that has in charge of calling all static
constructors stored in "ctors" array (see previous initialiZation by munch)

An example of this function is given here


--------_main() definition------------------

typedef int (*pfunc)();

extern pfunc _ctors[];

_main()
{
 int i ;
 for (i = 0; _ctors[i] ; (*_ctors[i++])());
}

------end of _main()------------------------


Bye

Rachid HIMMI

rh@jupiter.UUCP (#Rachid Himmi) writes:

>"munch" is an executable file provided (I suppose in standard) by SUN with
>the C++ environment.
>On my machines it is located in /usr/CC/sun4 (or /usr/CC/sun3).

"munch" is specific to C++ compilers derived from AT&T Cfront.  Some
compilers derived from Cfront use "patch" instead of munch.  I know
of no other C++ compilers which use either, as they are not required
when generating object code directly.

>If this executable is not available on your machine, you can easily build it
>if you know the rules of C++ mangling symbols.

These rules are different for every compiler.  Even different releases
of Cfront have used different name-mangling schemes.  Name mangling is
not part of the C++ language specification, but is merely a convenient
implementation technique.

>"_main.c" is a very simple function that has in charge of calling all static
>constructors stored in "ctors" array (see previous initialiZation by munch)

"_main.c" is used only by C++ compilers derived from Cfront.  Other
compilers use other techniques to call static constructors.

--

Steve Clamage, TauMetric Corp, steve@taumet.com