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Misery

How to hide global functions from C files in C++ program?

10 posts in this topic

Hello,

 

I am writing a program in C++ that uses also C sources. As this program is pretty big project and I work on it with different people I would like to hide thousands of internal functions that are defined in C source files and leave to other programmers only interfaces or wrappers written by me to those functions. Making long story short, the example defining this is given below, and a quiestion is: how to make g() and h() functions invisible to Cpp programs but to leave them visible to other C sources.

 

Cheader.h: defines one "usable" function for which I write a c++ wrapper

int F(int x);

Csource.c:

#include "Cheader.h"
/* helper functions */
int h(int x)  
{
   return x*x;
}

int g(int x) 
{
   return -x/x;
}

/*exported function*/
int F(int x)
{
  return h(x)+g(x);
}

My Cpp wrapper:

header:

extern "C"{
 #include "Cheader.h"
}

int Fpp(int x)
{
  return F(x);
}
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Just use two different headers, one containing the internal functions and one the public API. Then make only the external API header available.

So it meant I would need to create an additional header for g() and h() functions, or to put their declarations in CHeader.h?

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I would put all public declarations into the first header, let's call it CPublicAPI.h and include this header in Cheader.h which although contains the declarations of the internal functions. Therfore
 f -> CPublicAPI.h

 

g & h -> CHeader.h (which includes CPublicAPI.h)

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Correct me if I'm wrong, but if you omit h() and g() in the Cheader.h, aren't you essentially already hiding them from any other files?  You are only exposing F() from the header.  Other c/cpp files can't use h() nor g() because they are undeclared.

 

Therefore, these public/private header files are not necessary.

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Make the internal functions static.

 

Otherwise, they're still accessible if people know the names... and also the names are available in the global space to potentially causes clashes.

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Correct me if I'm wrong, but if you omit h() and g() in the Cheader.h, aren't you essentially already hiding them from any other files?  You are only exposing F() from the header.  Other c/cpp files can't use h() nor g() because they are undeclared.

 

Therefore, these public/private header files are not necessary.

Yes, but iw I want to use second function h() or g() defined somewhere else in the same manner, compiler returns an multiple declaration error

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Make the internal functions static.

 

Otherwise, they're still accessible if people know the names... and also the names are available in the global space to potentially causes clashes.

The problem is that I cannot make them static, as the other C files use them. And the problem is that in compeletly different folder there are exactly identical declarations of functions, but they do something compeletly different

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You can't do what you're proposing because the link-time ABI for C++ does not work that way.

 

Both C and C++ source code get compiled to a native link-format object (ie. a .OBJ or .o file on most common platforms).  These files have nothing to do with the source language used to generate them:  it could be FORTRAN or COBOL even.  The native link-format object consists of a collection of chunks of native machine code and some metainformation including some identifiers labelling the offsets of those chunks.  Those labels are available for the linker to resolve references in other native link-format files.  There is no way, using most common linkers, to make the labels available only when resolving referenced for object files compiled from a specific language.

 

The upside is that C++ performs name mangling, which is a way of encoding language-specific type information into the label at runtime.  That means that the label C++ is expecting for, say, int g(int), is different from the label C is expecting for the same function.  If you tried declaring that function in a C++ file (through a preprocessor header or no) there would be a link failure because the symbol would not be found.  You can rely on that to accomplish the task you required.

 

The one caveat is that C++ can be asked to use the C label naming scheme using extern "C".  If you do that for the C-only symbols, you can not achieve what you want.

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You can easily prevent name clashes when you not use stupidly short names like g and h for those internal functions(and probably also public functions) and prefix them manually with the library name to simulate namespaces in C like many librarys do.

Also as already said you can make as many as possible static and possibly reorganize some source files to facilitate this.

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I hope g() and h() are just example names of functions and not the ACTUAL names...

 

Source reorganisation really is the only way to go about this. If you write C programs you really need to use some strategy to avoid global namespace pollution (MODULENAME_funcname is the most usual way of doing that). And as others said use static functions for helper functions which aren't used outside the file (and provide a clean non-static 'public' interface for a module to clients in the header file).

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