How to hide helper functions from public API in c

Question

I'm working on a project and I need to create an API. I am using sockets to communicate between the server (my application) and the clients (the other applications using my API).

This project is in c not C++

I come from a linux background and this is my first project using Windows, Visual Studio 2008, and dll libraries.

I have communication working between the client and server, but I have some that is duplicated on both projects. I would like to create a library (probably a dll file), that both projects can link to so I don't have to maintain extra code.

I also have to create the library that has the API that I need to make available for my clients. Within the API functions that I want public are the calls to these helper functions that are "duplicated code", I don't want to expose these functions to my client, but I do want my server to be able to use those functions. How can I do this?

I will try to clarify with an example. This is what I started with.

Server Project:

int Server_GetPacket(SOCKET sd);
int ReceiveAll(SOCKET sd, char *buf, int len);
int VerifyLen(char *buf);

Client Project:

int Client_SendCommand(int command);
int Client_GetData(int command, char *buf, int len);
int ReceiveAll(SOCKET sd, char *buf, int len);
int VerifyLen(char *buf);

This is kind of what I would like to end up with:

//Server Project:
int Server_GetPacket(SOCKET sd);

// library with public and private types
//  private API (not exposed to my client)
int ReceiveAll(SOCKET sd, char *buf, int len);
int VerifyLen(char *buf);
//  public API (header file available for client)
int Client_SendCommand(int command);
int Client_GetData(int command, char *buf, int len);

Thanks any help would be appreciated.

Answer 1

Use

static int ReceiveAll(SOCKET sd, char *buf, int len);

etc to make the functions local to the file / compilation unit.

Answer 2

If you put the "private" functions in a DLL and make them externally callable by normal means (e.g., callable by a process that loads the library), then they will be "public" for others as well. It is possible to obfuscate the names and things like that, but that is probably not a good solution.

It might be better to put those into a statically linked library as opposed to a DLL. Note, though, that even in this case someone can disassemble the binary obviously and get to the code.

Answer 3

If you want to only expose certain functions in your DLL, you can create an exports file (dllname.def) that you give to the linker when you build the project. This exports file contains a list of the functions that you want to make publicly available to applications using your DLL.

See the MSDN article here for more information about this.

I think you can also achieve similar functionality by using the __declspec(dllexport) keyword on the functions you want to export.