Thread: Calling C++ function from C (yes I realise this has been done to death !)

I have my GatewayLT.hpp list class:

Code:

class GatewayList
{
private:
    typedef std::list<Gateway*> GatewayStoreType;
    GatewayStoreType store;

public:
    void AddGateway(Gateway * const gw);
    void Send(struct lgw_pkt_rx_s pkt_data);
};


extern "C" void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data)
{
    Global::gatewayList.Send(pkt_data);
    return;
}

This list resides in my GlobalDataLT.hpp class as below:

Code:

namespace Global
{
    //below defined in GlobalDataLT.cpp
    extern GatewayList gatewayList;
}

I want to call Global::gatewayList.Send(...).
1. Including the GlobalDataLT.hpp file in my C code and calling Global::gatewayList.Send() yields 100s of errors
2. Using the extern function call_GatewayList_Send as defined in my GatewayLT.hpp (as you can see above) seems to be the recommended way but when compiling the C file I still get an error:

Code:

sx1272.c:1982: undefined reference to `call_GatewayList_Send'

although it's definitley in the GatewayLT.hpp as you can see and I've defined the prototype for it in my .c file.
So what's the correct way to get around this ?

regards

Have you put the prototype in the C file?

Code:

void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data);

The C file will also need the definition of the struct.

You may want to read the following link: How to Mix C and C++.

Also I'd recommend against having the function implementation inside a header file when trying to mix these two languages.

Next I'd recommend trying to use non class member functions whenever possible. For example GatewayList.send() looks like it could be a non-class function since it doesn't appear to be using any class member variables.

Lastly when possible you should really consider using the C++ compiler to compile the program, rather than the C compiler.


Jim

Originally Posted by algorism

Have you put the prototype in the C file?

Code:

void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data);

The C file will also need the definition of the struct.

yes the prototype is definitely in the C file as I said above, but in any case here is the direct cut from the file:

Code:

void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data);
void MGWIfaceSendToGateways()
{
....
    call_GatewayList_Send(pkt_data);
}

Thanks for that, what you have said is pretty much what I have done now. Worth pointing out I'm building in the Eclipse IDE environment with a Cygwin compiler. The code is pretty much 50% C and 50% C++.
Still get the annoying error
undefined reference to `call_GatewayList_Send'
and I do have the prototype for it in the .c file.
I'll sleep on it and see if I come up with something in the morning,

That sounds like a linker error (as opposed to a compiler error). So you're probably not compiling it correctly.

Originally Posted by algorism

That sounds like a linker error (as opposed to a compiler error). So you're probably not compiling it correctly.

Turns out I needed the following around my extern "C":

Code:

#ifdef __cplusplus
extern "C" void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data);
#endif

then define the function in the cpp file as such:

Code:

void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data)
{
    Global::gatewayList.Send(pkt_data);
    return;
}

then add the prototype (wiithout the #ifdef __cplusplus and without extern "C" bit) to the .c file eg

Code:

void call_GatewayList_Send(struct lgw_pkt_rx_s pkt_data);

and simply call it.