Question

I’ve created some simple basic code to illustrate my problem.

header.h:

#ifdef __cplusplus
#   define API extern "C"
#else
#   define API
#endif

void callback();
API void libFunction();

testlib.c:

#include "header.h"

void libFunction()
{
    Callback();
}

I compile this as a static library like so:

gcc -c testlib.c
ar rsc libtest.a testlib.o

Then my sample c++ code is

main.cpp:

extern  "C"{
#include  <lib/header.h>
}
#include  <stdio.h>

main()
{
    libFunction();
}

void Callback()
{
    printf("Callback is called \n");
}

and I try to build my exe as so

g++ -I. -L. main.cpp -ltest

and get the following error

./lib/libtest.a(testlib.o): In function `libFunction':
testlib.c:(.text+0x7): undefined reference to `Callback'
collect2: ld returned 1 exit status

I have spent literally all day trying to figure out why. Can anyone please help?

Answer 1

If you want to call Callback from a C file, it needs to be built extern "C" in your C++ file – otherwise C++ name mangling will cause the symbols to not line up. You need to change the definition of Callback() in main.cpp to be:

extern "C" void Callback()

You have a case mismatch, too. The prototype in your header says callback, but everywhere else you use Callback. On re-reading your code, I think just fixing this case mismatch will solve all your problems. I didn’t see the extern "C" wrapper around the #include <lib/header.h> on first reading.