Call a function in shared .so from C - c++

Question

My goal is creating a shared library created from c++. And I want to call a function in that library from a C program
I have a compareImage.h:
#ifdef __cplusplus
#define EXTERNC extern "C"
#else
#define EXTERNC
#endif
EXTERNC int compareTwoImages();
#undef EXTERNC
and a compareImage.cpp file:
#include "SURF_Homography.h"
extern "C" int compareTwoImages(){
..
}
I have already created a shared library by using this command:
g++ -ggdb `pkg-config --cflags opencv` -fpic -shared compareImage.cpp -o libcompareImage.so `pkg-config --libs opencv`
Then, I write a c program to call compareTwoImages() function from that shared lib like this:
#include <stdio.h>
int main() {
/* my first program in C */
int test = compareTwoImages();
printf("Comparison Results: %d\n", test);
return 0;
}
and compile it with this command:
gcc -lcompareImage c_call_cpp.c -o callCpp.o
But it shows an error:
/tmp/cc0wuZTU.o: In function `main':
c_call_cpp.c:(.text+0xe): undefined reference to `compareTwoImages'
collect2: error: ld returned 1 exit status
So I don't know what the problem is.

Answer 1

The problem is not with C++ or with your shared library or anything like that.
Narrow down your problem to a simple example next time.
Here you simply put the link flag in the wrong place:
gcc -lcompareImage c_call_cpp.c -o callCpp.o
# ^^^^^^^^^^^^^^
It needs to go after the object that'll use its symbols.
gcc c_call_cpp.c -o callCpp.o -lcompareImage
This is clearly stated in the documentation for -l:
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, ‘foo.o -lz bar.o’ searches library ‘z’ after file foo.o but before bar.o. If bar.o refers to functions in ‘z’, those functions may not be loaded.