[C] How to make C call c++

/* b.h */
#ifndef _B_H_
#define _B_H_

#ifdef __cplusplus
extern "C" {
#endif
int add(int a, int b);
#ifdef __cplusplus
}
#endif
#endif

/* b.cpp */
#include "b.h"   <-- must include "b.h"

int add(int a, int b) {
    return a + b;
}

/* a.c */
#include <stdio.h>
#include "b.h"   <-- must include "b.h"

int main(void) {
    printf("%d\n", add(3, 5));
    return 0;
}

Reference:

1. 從 C 呼叫 C++ 函式的過程理解程式編譯、連結的原理