icebarf

$ g++ -o libcode.so code.cpp -ldl -rdynamic -shared -fPIC -std=c++20
$ g++ -o main main.cpp -rdynamic -std=c++20 -Os

The -rdynamic flag when compiling the executable adds the unused symbols to the table so we can use the definition from the main/executable file in the shared object's foo() call.

icebarf

/* FCFS Example Program by Amritpal Singh is marked with CC0 1.0 Universal.
 * To view a copy of this license, visit 
 * https://creativecommons.org/publicdomain/zero/1.0/
 * Copy of Program at:
 * https://gist.github.com/icebarf/8815aaed5bcca117518369c6428d7cb3/#file-fcfs_impl_from_class-cpp
 */

#include <cstdint>
#include <cstdio>

icebarf

#include "generic_list.h"
#include <string.h>

#define UNUSED(X) (void)X

// supply your own
typedef struct iNode
{
    size_t index;
    struct iNode *next;