A simple and probably flawed base for a plugin system using DSOs in C.

main.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>

#include "plugin_api.h"

plugin_repository *repo;

int load_plugin(plugin_repository *repo, char *name)
{
	char path[256], init_func_name[128];
	size_t len;
	int ret;
	void *handle;
	plugin *current, *new_plugin;
	plugin_init_func init_func;

	len = snprintf(path, 256, "%s/%s.so", PLUGIN_BASE_PATH, name);
	if (len >= 256) {
		fprintf(stderr, "Plugin path name overflowed\n");
		return -1;
	}

	len = snprintf(init_func_name, 128, "init_%s", name);
	if (len >= 128) {
		fprintf(stderr, "Plugin func name overflowed\n");
		return -1;
	}

	printf("Loading plugin %s from file %s\n", name, path);

	new_plugin = (plugin *) calloc(1, sizeof(plugin));

	if (repo->plugins == NULL) {
		repo->plugins = calloc(1, sizeof(plugin));
	}
	
	current = repo->plugins;
	while(current->next) {
		current = repo->plugins->next;
	}

	new_plugin->name = strdup(name);
	new_plugin->handle = dlopen(path, RTLD_NOW);

	if (!new_plugin->handle) {
		fprintf(stderr, "Failed to load plugin %s - object %s not found\n", name, path);
		free(new_plugin);
		return -1;
	}

	init_func = (plugin_init_func) (intptr_t) dlsym(new_plugin->handle, init_func_name);

	if (!init_func) {
		fprintf(stderr, "Failed to find plugin init function %s in %s\n", init_func_name, path);
		free(new_plugin);
		return -1;
	}

	ret = init_func(repo);
	if (ret < 0) {
		fprintf(stderr, "Failed to initialize plugin %s - function %s returned %d\n", name, init_func_name, ret);
		return -1;
	}

	current->next = new_plugin;

	fprintf(stderr, "Loaded plugin %s", name);

	return 0;
}

int main(int argc, char **argv)
{
	int i;
	repo = (plugin_repository *) calloc(1, sizeof(plugin_repository));
	
	for (i = 1; i < argc; i++)
	{
		load_plugin(repo, argv[i]);
	}

	return 0;
}

Makefile

all:
	gcc -g -o plugin main.c
	gcc -o test.so -g -fPIC -shared test.c

plugin_api.h

#define PLUGIN_BASE_PATH "/Users/michael/Code/plugins"

typedef struct _plugin plugin;
struct _plugin
{
	char *name;
	void *handle;
	plugin *next;
};

typedef struct _plugin_repository
{
	plugin *plugins;
} plugin_repository;

typedef int (*plugin_init_func)(plugin_repository *);

test.c

#include <stdio.h>
#include "plugin_api.h"

int init_test(plugin_repository *repo)
{
	printf("Hello from inside the test plugin\n");
	return 0;
}