September 15, 2019

Few years ago I stumbled upon this (well-established) technique for loading c-coded plugins at running time. Since I recently had to recall these concepts but I could not easily find back the sources I decided to describe it in few notes.

The problem

If you are designing a program to be highly modular, eventually you are gonna play with the idea to make it detect and load new parts at running time. E.g., if your software offer the user several algorithms to perform a common task, you would like to write and touch the main program only once and focus on adding new features as separate modules.

In the following I refer to these additional modules as plugins. These plugins are separated binary files that can be loaded and run by our software without any modification for the latter.

The plugin

The plugin is a piece of code which implements some well defined interface. For example, if our interface is made up by

• a plugin name PLUGIN-NAME;
• a function speak;

Our doggy plugin is gonna look like these:

#include<stdio.h>

char PLUGIN_NAME[]="doggy";

void speak()
{
printf("Woof woof!\n");
}


It is easy to imagine variations for this dumb example;

#include<stdio.h>

char PLUGIN_NAME[]="kitty";

void speak()
{
printf("Meow meow!\n");
}


The main program

Our platform must:

1. check for available plugins;
3. be able to execute their code.

It turns out it is quite simple, relying on ELF shared objects and the DL (dynamic linking, the one used with the -ldl flag) library.

1. list the shared object in the current directory;
2. open and load the plugin code.

The function opendir opens a location given as string and returns a handle DIR. Once we have the DIR handle we can list the files in the directory calling readdir until it returns NULL. (Open directories must be closed with closedir).

To open a shared object file we use dlopen which returns an opaque handler. Once we have the handler we can request the plugin variables and functions by name using dlsym. (Open shared object files must be closed with dlclose).

#include<stdio.h>
#include<dirent.h>
#include<libgen.h>
#include<string.h>
#include<dlfcn.h>

typedef void func_t(void);

int main(int argc, char **argv)
{
char *pwd, *str_pos;
DIR *pwdir;
struct dirent* file;
int len;
void *plugin_handler;
char path[512];
func_t *f;

printf("In main of %s\n", argv[0]);
pwd = dirname(argv[0]);
pwdir = opendir(pwd);

{
len = strlen(file->d_name);
if (strcmp(file->d_name+len-3, ".so")==0)
{
printf("found plugin file %s\n", file->d_name);
sprintf(path, "%s/%s", pwd, file->d_name);

plugin_handler = dlopen(path, RTLD_LAZY);
str_pos = (char*) dlsym(plugin_handler, "PLUGIN_NAME");
printf("Executing plugin %s\n", str_pos);
f = (func_t*) dlsym(plugin_handler, "speak");
f();  // here we call the plugin specific method!!
dlclose(plugin_handler);
}
}

closedir(pwdir);

return 0;
}


The code above can be compiled with meson and the following meson.build file:

project('plugin_fun', 'c')
dl = meson.get_compiler('c').find_library('dl')
executable('fun', 'main.c', dependencies : dl)
shared_library('doggy', 'doggy.c')
shared_library('kitty', 'kitty.c')