dlopen demo

dlopen_demo.c

#define _POSIX_C_SOURCE 200809L
#define _ISOC99_SOURCE
#define _XOPEN_SOURCE 700

#include "library.h"

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

#define LIB_FILE "liblib.so"
#define FUNC_NAME "libfunction"

int main(int argc, char **argv)
{
    int ret;
    void *lib_handle;
    int (*func_handle)(void);

    // Suppress unused warnings
    (void) argc;
    (void) argv;

    // Load the library
    lib_handle = dlopen(LIB_FILE, RTLD_NOW);
    if (lib_handle == NULL)
    {
        fprintf(stderr, "Failed to open library %s\n", dlerror());
        return EXIT_FAILURE;
    }

    // Load the function and assign it to a function pointer so that we can call it
    // See man page about possible compiler warnings about casting to function pointer
    func_handle = dlsym(lib_handle, FUNC_NAME);
    if (func_handle == NULL)
    {
        fprintf(stderr, "Failed to load function %s: %s\n", FUNC_NAME, dlerror());
        return EXIT_FAILURE;
    }

    // Call the function
    printf("Library function return value: %d\n", (*func_handle)());

    // Close the library
    ret = dlclose(lib_handle);
    if (ret != 0)
    {
        fprintf(stderr, "WARNING dlclose failed: %s\n", dlerror());
    }

    return EXIT_SUCCESS;
}

library.c

#include "library.h"

int libfunction()
{
    return 42;
}

library.h

#ifndef DLOPEN_DEMO_LIBRARY_H
#define DLOPEN_DEMO_LIBRARY_H

int libfunction();

#endif //DLOPEN_DEMO_LIBRARY_H

Makefile

CC=gcc -std=c99
LD=$(CC)
CFLAGS=-Wall -Wextra -Wfloat-equal -Wundef -Wcast-align -Wpointer-arith -Wcast-qual -g -O
# -ldl is necessary for dlopen to work and it ensures that the library is dynamically loaded
# and not loaded automatically by the application
# -Wl,-R. tells the application to look for libraries in the current directory during runtime
LFLAGS=-ldl -Wl,-R.

DEPS=library.h
LIB_NAME=liblib.so
APP_NAME=dlopen_demo

all: $(APP_NAME) $(LIB_NAME)

%.o: %.c $(DEPS)
	$(CC) -c -o $@ $< $(CFLAGS) $(IFLAGS) -fPIC

$(APP_NAME): % : %.o
	$(LD) -o $@ $^ $(LIBS) $(LFLAGS)

$(LIB_NAME): library.o
	$(LD) -o $@ $^ $(LIBS) $(LFLAGS) -shared

clean:
	rm -rf $(LIB_NAME) $(APP_NAME) *.o

.PHONY: all clean