Load libraries dynamically runtime

C++ linux

Application

#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h> // for dlopen, dlsym, dlclose
int main(int argc, char** argv) {
    if (argc < 3) {
        printf("usage: %s <library> <number>\n", argv[0]);
        exit(1);
    }
    char* libPath = argv[1];
    int number = atoi(argv[2]);
    void* libHandle = dlopen(libPath, RTLD_LAZY);
    if (libHandle == nullptr) {
        perror("dlopen failed");
        exit(1);
    }
    typedef int (*func)(int);
    func f;
    f = (func)dlsym(libHandle, "perform");
    if (f == nullptr) {
        perror("dlsym failed");
        exit(1);
    }
    printf("%d --> %d\n", number, f(number));
    dlclose(libHandle);
    return 0;
}

Library

extern "C" {
// C++ mangles function name. So, "C" binding is required to search a function for dlsym
int perform(int i);
}
const char* getLibraryName() {
    return "add1";
}
int perform(int i) {
    return i+1;
}

Makefile

make sure to add `-Wl, — no-as-needed -ldl`

CC=g++
CFLAGS=-Wall -g
BINS=libplus1.so test.elf
all: $(BINS)
%.so: %.cpp
 $(CC) $(CFLAGS) -fPIC -shared -o $@ $^
%.elf: %.cpp
 $(CC) $(CFLAGS) -Wl,--no-as-needed -ldl -o $@ $^
clean:
 rm $(BINS)
 rm -r *.so

Run

$ ./test.elf ./libplus1.so 4
4 --> 5