dlopen

Learn to load dynamic libraries with dlopen.

What is dlopen?

dlopen is the POSIX call that loads a shared library (.so) at runtime. It is declared in <dlfcn.h>. You load a library with void *h = dlopen(path, flags); if the return is NULL, an error occurred.

If you pass NULL for path, you get a handle to the main program's symbols; this is convenient for tests. flags is either RTLD_LAZY (symbols resolved on first use) or RTLD_NOW (all resolved immediately). RTLD_LAZY gives a fast startup. When done, close the library with dlclose(h).

Loading the main program

The simplest pattern for tests: void *h = dlopen(NULL, RTLD_LAZY);. This gives a handle to the main program's symbols. It usually succeeds because the main program is already loaded.

To print the success state, you can use printf("loaded\n"). Then close with if (h) dlclose(h);. dlclose closes the handle and decrements the reference count. This pattern's output is deterministic; it is environment-independent. Checking dlclose success is also a good habit.

Exercise Load a library

Get a handle to the main program with void *h = dlopen(NULL, RTLD_LAZY);. Print the state with printf("loaded\n");. Close with if (h) dlclose(h);.

Show example solution

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

int main(void) {
    void *h = dlopen(NULL, RTLD_LAZY);
    printf("loaded\n");
    if (h) dlclose(h);
    return 0;
}

Test yourself

Which flag resolves symbols on first use?
- RTLD_LAZY
- RTLD_NOW
- RTLD_GLOBAL
- RTLD_LOCAL
Show answer

RTLD_LAZY — RTLD_LAZY resolves symbols on first use; startup is fast.

dlopen call

void *h = dlopen(NULL, ____);

Show answer

void *h = dlopen(NULL, RTLD_LAZY);

Match dlopen parts

dlopen

Loads library
RTLD_LAZY

Lazy
RTLD_NOW

Immediate
dlclose

Closes

Next lesson: dlsym

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