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Date: Wed, 11 Dec 2019 13:09:36 +0300
From: Scherbatiy Alexander <>
Subject: RTLD_LAZY deferred symbol binding


musl libc release 1.1.17 has new feature [1]:
- RTLD_LAZY deferred symbol binding, functionally equivalent to lazy binding

The lazy bindings section [2] gives more details on it:
Newer versions of musl implement “deferred binding” in place of lazy binding, whereby binding is deferred until a subsequent dlopen call that introduces new symbols, rather than at the point of the function call.

It is still is not clear for me what is a difference between of lazy and deferred binding.

I wrote a simple example there a shared library with an unresolved symbols is loaded by dlopen with RTLD_LAZY option (source code is at the end of the email).
It works on my Ubuntu desktop but fails on Alpine linux  3.10.3 with musl libc  1.1.22 (x86_64) with message:
"dlopen failed: Error relocating bin/shared/ unresolved_function: symbol not found"

What is a good example that can show how the new "deferred symbol binding" feature works so it fails before muls libc 1.1.17 and starts working after it? 



Loading a shared library with unresolved symbols example:

--- include/resolved_lib.h ---
void resolved_function();

--- include/unresolved_lib.h ---
void unresolved_function();

--- include/shared_lib.h ---
void call_resolved_function();
void call_unresolved_function();

--- src/resolved_impl.c ---
#include <stdio.h>
#include "resolved_lib.h"

void resolved_function() {
    printf("call resolved function.\n");

---  src/shared_lib.c ---
#include "shared_lib.h"
#include "resolved_lib.h"
#include "unresolved_lib.h"

void call_resolved_function() {

void call_unresolved_function() {

--- src/main.c ---
#include <stdio.h>
#include <stdlib.h>
#include <dlfcn.h>

void call_resolved_function_dynamic() {

    const char *lib_path = "bin/shared/";
    void (*call_resolved_function)(void);

    void *handle = dlopen(lib_path, RTLD_LAZY);

    if (!handle) {
        fprintf(stderr, "dlopen failed: %s\n", dlerror());

    *(void **) (&call_resolved_function) = dlsym(handle, "call_resolved_function");

    char *error = dlerror();
    if (error != NULL)  {
        fprintf(stderr, "%s\n", error);


int main(int argc, char* argv[]) {
    printf("call main.\n");

--- ---
# build sources
gcc -c -fPIC src/resolved_impl.c -Iinclude -o bin/shared/resolved_impl.o
gcc -c -fPIC src/shared_lib.c    -Iinclude -o bin/shared/shared_lib.o
gcc -shared bin/shared/shared_lib.o bin/shared/resolved_impl.o -Iinclude -o bin/shared/
gcc -c src/main.c                -Iinclude -o bin/main.o
gcc bin/main.o -ldl   -o bin/main

# run
export LD_LIBRARY_PATH=./bin/shared
--- ---

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