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Date: Wed, 7 Nov 2018 18:00:14 -0500
From: Barry Flartus <>
Subject: Problems with pthreads from a shared object?


I am encountering some issues that I believe are musl related when trying to
use pthreads from a shared object. Below is a minimal example (2 files) of
the problem.

==================== test.c ====================

// Compile with
// musl-gcc -Wall -lpthread -shared -fPIC test.c -o

#include <pthread.h>
#include <stdio.h>

static void *launch(void *arg);
int start_thread(void);
void entry_point(void) __attribute__((constructor));

static pthread_t gthread;

static void *launch(void *arg)
    for (int i = 0; i < 4; i++){
        printf("[%d] Hello from the thread\n", i);
    return NULL;

int start_thread(void)
    printf("Starting the thread\n");
    return pthread_create(&gthread, NULL, launch, NULL);

void entry_point(void){
    puts("Starting Execution");
    int res = start_thread();
    printf("start_thread returned %d\n", res);
    if (res == 0){
        pthread_join(gthread, NULL);
    } else {
        printf("pthread_create() returned an error. Aborting.\n");

==================== launch.c ====================

// Compile with:
// gcc launch.c -ldl -o launch

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

int main(){
    void *handle;

    puts("\nLaunching test...");
    handle = dlopen("./", RTLD_NOW);
    if (!handle) {
        fprintf(stderr, "%s\n", dlerror());

    return EXIT_SUCCESS;

When the above examples are compiled with gcc, everything works correctly
produces the following output:

Launching test...
Starting Execution
Starting the threadstart_thread returned 0
[0] Hello from the thread
[1] Hello from the thread
[2] Hello from the thread
[3] Hello from the thread

When I switch to musl pthread_create() returns the integer '38', which is an
undocumented return value. I assume this corresponds with ENOSYS from
but I can't seem to figure out why this is happening.


Launching test...
Starting Execution
Starting the thread
start_thread returned 38
pthread_create() returned an error. Aborting.

Is this even supported by musl? If so, where have I gone wrong?

Thanks in advance!

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