Re: Problems with pthreads from a shared object?

[Rich Felker <dalias@libc.org>]

On Wed, Nov 07, 2018 at 06:00:14PM -0500, Barry Flartus wrote:
> Hello,
> 
> 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 test.so
> 
> #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("./test.so", RTLD_NOW);
>     if (!handle) {
>         fprintf(stderr, "%s\n", dlerror());
>         exit(EXIT_FAILURE);
>     }
>     dlerror();
> 
>     return EXIT_SUCCESS;
> }
> 
> When the above examples are compiled with gcc, everything works correctly
> and
> 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
> errno.h
> but I can't seem to figure out why this is happening.
> 
> Output:
> 
> 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!

Are you running on an ancient kernel or a kernel built with a lot of
default functionality configured out? These should be the only
conditions under which pthread_create can return ENOSYS.

Rich