# Interesting Facts of Pthread

## Fact 0

:::info
:bulb: Calling pthread_exit on main thread may block when the jobs executed by other threads do not complete
:::

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

void *looping(void *args)
{
    (void) args;

    for (;;) {
        // Looping
        printf("Running loop ...\n");
        sleep(1);
    }
}

int main(void)
{
    printf("Creating the thread ...\n");

    pthread_t loop;
    pthread_create(&loop, NULL, looping, NULL);

    pthread_exit(NULL);
}

It may exit immediately if we replace line 24 with `return 0`.

## Fact 1

:::info
:bulb: Checking the return values of pthread_attr_init and pthread_attr_destroy is unneccessary
:::

According to the manual pthread_attr_init(3):

> **RETURN VALUE**
> On success, these functions return 0; on error, they return a nonzero error number.
>
> **ERRORS**
> POSIX.1 documents an ENOMEM error for pthread_attr_init(); on Linux these functions always succeed (but portable and future-proof applications should nevertheless handle a possible error return).

Which means that it will always success on Linux

## Fact 2

:::info
:bulb: pthread_attr_t should be destroyed after used
:::

This is a feature introduced at v2.3.2 [^glibc2.3.2] to enqueue the address of each pthread_attr_t into a global list when debugging pthread library

[^glibc2.3.2]: [glibc-2.3.2/source/nptl/pthread_attr_init.c#L51](https://elixir.bootlin.com/glibc/glibc-2.3.2/source/nptl/pthread_attr_init.c#L51)

int
__pthread_attr_init_2_1 (attr)
     pthread_attr_t *attr;
{
  struct pthread_attr *iattr;

  /* Many elements are initialized to zero so let us do it all at
     once.  This also takes care of clearing the bytes which are not
     internally used.  */
  memset (attr, '\0', __SIZEOF_PTHREAD_ATTR_T);

  assert (sizeof (*attr) >= sizeof (struct pthread_attr));
  iattr = (struct pthread_attr *) attr;

  /* Default guard size specified by the standard.  */
  iattr->guardsize = __getpagesize ();

  /* Enqueue the attributes to the list of all known variables.  */
  if (DEBUGGING_P)
    {
      lll_lock (__attr_list_lock);

      iattr->next = __attr_list;
      __attr_list = iattr;

      lll_unlock (__attr_list_lock);
    }

  return 0;
}
versioned_symbol (libpthread, __pthread_attr_init_2_1, pthread_attr_init,
		  GLIBC_2_1);

But it seems to be removed at v.2.3.4 [^glibc2.3.4]. Leaving the global list `__attr_list` unused.

[^glibc2.3.4]: [glibc-2.3.4/source/nptl/pthread_attr_init.c#L49](https://elixir.bootlin.com/glibc/glibc-2.3.4/source/nptl/pthread_attr_init.c#L49)

## Fact 3

:::info
:bulb: Threads may be considered as zombie if you do not handle the exit of each thread before exiting the main thread
:::