tests: posix: common: check return value in pthread_join()

Ensure that the thread return value is set by `pthread_join()`
when `status` is non-NULL.

Signed-off-by: Christopher Friedt <cfriedt@meta.com>

tests/posix/common/src/pthread.c

@@ -793,9 +793,12 @@ ZTEST(posix_apis, test_pthread_equal)
  zassert_false(pthread_equal(pthread_self(), (pthread_t)4242));
 }
 
+/* A 32-bit value to use between threads for validation */
+#define BIOS_FOOD 0xB105F00D
+
 static void *fun(void *arg)
 {
- *((uint32_t *)arg) = 0xB105F00D;
+ *((uint32_t *)arg) = BIOS_FOOD;
  return NULL;
 }
 
@@ -810,7 +813,28 @@ ZTEST(posix_apis, test_pthread_dynamic_stacks)
 
  zassert_ok(pthread_create(&th, NULL, fun, &x));
  zassert_ok(pthread_join(th, NULL));
- zassert_equal(0xB105F00D, x);
+ zassert_equal(BIOS_FOOD, x);
+}
+
+static void *non_null_retval(void *arg)
+{
+ ARG_UNUSED(arg);
+
+ return (void *)BIOS_FOOD;
+}
+
+ZTEST(posix_apis, test_pthread_return_val)
+{
+ pthread_t pth;
+ void *ret = NULL;
+ pthread_attr_t attr;
+
+ zassert_ok(pthread_attr_init(&attr));
+ zassert_ok(pthread_attr_setstack(&attr, &stack_e[0][0], STACKS));
+
+ zassert_ok(pthread_create(&pth, &attr, non_null_retval, NULL));
+ zassert_ok(pthread_join(pth, &ret));
+ zassert_equal(ret, (void *)BIOS_FOOD);
 }
 
 static void *detached(void *arg)
@@ -831,7 +855,7 @@ ZTEST(posix_apis, test_pthread_join_detached)
  zassert_ok(pthread_create(&pth, &attr, detached, NULL));
  zassert_ok(pthread_detach(pth));
  /* note, this was required to be EINVAL previously but is now undefined behaviour */
- zassert_equal(EINVAL, pthread_join(pth, NULL));
+ zassert_not_equal(0, pthread_join(pth, NULL));
 
  /* need to allow this thread to be clean-up by the recycler */
  k_msleep(500);