Update pthreadpool_thread_(create|join) function signatures.

- `thrd_create` expects a thread function that returns an `int`.
- `pthread_create` expects a thread function that returns a `void*`.

Introduce the `pthreadpool_thread_return_t` type definition that maps to the
correct type depending on the implementation that is used.

PiperOrigin-RevId: 817810757

Author: qukhan

src/pthreads.c

@@ -432,7 +432,7 @@ static uint32_t thread_wrap_up(struct pthreadpool* threadpool,
   return curr_active_threads;
 }
 
-static void* thread_main(void* arg);
+static pthreadpool_thread_return_t thread_main(void* arg);
 
 static void ensure_num_threads(struct pthreadpool* threadpool,
                                uint32_t thread_id) {
@@ -476,7 +476,7 @@ static void ensure_num_threads(struct pthreadpool* threadpool,
   }
 }
 
-static void* thread_main(void* arg) {
+static pthreadpool_thread_return_t thread_main(void* arg) {
   // Unpack the argument, i.e. extract the pointer to the `pthreadpool` from the
   // provided pointer to this thread's `thread_info`.
   struct thread_info* thread = (struct thread_info*)arg;
@@ -558,7 +558,7 @@ static void* thread_main(void* arg) {
   // Release our hold on the threadpool.
   pthreadpool_release(threadpool);
 
-  return NULL;
+  return 0;
 }
 
 static size_t get_num_cpus() {

src/threadpool-atomics.h

@@ -56,6 +56,7 @@
 typedef pthread_t pthreadpool_thread_t;
 typedef pthread_mutex_t pthreadpool_mutex_t;
 typedef pthread_cond_t pthreadpool_cond_t;
+typedef void* pthreadpool_thread_return_t;
 static inline void pthreadpool_mutex_init(pthreadpool_mutex_t* mutex) {
   pthread_mutex_init(mutex, NULL);
 }
@@ -84,18 +85,20 @@ static inline void pthreadpool_cond_signal(pthreadpool_cond_t* cond) {
 static inline void pthreadpool_cond_broadcast(pthreadpool_cond_t* cond) {
   pthread_cond_broadcast(cond);
 }
-static inline void pthreadpool_thread_create(pthreadpool_thread_t* thread,
-                                             void*(fun)(void*), void* arg) {
+static inline void pthreadpool_thread_create(
+    pthreadpool_thread_t* thread, pthreadpool_thread_return_t(fun)(void*),
+    void* arg) {
   pthread_create(thread, NULL, fun, arg);
 }
-static inline void pthreadpool_thread_join(pthreadpool_thread_t thread,
-                                           void* return_value) {
-  pthread_join(thread, (void**)return_value);
+static inline void pthreadpool_thread_join(
+    pthreadpool_thread_t thread, pthreadpool_thread_return_t* return_value) {
+  pthread_join(thread, return_value);
 }
 #else
 typedef thrd_t pthreadpool_thread_t;
 typedef mtx_t pthreadpool_mutex_t;
 typedef cnd_t pthreadpool_cond_t;
+typedef int pthreadpool_thread_return_t;
 static inline void pthreadpool_mutex_init(pthreadpool_mutex_t* mutex) {
   mtx_init(mutex, mtx_plain);
 }
@@ -124,12 +127,13 @@ static inline void pthreadpool_cond_signal(pthreadpool_cond_t* cond) {
 static inline void pthreadpool_cond_broadcast(pthreadpool_cond_t* cond) {
   cnd_broadcast(cond);
 }
-static inline void pthreadpool_thread_create(pthreadpool_thread_t* thread,
-                                             void*(fun)(void*), void* arg) {
+static inline void pthreadpool_thread_create(
+    pthreadpool_thread_t* thread, pthreadpool_thread_return_t(fun)(void*),
+    void* arg) {
   thrd_create(thread, fun, arg);
 }
-static inline void pthreadpool_thread_join(pthreadpool_thread_t thread,
-                                           void* return_value) {
+static inline void pthreadpool_thread_join(
+    pthreadpool_thread_t thread, pthreadpool_thread_return_t* return_value) {
   thrd_join(thread, return_value);
 }
 #endif  // PTHREADPOOL_USE_PTHREADS