diff --git a/inc/os_win/threads_types.h b/inc/os_win/threads_types.h
index 1173355..1da440f 100644
--- a/inc/os_win/threads_types.h
+++ b/inc/os_win/threads_types.h
@@ -4,6 +4,8 @@
 
 #pragma once
 
+#define ONCE_FLAG_INIT ((once_flag){0})
+
 typedef struct thrd_t {
     void *handle;
 } thrd_t;
@@ -12,6 +14,13 @@ typedef struct tss_t {
     unsigned tls_index;
 } tss_t;
 
+// We pretend that once_flag defined the same way as INIT_ONCE
+// from winapi headers (aka _RTL_RUN_ONCE), which itself is defined
+// as a union
+typedef union once_flag {
+    void *ptr;
+} once_flag;
+
 typedef struct cnd_t {
     int idk_yet;
 } cnd_t;
diff --git a/inc/threads.h b/inc/threads.h
index 243c25e..c9b594d 100644
--- a/inc/threads.h
+++ b/inc/threads.h
@@ -1,8 +1,16 @@
 #pragma once
 
+// Note(bumbread): some of the macros and types are platform-dependent
+// and can be found in os_win/ subfolder.
+
+#if defined(_WIN32)
+    #include "os_win/threads_types.h"
+#else
+    #error "Not implemented"
+#endif
+
 // 7.28.1 p.3: Macros
 
-#define ONCE_FLAG_INIT      1
 #define TSS_DTOR_ITERATIONS 32
 
 // TODO(bumbread): check the spec for whether thread_local needs to be declared
@@ -20,19 +28,9 @@
     };
 #endif
 
-#if defined(_WIN32)
-    #include "os_win/threads_types.h"
-#else
-    #error "Not implemented"
-#endif
-
 typedef void(*tss_dtor_t)  (void*);
 typedef int (*thrd_start_t)(void*);
 
-// TODO(bumbread): this probably should be a mutex or a semaphore
-// also probably can be implemented with interlocked increment
-typedef int once_flag;
-
 // 7.28.1 p.5: Enumeration constants
 
 enum {
diff --git a/src/os_win/thread.c b/src/os_win/thread.c
index 810254c..8fe6c73 100644
--- a/src/os_win/thread.c
+++ b/src/os_win/thread.c
@@ -130,16 +130,21 @@ _Noreturn void thrd_exit(int res) {
 #define TSS_KEYS_MAX 1088
 
 static tss_dtor_t _tss_dtors[TSS_KEYS_MAX];
+static bool _tss_init[TSS_KEYS_MAX];
 
 static void _thread_cleanup() {
     for(int i = 0; i != TSS_DTOR_ITERATIONS; ++i) {
-        for(unsigned k = 0; k != TSS_KEYS_MAX; ++k) {
+        for(unsigned k = 1; k != TSS_KEYS_MAX; ++k) {
+            if(!_tss_init[k]) {
+                continue;
+            }
             void *data = TlsGetValue(k);
-            if(data != NULL) {
-                TlsSetValue(k, NULL);
-                if(_tss_dtors[k]) {
-                    _tss_dtors[k](data);
-                }
+            if(data == NULL) {
+                continue;
+            }
+            TlsSetValue(k, NULL);
+            if(_tss_dtors[k]) {
+                _tss_dtors[k](data);
             }
         }
     }
@@ -156,11 +161,13 @@ int tss_create(tss_t *key, tss_dtor_t dtor) {
         TlsFree(tls_index);
         return thrd_error;
     }
+    _tss_init[tls_index] = true;
     _tss_dtors[tls_index] = dtor;
     return thrd_success;
 }
 
 void tss_delete(tss_t key) {
+    _tss_init[key.tls_index] = false;
     _tss_dtors[key.tls_index] = NULL;
     TlsFree(key.tls_index);
 }
@@ -180,6 +187,18 @@ int tss_set(tss_t key, void *val) {
    return thrd_success;
 }
 
+// Call once
+
+static BOOL _call_once_trampoline(PINIT_ONCE init_once, PVOID param, PVOID *ctx) {
+    void (*user_func)(void) = *ctx;
+    user_func();
+    return TRUE;
+}
+
+void call_once(once_flag *flag, void (*func)(void)) {
+    InitOnceExecuteOnce((void *)flag, _call_once_trampoline, NULL, (void **)&func);
+}
+
 // Mutex functions
 
 void mtx_destroy(mtx_t *mtx) {
diff --git a/test/test_threads.c b/test/test_threads.c
index 965e6a2..9d11c8b 100644
--- a/test/test_threads.c
+++ b/test/test_threads.c
@@ -2,33 +2,36 @@
 #include <threads.h>
 #include <stdatomic.h>
 
+#define N_THREADS 1
+
 _Thread_local int counter;
-tss_t key;
+once_flag flag = ONCE_FLAG_INIT;
+
+void init() {
+    puts("Hey I got a call");
+}
 
 int f(void* thr_data) {
-    tss_set(key, "Thread 2 finished");
+    call_once(&flag, init);
     for(int n = 0; n < 5; ++n)
         counter++;
-    puts(tss_get(key));
+    puts("Finished");
     return 0;
 }
 
 int main(void)
 {
-    tss_create(&key, NULL);
-    thrd_t thread;
-    int status = thrd_create(&thread, f, NULL);
-    if(status == thrd_error) {
-        puts("Failed creating threads");
+    thrd_t thread[N_THREADS];
+    for(int i = 0; i != N_THREADS; ++i) {
+        int status = thrd_create(&thread[i], f, NULL);
+        if(status == thrd_error) {
+            puts("Failed creating threads");
+        }
     }
-    for(int n = 0; n < 10; ++n) {
-        counter++;
+    for(int i = 0; i != N_THREADS; ++i) {
+        int res;
+        if(thrd_join(thread[i], &res) == thrd_error) {
+            puts("Failed waiting on thread");
+        }
     }
-    tss_set(key, "Thread 1 finished");
-    int res;
-    if(thrd_join(thread, &res) == thrd_error) {
-        puts("Failed waiting on thread");
-    }
-    puts(tss_get(key));
-    tss_delete(key);
 }