1 files changed, 58 insertions, 0 deletions

diff --git a/zutil.c b/zutil.c
index 6e8a369..3a94e91 100644
--- a/zutil.c
+++ b/zutil.c
@@ -305,3 +305,61 @@ void ZLIB_INTERNAL zcfree(voidpf opaque, voidpf ptr) {
 #endif /* MY_ZCALLOC */
 
 #endif /* !Z_SOLO */
+
+#if defined(BUILDFIXED) || defined(DYNAMIC_CRC_TABLE)
+/*
+  Define a z_once() function depending on the availability of atomics.
+ */
+
+/* Check for the availability of atomics. */
+#if defined(__STDC__) && __STDC_VERSION__ >= 201112L && \
+    !defined(__STDC_NO_ATOMICS__)
+
+#include <stdatomic.h>
+
+/*
+  Run the provided init() function exactly once, even if multiple threads
+  invoke once() at the same time. The state must be a once_t initialized with
+  Z_ONCE_INIT.
+ */
+void z_once(z_once_t *state, void (*init)(void)) {
+    if (!atomic_load(&state->done)) {
+        if (atomic_flag_test_and_set(&state->begun))
+            while (!atomic_load(&state->done))
+                ;
+        else {
+            init();
+            atomic_store(&state->done, 1);
+        }
+    }
+}
+
+#else   /* no atomics */
+
+#warning zlib not thread-safe
+
+/* Test and set. Alas, not atomic, but tries to limit the period of
+   vulnerability. */
+local int test_and_set(int volatile *flag) {
+    int was;
+
+    was = *flag;
+    *flag = 1;
+    return was;
+}
+
+/* Run the provided init() function once. This is not thread-safe. */
+void z_once(z_once_t *state, void (*init)(void)) {
+    if (!state->done) {
+        if (test_and_set(&state->begun))
+            while (!state->done)
+                ;
+        else {
+            init();
+            state->done = 1;
+        }
+    }
+}
+
+#endif
+#endif