Add a compat dummy stdatomic.h used when threading is disabled

Adapted from the code by Rémi Denis-Courmont from VLC

compat/atomics/dummy/stdatomic.h

+/*
+ * This file is part of Libav.
+ *
+ * Libav is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2.1 of the License, or (at your option) any later version.
+ *
+ * Libav is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with Libav; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+/*
+ * based on vlc_atomic.h from VLC
+ * Copyright (C) 2010 Rémi Denis-Courmont
+ */
+
+#ifndef LIBAV_COMPAT_ATOMICS_DUMMY_STDATOMIC_H
+#define LIBAV_COMPAT_ATOMICS_DUMMY_STDATOMIC_H
+
+#include <stdint.h>
+
+#define ATOMIC_FLAG_INIT 0
+
+#define ATOMIC_VAR_INIT(value) (value)
+
+#define atomic_init(obj, value) \
+do {                            \
+    *(obj) = (value);           \
+} while(0)
+
+#define kill_dependency(y) ((void)0)
+
+#define atomic_thread_fence(order) \
+    ((void)0)
+
+#define atomic_signal_fence(order) \
+    ((void)0)
+
+#define atomic_is_lock_free(obj) 0
+
+typedef intptr_t atomic_flag;
+typedef intptr_t atomic_bool;
+typedef intptr_t atomic_char;
+typedef intptr_t atomic_schar;
+typedef intptr_t atomic_uchar;
+typedef intptr_t atomic_short;
+typedef intptr_t atomic_ushort;
+typedef intptr_t atomic_int;
+typedef intptr_t atomic_uint;
+typedef intptr_t atomic_long;
+typedef intptr_t atomic_ulong;
+typedef intptr_t atomic_llong;
+typedef intptr_t atomic_ullong;
+typedef intptr_t atomic_wchar_t;
+typedef intptr_t atomic_int_least8_t;
+typedef intptr_t atomic_uint_least8_t;
+typedef intptr_t atomic_int_least16_t;
+typedef intptr_t atomic_uint_least16_t;
+typedef intptr_t atomic_int_least32_t;
+typedef intptr_t atomic_uint_least32_t;
+typedef intptr_t atomic_int_least64_t;
+typedef intptr_t atomic_uint_least64_t;
+typedef intptr_t atomic_int_fast8_t;
+typedef intptr_t atomic_uint_fast8_t;
+typedef intptr_t atomic_int_fast16_t;
+typedef intptr_t atomic_uint_fast16_t;
+typedef intptr_t atomic_int_fast32_t;
+typedef intptr_t atomic_uint_fast32_t;
+typedef intptr_t atomic_int_fast64_t;
+typedef intptr_t atomic_uint_fast64_t;
+typedef intptr_t atomic_intptr_t;
+typedef intptr_t atomic_uintptr_t;
+typedef intptr_t atomic_size_t;
+typedef intptr_t atomic_ptrdiff_t;
+typedef intptr_t atomic_intmax_t;
+typedef intptr_t atomic_uintmax_t;
+
+#define atomic_store(object, desired)   \
+do {                                    \
+    *(object) = (desired);              \
+} while (0)
+
+#define atomic_store_explicit(object, desired, order) \
+    atomic_store(object, desired)
+
+#define atomic_load(object) \
+    (*(object))
+
+#define atomic_load_explicit(object, order) \
+    atomic_load(object)
+
+static inline intptr_t atomic_exchange(intptr_t *object, intptr_t desired)
+{
+    intptr_t ret = *object;
+    *object = desired;
+    return ret;
+}
+
+#define atomic_exchange_explicit(object, desired, order) \
+    atomic_exchange(object, desired)
+
+static inline int atomic_compare_exchange_strong(intptr_t *object, intptr_t *expected,
+                                                 intptr_t desired)
+{
+    int ret;
+    if (*object == *expected) {
+        *object = desired;
+        ret = 1;
+    } else {
+        *expected = *object;
+        ret = 0;
+    }
+    return ret;
+}
+
+#define atomic_compare_exchange_strong_explicit(object, expected, desired, success, failure) \
+    atomic_compare_exchange_strong(object, expected, desired)
+
+#define atomic_compare_exchange_weak(object, expected, desired) \
+    atomic_compare_exchange_strong(object, expected, desired)
+
+#define atomic_compare_exchange_weak_explicit(object, expected, desired, success, failure) \
+    atomic_compare_exchange_weak(object, expected, desired)
+
+#define FETCH_MODIFY(opname, op)                                            \
+static inline intptr_t atomic_fetch_ ## opname(intptr_t *object, intptr_t operand) \
+{                                                                    \
+    intptr_t ret;                                                    \
+    ret = *object;                                                   \
+    *object = *object op operand;                                    \
+    return ret;                                                      \
+}
+
+FETCH_MODIFY(add, +)
+FETCH_MODIFY(sub, -)
+FETCH_MODIFY(or,  |)
+FETCH_MODIFY(xor, ^)
+FETCH_MODIFY(and, &)
+
+#undef FETCH_MODIFY
+
+#define atomic_fetch_add_explicit(object, operand, order) \
+    atomic_fetch_add(object, operand)
+
+#define atomic_fetch_sub_explicit(object, operand, order) \
+    atomic_fetch_sub(object, operand)
+
+#define atomic_fetch_or_explicit(object, operand, order) \
+    atomic_fetch_or(object, operand)
+
+#define atomic_fetch_xor_explicit(object, operand, order) \
+    atomic_fetch_sub(object, operand)
+
+#define atomic_fetch_and_explicit(object, operand, order) \
+    atomic_fetch_and(object, operand)
+
+#define atomic_flag_test_and_set(object) \
+    atomic_exchange(object, 1)
+
+#define atomic_flag_test_and_set_explicit(object, order) \
+    atomic_flag_test_and_set(object)
+
+#define atomic_flag_clear(object) \
+    atomic_store(object, 0)
+
+#define atomic_flag_clear_explicit(object, order) \
+    atomic_flag_clear(object)
+
+#endif /* LIBAV_COMPAT_ATOMICS_DUMMY_STDATOMIC_H */

configure

@@ -5083,6 +5083,9 @@ if disabled stdatomic_h; then
     elif enabled pthreads; then
         add_compat atomics/pthread/stdatomic.o
         add_cppflags '-I\$(SRC_PATH)/compat/atomics/pthread'
+    else
+        enabled threads && die "Threading is enabled, but no atomics are available"
+        add_cppflags '-I\$(SRC_PATH)/compat/atomics/dummy'
     fi
 fi