Merge pull request #10797 from kaspar030/add_unaligned

sys: add unaligned.h (header for alignment safe value-from-pointer functions)

sys/checksum/fletcher32.c

@@ -18,6 +18,7 @@
  * @}
  */
 
+#include "unaligned.h"
 #include "checksum/fletcher32.h"
 
 uint32_t fletcher32(const uint16_t *data, size_t words)
@@ -28,7 +29,7 @@ uint32_t fletcher32(const uint16_t *data, size_t words)
         unsigned tlen = words > 359 ? 359 : words;
         words -= tlen;
         do {
-            sum2 += sum1 += *data++;
+            sum2 += sum1 += unaligned_get_u16(data++);
         } while (--tlen);
         sum1 = (sum1 & 0xffff) + (sum1 >> 16);
         sum2 = (sum2 & 0xffff) + (sum2 >> 16);

sys/include/unaligned.h

+/*
+ * Copyright (C) 2019 Kaspar Schleiser <kaspar@schleiser.de>
+ *
+ * This file is subject to the terms and conditions of the GNU Lesser
+ * General Public License v2.1. See the file LICENSE in the top level
+ * directory for more details.
+ */
+
+/**
+ * @defgroup    sys_unaligned unaligned memory access methods
+ * @ingroup     sys
+ * @brief       Provides functions for safe unaligned memory accesses
+ *
+ * This header provides functions to read values from pointers that are not
+ * necessarily aligned to the type's alignment requirements.
+ *
+ * E.g.,
+ *
+ *     uint16_t *foo = 0x123;
+ *     printf("%u\n", *foo);
+ *
+ * ... might cause an unaligned access, if `uint16_t` is usually aligned at
+ * 2-byte-boundaries, as foo has an odd address.
+ *
+ * The current implementation casts a pointer to a packed struct, which forces
+ * the compiler to deal with possibly unalignedness.  Idea taken from linux
+ * kernel sources.
+ *
+ * @{
+ *
+ * @file
+ * @brief       Unaligned but safe memory access functions
+ *
+ * @author      Kaspar Schleiser <kaspar@schleiser.de>
+ */
+
+#ifndef UNALIGNED_H
+#define UNALIGNED_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/** @brief Unaligned access helper struct (uint16_t version) */
+typedef struct __attribute__((packed)) {
+    uint16_t val;       /**< value */
+} uint16_una_t;
+
+/**
+ * @brief    Get uint16_t from possibly unaligned pointer
+ *
+ * @param[in]   ptr pointer to read from
+ *
+ * @returns value read from @p ptr
+ */
+static inline uint16_t unaligned_get_u16(const void *ptr)
+{
+    const uint16_una_t *tmp = ptr;
+    return tmp->val;
+}
+
+#ifdef __cplusplus
+}
+#endif
+
+/** @} */
+#endif /* UNALIGNED_H */