sys/libc: add endian.h

This provides glibc, NetBSD, FreeBSD compatible endian.h header with a
lean and simple API to convert between host byte order to little endian
and big endian and the other way around.

sys/libc/include/endian.h

@@ -0,0 +1,114 @@
+/*
+ * Copyright (C) 2024 Otto-von-Guericke-Universität Magdeburg
+ *
+ * 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.
+ */
+
+/**
+ * @addtogroup posix
+ * @{
+ */
+/**
+ * @file
+ * @brief       libc header for endian conversion
+ *
+ * @author      Marian Buschsieweke <[email protected]>
+ */
+
+#ifndef ENDIAN_H
+#define ENDIAN_H
+
+#include <stdint.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifdef DOXYGEN
+/**
+ * @brief   A numeric constant representing little endian byte order
+ */
+#define LITTLE_ENDIAN   implementation_defined
+
+/**
+ * @brief   A numeric constant representing big endian byte order
+ */
+#define BIG_ENDIAN      implementation_defined
+
+/**
+ * @brief   A numeric constant representing PDP endian byte order
+ */
+#define PDP_ENDIAN      implementation_defined
+
+/**
+ * @brief   The byte order of this machines indicated by the constant
+ *          @ref BIG_ENDIAN or @ref LITTLE_ENDIAN
+ *
+ * @note    This numeric constant is available at preprocessor time, so you
+ *          can compare this to @ref BIG_ENDIAN or @ref LITTLE_ENDIAN in
+ *          `#if` directives.
+ */
+#define BYTE_ORDER      <LITTLE_ENDIAN or BIG_ENDIAN>
+
+uint16_t htobe16(uint16_t host_16bits);         /**< host to big endian, 16 bit */
+uint16_t htole16(uint16_t host_16bits);         /**< host to little endian, 16 bit */
+uint16_t be16toh(uint16_t big_endian_16bits);   /**< big endian to host, 16 bit */
+uint16_t le16toh(uint16_t little_endian_16bits);/**< little endian to host, 16 bit */
+
+uint32_t htobe32(uint32_t host_32bits);         /**< host to big endian, 32 bit */
+uint32_t htole32(uint32_t host_32bits);         /**< host to little endian, 32 bit */
+uint32_t be32toh(uint32_t big_endian_32bits);   /**< big endian to host, 32 bit */
+uint32_t le32toh(uint32_t little_endian_32bits);/**< little endian to host, 32 bit */
+
+uint64_t htobe64(uint64_t host_64bits);         /**< host to big endian, 64 bit */
+uint64_t htole64(uint64_t host_64bits);         /**< host to little endian, 64 bit */
+uint64_t be64toh(uint64_t big_endian_64bits);   /**< big endian to host, 64 bit */
+uint64_t le64toh(uint64_t little_endian_64bits);/**< little endian to host, 64 bit */
+
+#else /* DOXYGEN */
+
+#define LITTLE_ENDIAN   _LITTLE_ENDIAN
+#define BIG_ENDIAN      _BIG_ENDIAN
+#define PDP_ENDIAN      _PDP_ENDIAN
+#define BYTE_ORDER      _BYTE_ORDER
+
+#if BYTE_ORDER == LITTLE_ENDIAN
+#  define htobe16(_x) __builtin_bswap16(_x)
+#  define htole16(_x) ((__uint16_t)(_x))
+#  define be16toh(_x) __builtin_bswap16(_x)
+#  define le16toh(_x) ((__uint16_t)(_x))
+#  define htobe32(_x) __builtin_bswap32(_x)
+#  define htole32(_x) ((__uint32_t)(_x))
+#  define be32toh(_x) __builtin_bswap32(_x)
+#  define le32toh(_x) ((__uint32_t)(_x))
+#  define htobe64(_x) __builtin_bswap64(_x)
+#  define htole64(_x) ((__uint64_t)(_x))
+#  define be64toh(_x) __builtin_bswap64(_x)
+#  define le64toh(_x) ((__uint64_t)(_x))
+#elif BYTE_ORDER == BIG_ENDIAN
+#  define htole16(_x) __builtin_bswap16(_x)
+#  define htobe16(_x) ((__uint16_t)(_x))
+#  define le16toh(_x) __builtin_bswap16(_x)
+#  define be16toh(_x) ((__uint16_t)(_x))
+#  define htole32(_x) __builtin_bswap32(_x)
+#  define htobe32(_x) ((__uint32_t)(_x))
+#  define le32toh(_x) __builtin_bswap32(_x)
+#  define be32toh(_x) ((__uint32_t)(_x))
+#  define htole64(_x) __builtin_bswap64(_x)
+#  define htobe64(_x) ((__uint64_t)(_x))
+#  define le64toh(_x) __builtin_bswap64(_x)
+#  define be64toh(_x) ((__uint64_t)(_x))
+#else
+#  error "Byte order not supported"
+#endif
+
+#endif /* DOXYGEN */
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* ENDIAN_H */
+/** @} */