kernel: types: add k_off_t and k_ssize_t types

Define k_off_t to have native support for measuring file offsets
within any kind of file that is supportable in Zephyr.

CONFIG_OFFSET_64BIT is added to allow 32-bit users to force
64-bit offsets, when trading space and speed for flexibility
makes sense for the application.

Define k_ssize_t to have native support for reporting file I/O
sizes along with negative error codes.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

include/zephyr/types.h

@@ -29,6 +29,44 @@ typedef union {
 	void            (*thepfunc)(void);
 } z_max_align_t;
 
+/*
+ * Offset type
+ *
+ * k_off_t, much like the POSIX off_t, is a signed integer type used to represent file sizes and
+ * offsets.
+ *
+ * k_off_t effectively limits the size of files that can be handled by the system, it is
+ * therefore not tied to the word-size of the architecture.
+ *
+ * In order to overcome historical limitations of the off_t type, Zephyr may be configured to
+ * always define k_off_t as 64-bits.
+ */
+#ifdef CONFIG_OFFSET_64BIT
+typedef int64_t k_off_t;
+#else
+typedef long k_off_t;
+#endif
+
+/*
+ * Signed size type
+ *
+ * k_ssize_t, much like the POSIX ssize_t, is a signed integer type that effectively limits the
+ * size of I/O operations on files while still supporting negative return values.
+ *
+ * k_ssize_t is usually tied to the word-size of the architecture.
+ */
+#ifdef __SIZE_TYPE__
+#define unsigned signed /* parasoft-suppress MISRAC2012-RULE_20_4-a MISRAC2012-RULE_20_4-b */
+typedef __SIZE_TYPE__ k_ssize_t;
+#undef unsigned
+#else
+#ifdef CONFIG_64BIT
+typedef long k_ssize_t;
+#else
+typedef int k_ssize_t;
+#endif
+#endif
+
 #ifdef __cplusplus
 /* Zephyr requires an int main(void) signature with C linkage for the application main if present */
 extern int main(void);

kernel/Kconfig

@@ -866,6 +866,23 @@ config XIP
 	  supply a linker command file when building your image. Enabling this
 	  option increases both the code and data footprint of the image.
 
+config OFFSET_64BIT
+	bool "Use 64-bit offsets"
+	help
+	  Select 'y' here to ensure k_off_t is always 64-bits. This may be
+	  useful on platforms with files, disks, and memories larger than 2 GiB.
+
+	  If this option is not selected (the default), then the size of k_off_t
+	  will match the architecture word size (like size_t does currently).
+
+	  On 32-bit platforms, enabling this feature trades space and speed for
+	  flexibiltity, since using 64-bit offsets may require more instructions
+	  for a given operation. For example, a 64-bit load may require two 32-bit
+	  loads, and a 64-bit divide may need to be completed with several
+	  32-bit instructions (the operation is emulated via a compiler built-in
+	  function).
+
+	  Note: offsets have a signed integer representation.
 
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