lib: add libxpc to implement functionality needed by xpc interfaces

lib/libxpc/include/xpc/string.h

@@ -0,0 +1,85 @@
+#ifndef XPC_STRING_H_
+#define XPC_STRING_H_
+
+#include <stddef.h>
+#include <xpc/status.h>
+
+#define XPC_STRING_NPOS ((size_t)-1)
+
+#define XPC_STRING_IN(msg, offset, size)                                       \
+	{                                                                      \
+		.s_flags = XPC_STRING_F_IN | XPC_STRING_F_REMOTE,              \
+		.s_origin = (msg),                                             \
+		.s_offset = (offset),                                          \
+		.s_len = (size),                                               \
+	}
+#define XPC_STRING_OUT(msg, offset, size)                                      \
+	{                                                                      \
+		.s_flags = XPC_STRING_F_OUT | XPC_STRING_F_REMOTE,             \
+		.s_origin = (msg),                                             \
+		.s_offset = (offset),                                          \
+		.s_len = (size),                                               \
+	}
+
+struct xpc_msg;
+
+typedef enum xpc_string_flags {
+	/* the string can be read from */
+	XPC_STRING_F_IN = 0x01u,
+	/* the string can be written to */
+	XPC_STRING_F_OUT = 0x02u,
+	/* the string is backed by a buffer located in another address space.
+	 * the string can only be accessed via xpc_string_read and/or
+	 * xpc_string_write */
+	XPC_STRING_F_REMOTE = 0x04u,
+	/* free the buffer backing this string when the string is discarded.
+	 * this is only used for out-strings. the buffer must have been
+	 * allocated using xpc_context_alloc, as it will be freed via a call
+	 * to xpc_context_free */
+	XPC_STRING_F_FREE_ON_DISCARD = 0x08u,
+} xpc_string_flags_t;
+
+typedef struct xpc_string {
+	xpc_string_flags_t s_flags;
+	union {
+		struct {
+			/* only valid if F_OUT is set. specifies the maximum
+			 * number of chars that can be written to s_buf,
+			 * including the null terminator. */
+			size_t s_max;
+			/* only valid if F_OUT is set.
+			 * if F_FREE_ON_DISCARD is set, must be either NULL or
+			 * allocated via xpc_context_alloc */
+			const char *s_buf;
+		};
+
+		struct {
+			/* only valid if F_IN is set. offset of the string data
+			 * within the associated message. used when reading
+			 * string data from a message. */
+			size_t s_offset;
+		};
+	};
+
+	/* only valid if F_REMOTE is set.
+	 * used to read/write string data from/to the sender's address space. */
+	const struct xpc_msg *s_origin;
+
+	/* valid for both F_IN and F_OUT strings.
+	 * F_IN: specifies the length of the incoming string data.
+	 * F_OUT: specifies how many characters from s_buf to send. */
+	size_t s_len;
+} xpc_string_t;
+
+extern xpc_status_t xpc_string_read(
+	const xpc_string_t *s,
+	char *out,
+	size_t max,
+	size_t *nr_read);
+extern xpc_status_t xpc_string_write(
+	xpc_string_t *s,
+	const char *in,
+	size_t len,
+	size_t *nr_written);
+
+#endif