Add 'String<SIZE>' buffer type to 'util/string.h'

The new 'String' buffer type is meant to replace the manually created
character buffers that are scattered throughout Genode. It plainly holds
a null-terminated string to be stored as a member variable (e.g., a
session label) or passed as RPC argument. It is not intended to become a
string API.

base/include/base/trace/types.h

@@ -25,26 +25,6 @@ namespace Genode { namespace Trace {
 	struct Traced_by_other_session : Exception { };
 	struct Subject_not_traced      : Exception { };
 
-	template <size_t MAX>
-	struct String
-	{
-		enum { MAX_SIZE = MAX };
-		char   buf[MAX];
-		size_t length;
-
-		String() : length(0) { }
-
-		String(char const *str) : length(min(strlen(str) + 1, MAX))
-		{
-			strncpy(buf, str, length);
-		}
-
-		bool valid() const {
-			return (length < MAX) && (length > 0) && (buf[length - 1] == '\0'); }
-
-		char const *string() const { return valid() ? buf : ""; }
-	};
-
 	typedef String<160> Session_label;
 	typedef String<64>  Thread_name;
 

base/include/util/string.h

@@ -447,6 +447,36 @@ namespace Genode {
 
 		return i;
 	}
+
+	/**
+	 * Buffer that contains a null-terminated string
+	 *
+	 * \param SIZE  buffer size
+	 */
+	template <size_t SIZE>
+	class String
+	{
+		private:
+
+			char   _buf[SIZE];
+			size_t _length;
+
+		public:
+
+			constexpr static size_t size() { return SIZE; }
+
+			String() : _length(0) { }
+
+			String(char const *str) : _length(min(strlen(str) + 1, SIZE))
+			{
+				strncpy(_buf, str, _length);
+			}
+
+			bool valid() const {
+				return (_length <= SIZE) && (_buf[_length - 1] == '\0'); }
+
+			char const *string() const { return valid() ? _buf : ""; }
+	};
 }
 
 #endif /* _INCLUDE__UTIL__STRING_H_ */

base/src/core/cpu_session_component.cc

@@ -301,7 +301,7 @@ Cpu_session_component::Cpu_session_component(Rpc_entrypoint         *thread_ep,
 	_trace_sources(trace_sources)
 {
 	/* remember session label */
-	char buf[Session_label::MAX_SIZE];
+	char buf[Session_label::size()];
 	Arg_string::find_arg(args, "label").string(buf, sizeof(buf), "");
 	_label = Session_label(buf);
 

base/src/core/include/trace/root.h

@@ -38,7 +38,7 @@ class Genode::Trace::Root : public Genode::Root_component<Session_component>
 			size_t arg_buffer_size = Arg_string::find_arg(args, "arg_buffer_size").ulong_value(0);
 			unsigned parent_levels = Arg_string::find_arg(args, "parent_levels").ulong_value(0);
 
-			char label[Trace::Session_label::MAX_SIZE];
+			char label[Trace::Session_label::size()];
 			Arg_string::find_arg(args, "label").string(label, sizeof(label), "");
 
 			if (arg_buffer_size > ram_quota)