gdb/

* common/linux-procfs.c (linux_proc_get_int): New, from
	linux_proc_get_tgid, change its LWPID type to pid_t, add parameter
	field.
	(linux_proc_get_tgid): Only call linux_proc_get_int.
	(linux_proc_get_tracerpid): New.
	(linux_proc_pid_has_state): New, from linux_proc_pid_is_zombie.
	(linux_proc_pid_is_stopped, linux_proc_pid_is_zombie): Only call
	linux_proc_pid_has_state.
	* common/linux-procfs.h (linux_proc_get_tracerpid): New declaration.
	* common/linux-ptrace.c: Include linux-procfs.h and buffer.h.
	(linux_ptrace_attach_warnings): New.
	* common/linux-ptrace.h (struct buffer, linux_ptrace_attach_warnings):
	New declaration.
	* linux-nat.c: Include exceptions.h, linux-ptrace.h and buffer.h.
	(linux_nat_attach): New variables ex, buffer, message and message_s.
	Wrap to_attach by TRY_CATCH and call linux_ptrace_attach_warnings.

gdb/gdbserver/
	* linux-low.c (linux_attach_lwp_1): New variable buffer.  Call
	linux_ptrace_attach_warnings.

gdb/testsuite/
	* gdb.base/attach-twice.c: New files.
	* gdb.base/attach-twice.exp: New files.

gdb/common/linux-ptrace.c

@@ -24,3 +24,26 @@
 #endif
 
 #include "linux-ptrace.h"
+#include "linux-procfs.h"
+#include "buffer.h"
+
+/* Find all possible reasons we could fail to attach PID and append these
+   newline terminated reason strings to initialized BUFFER.  '\0' termination
+   of BUFFER must be done by the caller.  */
+
+void
+linux_ptrace_attach_warnings (pid_t pid, struct buffer *buffer)
+{
+  pid_t tracerpid;
+
+  tracerpid = linux_proc_get_tracerpid (pid);
+  if (tracerpid > 0)
+    buffer_xml_printf (buffer, _("warning: process %d is already traced "
+				 "by process %d\n"),
+		       (int) pid, (int) tracerpid);
+
+  if (linux_proc_pid_is_zombie (pid))
+    buffer_xml_printf (buffer, _("warning: process %d is a zombie "
+				 "- the process has already terminated\n"),
+		       (int) pid);
+}