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-procfs.c

@@ -28,67 +28,54 @@
 /* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
    found.  */
 
-int
-linux_proc_get_tgid (int lwpid)
+static int
+linux_proc_get_int (pid_t lwpid, const char *field)
 {
+  size_t field_len = strlen (field);
   FILE *status_file;
   char buf[100];
-  int tgid = -1;
+  int retval = -1;
 
   snprintf (buf, sizeof (buf), "/proc/%d/status", (int) lwpid);
   status_file = fopen (buf, "r");
-  if (status_file != NULL)
+  if (status_file == NULL)
     {
-      while (fgets (buf, sizeof (buf), status_file))
-	{
-	  if (strncmp (buf, "Tgid:", 5) == 0)
-	    {
-	      tgid = strtoul (buf + strlen ("Tgid:"), NULL, 10);
-	      break;
-	    }
-	}
-
-      fclose (status_file);
+      warning (_("unable to open /proc file '%s'"), buf);
+      return -1;
     }
 
-  return tgid;
-}
+  while (fgets (buf, sizeof (buf), status_file))
+    if (strncmp (buf, field, field_len) == 0 && buf[field_len] == ':')
+      {
+	retval = strtol (&buf[field_len + 1], NULL, 10);
+	break;
+      }
 
-/* Detect `T (stopped)' in `/proc/PID/status'.
-   Other states including `T (tracing stop)' are reported as false.  */
-
-int
-linux_proc_pid_is_stopped (pid_t pid)
-{
-  FILE *status_file;
-  char buf[100];
-  int retval = 0;
-
-  snprintf (buf, sizeof (buf), "/proc/%d/status", (int) pid);
-  status_file = fopen (buf, "r");
-  if (status_file != NULL)
-    {
-      int have_state = 0;
-
-      while (fgets (buf, sizeof (buf), status_file))
-	{
-	  if (strncmp (buf, "State:", 6) == 0)
-	    {
-	      have_state = 1;
-	      break;
-	    }
-	}
-      if (have_state && strstr (buf, "T (stopped)") != NULL)
-	retval = 1;
-      fclose (status_file);
-    }
+  fclose (status_file);
   return retval;
 }
 
-/* See linux-procfs.h declaration.  */
+/* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
+   found.  */
 
 int
-linux_proc_pid_is_zombie (pid_t pid)
+linux_proc_get_tgid (pid_t lwpid)
+{
+  return linux_proc_get_int (lwpid, "Tgid");
+}
+
+/* See linux-procfs.h.  */
+
+pid_t
+linux_proc_get_tracerpid (pid_t lwpid)
+{
+  return linux_proc_get_int (lwpid, "TracerPid");
+}
+
+/* Return non-zero if 'State' of /proc/PID/status contains STATE.  */
+
+static int
+linux_proc_pid_has_state (pid_t pid, const char *state)
 {
   char buffer[100];
   FILE *procfile;
@@ -110,8 +97,24 @@ linux_proc_pid_is_zombie (pid_t pid)
 	have_state = 1;
 	break;
       }
-  retval = (have_state
-	    && strcmp (buffer, "State:\tZ (zombie)\n") == 0);
+  retval = (have_state && strstr (buffer, state) != NULL);
   fclose (procfile);
   return retval;
 }
+
+/* Detect `T (stopped)' in `/proc/PID/status'.
+   Other states including `T (tracing stop)' are reported as false.  */
+
+int
+linux_proc_pid_is_stopped (pid_t pid)
+{
+  return linux_proc_pid_has_state (pid, "T (stopped)");
+}
+
+/* See linux-procfs.h declaration.  */
+
+int
+linux_proc_pid_is_zombie (pid_t pid)
+{
+  return linux_proc_pid_has_state (pid, "Z (zombie)");
+}

gdb/common/linux-procfs.h

@@ -24,7 +24,12 @@
 /* Return the TGID of LWPID from /proc/pid/status.  Returns -1 if not
    found.  */
 
-extern int linux_proc_get_tgid (int lwpid);
+extern int linux_proc_get_tgid (pid_t lwpid);
+
+/* Return the TracerPid of LWPID from /proc/pid/status.  Returns -1 if not
+   found.  */
+
+extern pid_t linux_proc_get_tracerpid (pid_t lwpid);
 
 /* Detect `T (stopped)' in `/proc/PID/status'.
    Other states including `T (tracing stop)' are reported as false.  */

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);
+}

gdb/common/linux-ptrace.h

@@ -18,6 +18,8 @@
 #ifndef COMMON_LINUX_PTRACE_H
 #define COMMON_LINUX_PTRACE_H
 
+struct buffer;
+
 #include <sys/ptrace.h>
 
 #ifndef PTRACE_GETSIGINFO
@@ -65,4 +67,6 @@
 #define __WALL          0x40000000 /* Wait for any child.  */
 #endif
 
+extern void linux_ptrace_attach_warnings (pid_t pid, struct buffer *buffer);
+
 #endif /* COMMON_LINUX_PTRACE_H */