libbacktrace: better backtrace_print when no debug info

Fixes https://github.com/ianlancetaylor/libbacktrace/issues/59

	* print.c (print_syminfo_callback): New static function.
	(print_callback): Call backtrace_syminfo if there is no function
	or file name.

libbacktrace/print.c

@@ -47,6 +47,39 @@ struct print_data
   FILE *f;
 };
 
+/* Print errors to stderr.  */
+
+static void
+error_callback (void *data, const char *msg, int errnum)
+{
+  struct print_data *pdata = (struct print_data *) data;
+
+  if (pdata->state->filename != NULL)
+    fprintf (stderr, "%s: ", pdata->state->filename);
+  fprintf (stderr, "libbacktrace: %s", msg);
+  if (errnum > 0)
+    fprintf (stderr, ": %s", strerror (errnum));
+  fputc ('\n', stderr);
+}
+
+/* Print one level of a backtrace if we couldn't get a file or function name.
+   Use syminfo to try to get a symbol name.  */
+
+static void print_syminfo_callback (void *data, uintptr_t pc,
+				    const char *symname, uintptr_t symval,
+				    uintptr_t symsize ATTRIBUTE_UNUSED)
+{
+  struct print_data *pdata = (struct print_data *) data;
+
+  if (symname == NULL)
+    fprintf (pdata->f, "0x%lx ???\n\t???:0\n", (unsigned long) pc);
+  else
+    fprintf (pdata->f, "0x%lx ???\n\t%s+0x%lx:0\n",
+	     (unsigned long) pc,
+	     symname,
+	     pc - symval);
+}
+
 /* Print one level of a backtrace.  */
 
 static int
@@ -55,6 +88,13 @@ print_callback (void *data, uintptr_t pc, const char *filename, int lineno,
 {
   struct print_data *pdata = (struct print_data *) data;
 
+  if (function == NULL && filename == NULL)
+    {
+      backtrace_syminfo (pdata->state, pc, print_syminfo_callback,
+			 error_callback, data);
+      return 0;
+    }
+
   fprintf (pdata->f, "0x%lx %s\n\t%s:%d\n",
 	   (unsigned long) pc,
 	   function == NULL ? "???" : function,
@@ -63,21 +103,6 @@ print_callback (void *data, uintptr_t pc, const char *filename, int lineno,
   return 0;
 }
 
-/* Print errors to stderr.  */
-
-static void
-error_callback (void *data, const char *msg, int errnum)
-{
-  struct print_data *pdata = (struct print_data *) data;
-
-  if (pdata->state->filename != NULL)
-    fprintf (stderr, "%s: ", pdata->state->filename);
-  fprintf (stderr, "libbacktrace: %s", msg);
-  if (errnum > 0)
-    fprintf (stderr, ": %s", strerror (errnum));
-  fputc ('\n', stderr);
-}
-
 /* Print a backtrace.  */
 
 void __attribute__((noinline))