update README.md

Author: satanson

README.md

@@ -5,6 +5,18 @@ built by both clang and gcc. Some interesting perl scripts are also included.
 
 
 ## Scripts
+### calltree.pl: A smart tool to surf C++ source code
+
+run commands to get usage of calltree.pl as follows:
+```
+calltree.pl
+calltree.pl help
+calltree.pl -h
+calltree.pl --help
+```
+
+![image](./images/calltree.png)
+
 
 ### csvtable.pl: create a ascii-border table from CSV format.
 

calltree.pl

@@ -189,9 +189,6 @@ ()
  die "Never run calltree.pl in a directory whose depth <= 2" if scalar(@comp) <= 3;
 }
 
-ensure_safe;
-ensure_ag_installed;
-
 my $ignore_pattern = join "", map {" --ignore '$_' "}
  qw(*test* *benchmark* *CMakeFiles* *contrib/* *third_party/*
  *thirdparty/* *3rd-[pP]arty/* *3rd[pP]arty/* *deps/*);
@@ -1957,6 +1954,8 @@ ($)
 }
 
 sub show_tree() {
+ ensure_safe;
+ ensure_ag_installed;
  ($Global_calling, $Global_called, $Global_calling_names, $Global_called_names) =
  get_cached_or_extract_all_funcs(%ignored, $env_trivial_threshold, $env_length_threshold);
  if ($Opt_mode == 0) {
@@ -1992,7 +1991,7 @@ ()
 
 
 __DATA__
-<cs2>calltree.pl<end> - <cs2>A tool used to surf C++ code source.<end>
+<cs2>calltree.pl<end> - <cs2>A tool used to surf C++ source code.<end>
 -
 - <cs2>prerequisites<end>
 -- perl 5.10+: Most Linux distributions and MacOS have intalled perl 5.10+.