Add the web page for the 'Using Python Scripting to
Debug a Program in LLDB' example.
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+<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
+<html xmlns="http://www.w3.org/1999/xhtml">
+<head>
+<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />
+<link href="style.css" rel="stylesheet" type="text/css" />
+<title>LLDB Example - Python Scripting to Debug a Problem</title>
+</head>
+
+<body>
+ <div class="www_title">
+ Example - Using Scripting and Python to Debug in LLDB
+ </div>
+
+<div id="container">
+ <div id="content">
+ <!--#include virtual="sidebar.incl"-->
+ <div id="middle">
+ <div class="post">
+ <h1 class ="postheader">The Test Program and Input</h1>
+ <div class="postcontent">
+
+ <p>We have a simple C program (dictionary.c) that reads in a text file, and
+ stores all the words from the file in a Binary Search Tree, sorted
+ alphabetically. It then enters a loop prompting the user for a word, searching
+ for the word in the tree (using Binary Search), and reporting to the user
+ whether or not it found the word in the tree.</p>
+
+ <p>The input text file we are using to test our program contains the text for
+ William Shakespeare's famous tragedy "Romeo and Juliet".</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+ <div class="post">
+ <h1 class ="postheader">The Bug</h1>
+ <div class="postcontent">
+
+ <p>When we try running our program, we find there is a problem. While it
+ successfully finds some of the words we would expect to find, such as "love"
+ or "sun", it fails to find the word "Romeo", which MUST be in the input text
+ file:</p>
+
+ <code color=#ff0000>
+ % ./dictionary Romeo-and-Juliet.txt<br>
+ Dictionary loaded.<br>
+ Enter search word: love<br>
+ Yes!<br>
+ Enter search word: sun<br>
+ Yes!<br>
+ Enter search word: Romeo<br>
+ No!<br>
+ Enter search word: ^D<br>
+ %<br>
+ </code>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader">Is the word in our tree: Using Depth First Search</h1>
+ <div class="postcontent">
+
+ <p>Our first job is to determine if the word "Romeo" actually got inserted into
+ the tree or not. Since "Romeo and Juliet" has thousands of words, trying to
+ examine our binary search tree by hand is completely impractical. Therefore we
+ will write a Python script to search the tree for us. We will write a recursive
+ Depth First Search function that traverses the entire tree searching for a word,
+ and maintaining information about the path from the root of the tree to the
+ current node. If it finds the word in the tree, it returns the path from the
+ root to the node containing the word. This is what our DFS function in Python
+ would look like, with line numbers added for easy reference in later
+ explanations:</p>
+
+ <code>
+<pre><tt>
+ 1: def DFS (root, word, cur_path):
+ 2: root_word_ptr = root.GetChildMemberWithName ("word")
+ 3: left_child_ptr = root.GetChildMemberWithName ("left")
+ 4: right_child_ptr = root.GetChildMemberWithName ("right")
+ 5: root_word = root_word_ptr.GetSummary()
+ 6: end = len (root_word) - 1
+ 7: if root_word[0] == '"' and root_word[end] == '"':
+ 8: root_word = root_word[1:end]
+ 9: end = len (root_word) - 1
+10: if root_word[0] == '\'' and root_word[end] == '\'':
+11: root_word = root_word[1:end]
+12: if root_word == word:
+13: return cur_path
+14: elif word < root_word:
+15: if left_child_ptr.GetValue() == None:
+16: return ""
+17: else:
+18: cur_path = cur_path + "L"
+19: return DFS (left_child_ptr, word, cur_path)
+20: else:
+21: if right_child_ptr.GetValue() == None:
+22: return ""
+23: else:
+24: cur_path = cur_path + "R"
+25: return DFS (right_child_ptr, word, cur_path)
+</tt></pre>
+ </code>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader"><a name="accessing-variables">Accessing & Manipulating <strong>Program</strong> Variables in Python</a>
+</h1>
+ <div class="postcontent">
+
+ <p>Before we can call any Python function on any of our program's variables, we
+ need to get the variable into a form that Python can access. To show you how to
+ do this we will look at the parameters for the DFS function. The first
+ parameter is going to be a node in our binary search tree, put into a Python
+ variable. The second parameter is the word we are searching for (a string), and
+ the third parameter is a string representing the path from the root of the tree
+ to our current node.</p>
+
+ <p>The most interesting parameter is the first one, the Python variable that
+ needs to contain a node in our search tree. How can we take a variable out of
+ our program and put it into a Python variable? What kind of Python variable
+ will it be? The answers are to use the LLDB API functions, provided as part of
+ the LLDB Python module. Running Python from inside LLDB, LLDB will
+ automatically give us our current frame object as a Python variable,
+ "lldb.frame". This variable has the type "SBFrame" (see the LLDB API for
+ more information about SBFrame objects). One of the things we can do with a
+ frame object, is to ask it to find and return its local variable. We will call
+ the API function "FindVariable" on the lldb.frame object to give us our
+ dictionary variable as a Python variable:</p>
+
+ <code>
+ root = lldb.frame.FindVariable ("dictionary")
+ </code>
+
+ <p>The line above, executed in the Python script interpreter in LLDB, asks the
+ current frame to find the variable named "dictionary" and return it. We then
+ store the returned value in the Python variable named "root". This answers the
+ question of HOW to get the variable, but it still doesn't explain WHAT actually
+ gets put into "root". If you examine the LLDB API, you will find that the
+ SBFrame method "FindVariable" returns an object of type SBValue. SBValue
+ objects are used, among other things, to wrap up program variables and values.
+ There are many useful methods defined in the SBValue class to allow you to get
+ information or children values out of SBValues. For complete information, see
+ the header file <a href="http://llvm.org/svn/llvm-project/lldb/trunk/include/lldb/API/SBValue.h">SBValue.h</a>. The
+ SBValue methods that we use in our DFS function are
+ <code>GetChildMemberWithName()</code>,
+ <code>GetSummary()</code>, and <code>GetValue()</code>.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader">Explaining Depth First Search Script in Detail</h1>
+ <div class="postcontent">
+
+ <p><strong>"DFS" Overview.</strong> Before diving into the details of this
+ code, it would be best to give a high-level overview of what it does. The nodes
+ in our binary search tree were defined to have type <code>tree_node *</code>,
+ which is defined as:
+
+ <code>
+<pre><tt>typedef struct tree_node
+{
+ const char *word;
+ struct tree_node *left;
+ struct tree_node *right;
+} tree_node;</tt></pre></code>
+
+ <p>Lines 2-11 of DFS are getting data out of the current tree node and getting
+ ready to do the actual search; lines 12-25 are the actual depth-first search.
+ Lines 2-4 of our DFS function get the <code>word</code>, <code>left</code> and
+ <code>right</code> fields out of the current node and store them in Python
+ variables. Since <code>root_word_ptr</code> is a pointer to our word, and we
+ want the actual word, line 5 calls <code>GetSummary()</code> to get a string
+ containing the value out of the pointer. Since <code>GetSummary()</code> adds
+ quotes around its result, lines 6-11 strip surrounding quotes off the word.</p>
+
+ <p>Line 12 checks to see if the word in the current node is the one we are
+ searching for. If so, we are done, and line 13 returns the current path.
+ Otherwise, line 14 checks to see if we should go left (search word comes before
+ the current word). If we decide to go left, line 15 checks to see if the left
+ pointer child is NULL ("None" is the Python equivalent of NULL). If the left
+ pointer is NULL, then the word is not in this tree and we return an empty path
+ (line 16). Otherwise, we add an "L" to the end of our current path string, to
+ indicate we are going left (line 18), and then recurse on the left child (line
+ 19). Lines 20-25 are the same as lines 14-19, except for going right rather
+ than going left.</p>
+
+ <p>One other note: Typing something as long as our DFS function directly into
+ the interpreter can be difficult, as making a single typing mistake means having
+ to start all over. Therefore we recommend doing as we have done: Writing your
+ longer, more complicated script functions in a separate file (in this case
+ tree_utils.py) and then importing it into your LLDB Python interpreter.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader">Seeing the DFS Script in Action</h1>
+ <div class="postcontent">
+
+
+ <p>At this point we are ready to use the DFS function to see if the word "Romeo"
+ is in our tree or not. To actually use it in LLDB on our dictionary program,
+ you would do something like this:</p>
+
+ <code>
+ % lldb<br>
+ (lldb) process attach -n "dictionary"<br>
+ Architecture set to: x86_64.<br>
+ Process 521 stopped<br>
+ * thread #1: tid = 0x2c03, 0x00007fff86c8bea0 libSystem.B.dylib`read$NOCANCEL + 8, stop reason = signal SIGSTOP<br>
+ frame #0: 0x00007fff86c8bea0 libSystem.B.dylib`read$NOCANCEL + 8<br>
+ (lldb) br s -n find_word<br>
+ Breakpoint created: 1: name = 'find_word', locations = 1, resolved = 1<br>
+ (lldb) c<br>
+ Process 521 resuming<br>
+ Process 521 stopped<br>
+ * thread #1: tid = 0x2c03, 0x0000000100001830 dictionary`find_word + 16 <br>
+ at dictionary.c:105, stop reason = breakpoint 1.1<br>
+ frame #0: 0x0000000100001830 dictionary`find_word + 16 at dictionary.c:105<br>
+ 102 int<br>
+ 103 find_word (tree_node *dictionary, char *word)<br>
+ 104 {<br>
+ -> 105 if (!word || !dictionary)<br>
+ 106 return 0;<br>
+ 107 <br>
+ 108 int compare_value = strcmp (word, dictionary->word);<br>
+ (lldb) script<br>
+ Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.<br>
+ >>> import tree_utils<br>
+ >>> root = lldb.frame.FindVariable ("dictionary")<br>
+ >>> current_path = ""<br>
+ >>> path = tree_utils.DFS (root, "Romeo", current_path)<br>
+ >>> print path<br>
+ LLRRL<br>
+ >>> ^D<br>
+ (lldb) <br>
+ </code>
+
+ <p>The first bit of code above shows starting lldb, attaching to the dictionary
+ program, and getting to the find_word function in LLDB. The interesting part
+ (as far as this example is concerned) begins when we enter the
+ <code>script</code> command and drop into the embedded interactive Python
+ interpreter. We will go over this Python code line by line. The first line</p>
+
+ <code>
+ import tree_utils
+ </code>
+
+ <p>imports the file where we our DFS function, tree_utils.py, into Python.
+ Notice that to import the file we leave off the ".py" extension. We can now
+ call any function in that file, giving it the prefix "tree_utils.", so that
+ Python knows where to look for the function. The line</p>
+
+ <code>
+ root = lldb.frame.FindVariable ("dictionary")
+ </code>
+
+ <p>gets our program variable "dictionary" (which contains the binary search
+ tree) and puts it into the Python variable "root". See
+ <a href="#accessing-variables">Accessing & Manipulating Program Variables in Python</a>
+ above for more details about how this works. The next line is</p>
+
+ <code>
+ current_path = ""
+ </code>
+
+ <p>This line initializes the current_path from the root of the tree to our
+ current node. Since we are starting at the root of the tree, our current path
+ starts as an empty string. As we go right and left through the tree, the DFS
+ function will append an 'R' or an 'L' to the current path, as appropriate. The
+ line</p>
+
+ <code>
+ path = tree_utils.DFS (root, "Romeo", current_path)
+ </code>
+
+ <p>calls our DFS function (prefixing it with the module name so that Python can
+ find it). We pass in our binary tree stored in the variable <code>root</code>,
+ the word we are searching for, and our current path. We assign whatever path
+ the DFS function returns to the Python variable <code>path</code>.</p>
+
+
+ <p>Finally, we want to see if the word was found or not, and if so we want to
+ see the path through the tree to the word. So we do</p>
+
+ <code>
+ print path
+ </code>
+
+ <p>From this we can see that the word "Romeo" was indeed found in the tree, and
+ the path from the root of the tree to the node containing "Romeo" is
+ left-left-right-right-left.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader">What next? Using Breakpoint Command Scripts...</h1>
+ <div class="postcontent">
+
+ <p>We are halfway to figuring out what the problem is. We know the word we are
+ looking for is in the binary tree, and we know exactly where it is in the binary
+ tree. Now we need to figure out why our binary search algorithm is not finding
+ the word. We will do this using breakpoint command scripts.</p>
+
+
+ <p>The idea is as follows. The binary search algorithm has two main decision
+ points: the decision to follow the right branch; and, the decision to follow
+ the left branch. We will set a breakpoint at each of these decision points, and
+ attach a Python breakpoint command script to each breakpoint. The breakpoint
+ commands will use the global <code>path</code> Python variable that we got from
+ our DFS function. Each time one of these decision breakpoints is hit, the script
+ will compare the actual decision with the decision the front of the
+ <code>path</code> variable says should be made (the first character of the
+ path). If the actual decision and the path agree, then the front character is
+ stripped off the path, and execution is resumed. In this case the user never
+ even sees the breakpoint being hit. But if the decision differs from what the
+ path says it should be, then the script prints out a message and does NOT resume
+ execution, leaving the user sitting at the first point where a wrong decision is
+ being made.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader">Side Note: Python Breakpoint Command Scripts are NOT What They Seem</h1>
+ <div class="postcontent">
+
+ </div>
+ <div class="postfooter"></div>
+
+ <p>What do we mean by that? When you enter a Python breakpoint command in LLDB,
+ it appears that you are entering one or more plain lines of Python. BUT LLDB
+ then takes what you entered and wraps it into a Python FUNCTION (just like using
+ the "def" Python command). It automatically gives the function an obscure,
+ unique, hard-to-stumble-across function name, and gives it two parameters:
+ <code>frame</code> and <code>bp_loc</code>. When the breakpoint gets hit, LLDB
+ wraps up the frame object where the breakpoint was hit, and the breakpoint
+ location object for the breakpoint that was hit, and puts them into Python
+ variables for you. It then calls the Python function that was created for the
+ breakpoint command, and passes in the frame and breakpoint location objects.</p>
+
+ <p>So, being practical, what does this mean for you when you write your Python
+ breakpoint commands? It means that there are two things you need to keep in
+ mind: 1. If you want to access any Python variables created outside your script,
+ <strong>you must declare such variables to be global</strong>. If you do not
+ declare them as global, then the Python function will treat them as local
+ variables, and you will get unexpected behavior. 2. <strong>All Python
+ breakpoint command scripts automatically have a <code>frame</code> and a
+ <code>bp_loc</code> variable.</strong> The variables are pre-loaded by LLDB
+ with the correct context for the breakpoint. You do not have to use these
+ variables, but they are there if you want them.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+
+ <div class="post">
+ <h1 class ="postheader">The Decision Point Breakpoint Commands</h1>
+ <div class="postcontent">
+
+ <p>This is what the Python breakpoint command script would look like for the
+ decision to go right:<p>
+
+<code><pre><tt>
+global path
+if path[0] == 'R':
+ path = path[1:]
+ thread = frame.GetThread()
+ process = thread.GetProcess()
+ process.Continue()
+else:
+ print "Here is the problem; going right, should go left!"
+</tt></pre></code>
+
+ <p>Just as a reminder, LLDB is going to take this script and wrap it up in a
+ function, like this:</p>
+
+<code><pre><tt>
+def some_unique_and_obscure_function_name (frame, bp_loc):
+ global path
+ if path[0] == 'R':
+ path = path[1:]
+ thread = frame.GetThread()
+ process = thread.GetProcess()
+ process.Continue()
+ else:
+ print "Here is the problem; going right, should go left!"
+</tt></pre></code>
+
+ <p>LLDB will call the function, passing in the correct frame and breakpoint
+ location whenever the breakpoint gets hit. There are several things to notice
+ about this function. The first one is that we are accessing and updating a
+ piece of state (the <code>path</code> variable), and actually conditioning our
+ behavior based upon this variable. Since the variable was defined outside of
+ our script (and therefore outside of the corresponding function) we need to tell
+ Python that we are accessing a global variable. That is what the first line of
+ the script does. Next we check where the path says we should go and compare it to
+ our decision (recall that we are at the breakpoint for the decision to go
+ right). If the path agrees with our decision, then we strip the first character
+ off of the path.</p>
+
+ <p>Since the decision matched the path, we want to resume execution. To do this
+ we make use of the <code>frame</code> parameter that LLDB guarantees will be
+ there for us. We use LLDB API functions to get the current thread from the
+ current frame, and then to get the process from the thread. Once we have the
+ process, we tell it to resume execution (using the <code>Continue()</code> API
+ function).</p>
+
+ <p>If the decision to go right does not agree with the path, then we do not
+ resume execution. We allow the breakpoint to remain stopped (by doing nothing),
+ and we print an informational message telling the user we have found the
+ problem, and what the problem is.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+ <div class="post">
+ <h1 class ="postheader">Actually Using the Breakpoint Commands</h1>
+ <div class="postcontent">
+
+ <p>Now we will look at what happens when we actually use these breakpoint
+ commands on our program. Doing a <code>source list -n find_word</code> shows
+ us the function containing our two decision points. Looking at the code below,
+ we see that we want to set our breakpoints on lines 113 and 115:</p>
+
+<code><pre><tt>
+(lldb) source list -n find_word
+File: /Volumes/Data/HD2/carolinetice/Desktop/LLDB-Web-Examples/dictionary.c.
+101
+102 int
+103 find_word (tree_node *dictionary, char *word)
+104 {
+105 if (!word || !dictionary)
+106 return 0;
+107
+108 int compare_value = strcmp (word, dictionary->word);
+109
+110 if (compare_value == 0)
+111 return 1;
+112 else if (compare_value < 0)
+113 return find_word (dictionary->left, word);
+114 else
+115 return find_word (dictionary->right, word);
+116 }
+117
+</tt></pre></code>
+
+ <p>So, we set our breakpoints, enter our breakpoint command scripts, and see
+ what happens:<p>
+
+<code><pre><tt>
+(lldb) breakpoint set -l 113
+Breakpoint created: 2: file ='dictionary.c', line = 113, locations = 1, resolved = 1
+(lldb) breakpoint set -l 115
+Breakpoint created: 3: file ='dictionary.c', line = 115, locations = 1, resolved = 1
+(lldb) breakpoint command add -s python 2
+Enter your Python command(s). Type 'DONE' to end.
+> global path
+> if (path[0] == 'L'):
+> path = path[1:]
+> thread = frame.GetThread()
+> process = thread.GetProcess()
+> process.Continue()
+> else:
+> print "Here is the problem. Going left, should go right!"
+> DONE
+(lldb) breakpoint command add -s python 3
+Enter your Python command(s). Type 'DONE' to end.
+> global path
+> if (path[0] == 'R'):
+> path = path[1:]
+> thread = frame.GetThread()
+> process = thread.GetProcess()
+> process.Continue()
+> else:
+> print "Here is the problem. Going right, should go left!"
+> DONE
+(lldb) continue
+Process 696 resuming
+Here is the problem. Going right, should go left!
+Process 696 stopped
+* thread #1: tid = 0x2d03, 0x000000010000189f dictionary`find_word + 127 at dictionary.c:115, stop reason = breakpoint 3.1
+ frame #0: 0x000000010000189f dictionary`find_word + 127 at dictionary.c:115
+ 112 else if (compare_value < 0)
+ 113 return find_word (dictionary->left, word);
+ 114 else
+ -> 115 return find_word (dictionary->right, word);
+ 116 }
+ 117
+ 118 void
+(lldb)
+</tt></pre></code>
+
+
+ <p>After setting our breakpoints, adding our breakpoint commands and continuing,
+ we run for a little bit and then hit one of our breakpoints, printing out the
+ error message from the breakpoint command. Apparently at this point the the
+ tree, our search algorithm decided to go right, but our path says the node we
+ want is to the left. Examining the word at the node where we stopped, and our
+ search word, we see:</p>
+
+ <code>
+ (lldb) expr dictionary->word<br>
+ (const char *) $1 = 0x0000000100100080 "dramatis"<br>
+ (lldb) expr word<br>
+ (char *) $2 = 0x00007fff5fbff108 "romeo"<br>
+ </code>
+
+ <p>So the word at our current node is "dramatis", and the word we are searching
+ for is "romeo". "romeo" comes after "dramatis" alphabetically, so it seems like
+ going right would be the correct decision. Let's ask Python what it thinks the
+ path from the current node to our word is:</p>
+
+ <code>
+ (lldb) script print path<br>
+ LLRRL<br>
+ </code>
+
+ <p>According to Python we need to go left-left-right-right-left from our current
+ node to find the word we are looking for. Let's double check our tree, and see
+ what word it has at that node:</p>
+
+ <code>
+ (lldb) expr dictionary->left->left->right->right->left->word<br>
+ (const char *) $4 = 0x0000000100100880 "Romeo"<br>
+ </code>
+
+ <p>So the word we are searching for is "romeo" and the word at our DFS location
+ is "Romeo". Aha! One is uppercase and the other is lowercase: We seem to have
+ a case conversion problem somewhere in our program (we do).</p>
+
+ <p>This is the end of our example on how you might use Python scripting in LLDB
+ to help you find bugs in your program.</p>
+
+ </div>
+ <div class="postfooter"></div>
+
+ <div class="post">
+ <h1 class ="postheader">Source Files for The Example</h1>
+ <div class="postcontent">
+
+
+ </div>
+ <div class="postfooter"></div>
+
+ <p> The complete code for the Dictionary program (with case-conversion bug),
+ the DFS function and other Python script examples (tree_utils.py) used for this
+ example are available via following file links:</p>
+
+<a href="http://llvm.org/svn/llvm-project/lldb/trunk/examples/scripting/tree_utils.py">tree_utils.py</a> - Example Python functions using LLDB's API, including DFS<br>
+<a href="http://llvm.org/svn/llvm-project/lldb/trunk/examples/scripting/dictionary.c">dictionary.c</a> - Sample dictionary program, with bug<br>
+
+ <p>The text for "Romeo and Juliet" can be obtained from the Gutenberg Project
+ (http://www.gutenberg.org).</p>
+ </div>
+ </div>
+ </div>
+</div>
+</body>
+</html>