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diff --git a/www/scripting.html b/www/scripting.html new file mode 100755 index 0000000000000..10ba05b6a1090 --- /dev/null +++ b/www/scripting.html @@ -0,0 +1,586 @@ +<!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">Introduction</h1> + <div class="postcontent"> + + <p>LLDB has been structured from the beginning to be scriptable in two ways + -- a Unix Python session can initiate/run a debug session non-interactively + using LLDB; and within the LLDB debugger tool, Python scripts can be used to + help with many tasks, including inspecting program data, iterating over + containers and determining if a breakpoint should stop execution or continue. + This document will show how to do some of these things by going through an + example, explaining how to use Python scripting to find a bug in a program + that searches for text in a large binary tree.</p> + + </div> + <div class="postfooter"></div> + + <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> + % <strong>lldb</strong><br> + (lldb) <strong>process attach -n "dictionary"</strong><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) <strong>breakpoint set -n find_word</strong><br> + Breakpoint created: 1: name = 'find_word', locations = 1, resolved = 1<br> + (lldb) <strong>continue</strong><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) <strong>script</strong><br> + Python Interactive Interpreter. To exit, type 'quit()', 'exit()' or Ctrl-D.<br> + >>> <strong>import tree_utils</strong><br> + >>> <strong>root = lldb.frame.FindVariable ("dictionary")</strong><br> + >>> <strong>current_path = ""</strong><br> + >>> <strong>path = tree_utils.DFS (root, "Romeo", current_path)</strong><br> + >>> <strong>print path</strong><br> + LLRRL<br> + >>> <strong>^D</strong><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 wrote 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 in 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> |