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diff --git a/www/scripting.html b/www/scripting.html deleted file mode 100755 index 10ba05b6a109..000000000000 --- a/www/scripting.html +++ /dev/null @@ -1,586 +0,0 @@ -<!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> |