Intermediate LPC Descartes of Borg November 1993 Chapter 7: Debugging 7.1 Types of Errors By now, you have likely run into errors here, there, and everywhere. In general, there are three sorts of errors you might see: compile time errors, run time errors, and malfunctioning code. On most muds you will find a personal file where your compile time errors are logged. For the most part, this file can be found either in your home directory as the file named "log" or ".log", or somewhere in the directory "/log" as a file with your name.. In addition, muds tend to keep a log of run time errors which occur while the mud is up. Again, this is generally found in "/log". On MudOS muds it is called "debug.log". On other muds it may be called something different like "lpmud.log". Ask your administrators where compile time and run time errors are each logged if you do not already know. Compile time errors are errors which occur when the driver tries to load an object into memory. If, when the driver is trying to load an object into memory, it encounters things which it simply does not understand with respect to what you wrote, it will fail to load it into memory and log why it could not load the object into your personal error log. The most common compile time errors are typos, missing or extra (), {}. [], or "", and failure to declare properly functions and variables used by the object. Run time errors occur when something wrong happens to an object in memory while it is executing a statement. For example, the driver cannot tell whether the statement "x/y" will be valid in all circumstances. In fact, it is a valid LPC expression. Yet, if the value of y is 0, then a run time error will occur since you cannot divide by 0. When the driver runs across an error during the execution of a function, it aborts execution of the function and logs an error to the game's run time error log. It will also show the error to this_player(), if defined, if the player is a creator, or it will show "What?" to players. Most common causes for run time errors are bad values and trying to perform operations with data types for which those operations are not defined. The most insideous type of error, however, is plain malfunctioning code. These errors do not log, since the driver never really realizes that anything is wrong. In short, this error happens when you think the code says one thing, but in fact it says another thing. People too often encounter this bug and automatically insist that it must be a mudlib or driver bug. Everyone makes all types of errors though, and more often than not when code is not functioning the way you should, it will be because you misread it. 7.2 Debugging Compile Time Errors Compile time errors are certainly the most common and simplest bugs to debug. New coders often get frustrated by them due to the obscure nature of some error messages. Nevertheless, once a person becomes used to the error messages generated by their driver, debugging compile time errors becomes utterly routine. In your error log, the driver will tell you the type of error and on which line it finally noticed there was an error. Note that this is not on which line the actual error necessarily exists. The most common compile time error, besides the typo, is the missing or superfluous parentheses, brackets, braces, or quotes. Yet this error is the one that most baffles new coders, since the driver will not notice the missing or extra piece until well after the original. Take for example the following code: 1 int test(string str) { 2 int x; 3 for(x =0; x<10; x++) 4 write(x+"\n"); 5 } 6 write("Done.\n"); 7 } Depending on what you intended, the actual error here is either at line 3 (meaning you are missing a {) or at line 5 (meaing you have an extra }). Nevertheless, the driver will report that it found an error when it gets to line 6. The actual driver message may vary from driver to driver, but no matter which driver, you will see an error on line 6, since the } in line 5 is interpreted as ending the function test(). At line 6, the driver sees that you have a write() sitting outside any function definition, and thus reports an error. Generally, the driver will also go on to report that it found an error at line 7 in the form of an extra }. The secret to debugging these is coding style. Having closing } match up vertically with the clauses they close out helps you see where you are missing them when you are debugging code. Similarly, when using multiple sets of parentheses, space out different groups like this: if( (x=sizeof(who=users()) > ( (y+z)/(a-b) + (-(random(7))) ) ) As you can see, the parentheses for the for() statement, are spaced out from the rest of the statement. In addition, individual sub-groups are spaced so they can easily be sorted out in the event of an error. Once you have a coding style which aids in picking these out, you learn which error messages tend to indicate this sort of error. When debugging this sort of error, you then view a section of code before and after the line in question. In most all cases, you will catch the bug right off. Another common compile time error is where the driver reports an unknown identifier. Generally, typos and failure to declare variables causes this sort of error. Fortunately, the error log will almost always tell you exactly where the error is. So when debugging it, enter the editor and find the line in question. If the problem is with a variable and is not a typo, make sure you declared it properly. On the other hand, if it is a typo, simply fix it! One thing to beware of, however, is that this error will sometimes be reported in conjunction with a missing parentheses, brackets, or braces type error. In these situations, your problem with an unknown identifier is often bogus. The driver misreads the way the {} or whatever are setup, and thus gets variable declarations confused. Therefore make sure all other compile time errors are corrected before bothering with these types of errors. In the same class with the above error, is the general syntax error. The driver generates this error when it simply fails to understand what you said. Again, this is often caused by typos, but can also be caused by not properly understanding the syntax of a certain feature like writing a for() statement: for(x=0, x<10, x++). If you get an error like this which is not a syntax error, try reviewing the syntax of the statement in which the error is occurring. 7.3 Debugging Run Time Errors Run time errors are much more complex than their compile time counterparts. Fortunately these errors do get logged, though many creators do not realise or they do not know where to look. The error log for run time errors are also generally much more detailed than compile time errors, meaning that you can trace the history of the execution train from where it started to where it went wrong. You therefore can setup debugging traps using precompiler statements much easier using these logs. Run time errors, however, tend to result from using more complex codign techniques than beginners tend to use, which means you are left with errors which are generally more complicated than simple compile time errors. Run time errors almost always result from misusing LPC data types. Most commonly, trying to do call others using object variables which are NULL, indexing on mapping, array, or string variables which are NULL, or passing bad arguments to functions. We will look at a real run time error log from Nightmare: Bad argument 1 to explode() program: bin/system/_grep.c, object: bin/system/_grep line 32 ' cmd_hook' in ' std/living.c' (' std/user#4002')line 83 ' cmd_grep' in ' bin/system/_grep.c' (' bin/system/_grep')line 32 Bad argument 2 to message() program: adm/obj/simul_efun.c, object: adm/obj/simul_efun line 34 ' cmd_hook' in ' std/living.c' (' std/user#4957')line 83 ' cmd_look' in ' bin/mortal/_look.c' (' bin/mortal/_look')line 23 ' examine_object' in ' bin/mortal/_look.c' (' bin/mortal/_look')line 78 ' write' in 'adm/obj/simul_efun.c' (' adm/obj/simul_efun')line 34 Bad argument 1 to call_other() program: bin/system/_clone.c, object: bin/system/_clone line 25 ' cmd_hook' in ' std/living.c' (' std/user#3734')line 83 ' cmd_clone' in ' bin/system/_clone.c' (' bin/system/_clone')line 25 Illegal index program: std/monster.c, object: wizards/zaknaifen/spy#7205 line 76 ' heart_beat' in ' std/monster.c' ('wizards/zaknaifen/spy#7205')line 76 All of the errors, except the last one, involve passing a bad argument to a function. The first bug, involves passing a bad first arument to the efun explode(). This efun expects a string as its first argment. In debugging these kinds of errors, we would therefore go to line 32 in /bin/system/_grep.c and check to see what the data type of the first argument being passed in fact is. In this particular case, the value being passed should be a string. If for some reason I has actually passed something else, I would be done debugging at that point and fix it simply by making sure that I was passing a string. This situation is more complex. I now need to trace the actual values contained by the variable being passed to explode, so that I can see what it is the explode() efun sees that it is being passed. The line is question is this: borg[files[i]] = regexp(explode(read_file(files[i]), "\n"), exp); where files is an array for strings, i is an integer, and borg is a mapping. So clearly we need to find out what the value of read_file(files[i]) is. Well, this efun returns a string unless the file in question does not exist, the object in question does not have read access to the file in question, or the file in question is an empty file, in which cases the function will return NULL. Clearly, our problem is that one of these events must have happened. In order to see which, we need to look at files[i]. Examining the code, the files array gets its value through the get_dir() efun. This returns all the files in a directory if the object has read access to the directory. Therefore the problem is neither lack of access or non- existent files. The file which caused this error then must have been an empty file. And, in fact, that is exactly what caused this error. To debug that, we would pass files through the filter_array() efun and make sure that only files with a file size greater than 0 were allowed into the array. The key to debugging a run time error is therefore knowing exactly what the values of all variables in question are at the exact moment where the bug created. When reading your run time log, be careful to separate the object from the file in which the bug occurred. For example, the indexing error above came about in the object /wizards/zaknaifen/spy, but the error occured while running a function in /std/monster.c, which the object inherited. 7.4 Malfunctioning Code The nastiest problem to deal with is when your code does not behave the way you intended it to behave. The object loads fine, and it produces no run time errors, but things simply do not happen the way they should. Since the driver does not see a problem with this type of code, no logs are produced. You therefore need to go through the code line by line and figure out what is happening. Step 1: Locate the last line of code you knew successfully executed Step 2: Locate the first line of code where you know things are going wrong Step 3: Examine the flow of the code from the known successful point to the first known unsuccessful point. More often than not, these problems occurr when you are using if() statements and not accounting for all possibilities. For example: int cmd(string tmp) { if(stringp(tmp)) return do_a() else if(intp(tmp)) return do_b() return 1; } In this code, we find that it compiles and runs fine. Problem is nothing happens when it is executed. We know for sure that the cmd() function is getting executed, so we can start there. We also know that a value of 1 is in fact being returned, since we do not see "What?" when we enter the command. Immediately, we can see that for some reason the variable tmp has a value other than string or int. As it turns out, we issued the command without parameters, so tmp was NULL and failed all tests. The above example is rather simplistic, bordering on silly. Nevertheless, it gives you an idea of how to examine the flow of the code when debugging malfunctioning code. Other tools are available as well to help in debugging code. The most important tool is the use of the precompiler to debug code. With the code above, we have a clause checking for integers being passed to cmd(). When we type "cmd 10", we are expecting do_b() to execute. We need to see what the value of tmp is before we get into the loop: #define DEBUG int cmd(string tmp) { #ifdef DEBUG write(tmp); #endif if(stringp(tmp)) return do_a(); else if(intp(tmp)) return do_b(); else return 1; } We find out immediately upon issuing the command, that tmp has a value of "10". Looking back at the code, we slap ourselves silly, forgetting that we have to change command arguments to integers using sscanf() before evaluating them as integers. 7.5 Summary The key to debugging any LPC problem is always being aware of what the values of your variables are at any given step in your code. LPC execution reduces on the simplest level to changes in variable values, so bad values are what causes bad things to happen once code has been loaded into memory. If you get errors about bad arguments to functions, more likely than not you are passing a NULL value to a function for that argument. This happens most often with objects, since people will do one of the following: 1) use a value that was set to an object that has since destructed 2) use the return value of this_player() when there is no this_player() 3) use the return value of this_object() just after this_object() was destructed In addition, people will often run into errors involving illegal indexing or indexing on illegal types. Most often, this is because the mapping or array in question was not initialized, and therefore cannot be indexed. The key is to know exactly what the full value of the array or mapping should be at the point in question. In addition, watch for using index numbers larger than the size of given arrays Finally, make use of the precompiler to temporarly throw out code, or introduce code which will show you the values of variables. The precompiler makes it easy to get rid of debugging code quickly once you are done. You can simply remove the DEBUG define when you are done. Copyright (c) George Reese 1993