Guide to Faster, Less Frustrating Debugging

Norman Matloff

University of California at Davis

(530) 752-1953

matloff@cs.ucdavis.edu

©1992-2001, N. Matloff

April 4, 2002

Contents

1 Save Time and Frustration!

An integral part of your programming skills should be high proficiency in debugging. This document is intended as a first step toward that goal.

2 General Debugging Strategies

When your program contains a bug, it is of course because somewhere there is something which you believe to be true but actually is not true. In other words:

Finding your bug is a process of confirming the many things you believe are true, until you find one which is not true.

You believe that at a certain point in your source file, a certain variable has a certain value.

You believe that in a given if-then-else statement, the ``else'' part is the one that is executed.

You believe that when you call a certain function, the function receives its parameters correctly.

Usually your belief will be confirmed, but eventually you will find a case where your belief will not be confirmed-and you will then know the location of the bug.

2.2 Binary Search

In this confirmation process, use a ``binary search'' strategy. To explain this, suppose for the moment your program were one long file, say 200 lines long, with no function calls. (This would be terrible style, but again it will be easier to explain in this setting.)

Suppose you have an array x, and that you believe that x[4] = 9 for almost the entire execution of the program. To check this, first check the value of x[4] at line 100. Say the value is 9. That means you have narrowed down the location of the bug to lines 101-200! Now check at line 150. Say there x[4] = -127, which is wrong. So, the location of the bug is now narrowed down to lines 101-150, and you will next check at line 125, and so on.

Of course, this is an oversimplified view, because hopefully your program does consist of function calls and thus we cannot simply divide numbers of lines by 2 in this manner, but you can see how you can quickly pinpoint the location of the bug but carefully choosing your checkpoints in something like a ``binary search'' manner.

2.3 What If It Doesn't Even Compile?

Most compilation errors are obvious and easily fixed. But in some cases, you will just have no idea even where the error is. The compiler may tell you that the error is at the very last line of the definition of a function (or a class, in C++ or Java) , even though all you have on that line is, say, `}'. That means the true location of the error could be anywhere in the function.

To deal with this, again use binary search! First, temporarily delete, say, the second half of the function. (Or, if done easily, comment-out that half.) You'll have to be careful in doing this, as otherwise you could introduce even more errors. If the error message disappears, then you know the problem is in that half; restore the deleted lines, and now delete half of that half, etc., until you pinpoint the location of the error.

3 Use a Debugging Tool and a Good Text Editor!

3.1 Don't Use printf()/cout As Your Main Debugging Devices

As seen above, the confirmation process often involves checking the values of variables at different points in your code and different points in time. In the past, you probably did this by adding temporary printf or cout statements to your C or C++ source file. This is very inconvenient: You have to recompile your program after you add them, and you may have to add a large number of them, say if you want to check the value of some variable at several different places in your program. Then after recompiling and running the program, you realize that there are even more places at which you need printf/cout statements, so you have to recompile again! Then after you fix that bug, you have to remove all these printf/couts, and start adding them to track down the next bug. Not only is this annoying, but it is distracting, making it difficult to concentrate, i.e. you will be apt to lose your train of thought on finding the bug.

A debugging tool allows you to print out values of variables much more conveniently: You don't have to recompile, and you can ask the debugging tool to monitor any variables you like, automatically. Again, the ``convenience factor'' here is quite important. The less time you spend on recompiles, etc., the more time-and mental energy-you have available to concentrate on hunting down the bug.

Even more important, the debugging tool will tell you where in your program a disastrous error (e.g. a ``segmentation fault'') occurs. In addition, it can tell you where the function generating the seg fault was called from. This is extremely useful.

There are times when printf/couts are useful in conjunction with a debugging tool, but you'll be doing yourself a favor if you use the debugging tool, not printf/couts, as your main debugging device.

3.2 Which Debugging Tools?

3.2.1 The Ubiquitous (on Unix) gdb

Most computer systems have one or more debugging tools available. These can save you a tremendous amount of time and frustration in the debugging process. The tool available on almost all Unix systems is gdb.

Of course, once you become adept at using one debugging tool, it will be very simple to learn others. So, although we use gdb as our example here, the principles apply to any debugging tool. (I am not saying that gdb is the best debugger around, but since it is so common I am using it as my example.)

3.2.2 ddd: A Better View of gdb

This is a text-based tool, and a number of GUI ``front ends'' have been developed for it, such as ddd. I strongly recommend using a GUI-based interface like this for gdb; see my debugging home page for how to obtain and use these: http://heather.cs.ucdavis.edu/~matloff/debug.html

It's much easier and pleasurable to use gdb through the ddd interface. For example, to set a breakpoint we just click the mouse, and a little stop sign appears to show that we will stop there.

But I do recommend learning the text-based version first.

3.2.3 A Good Text Editor Can Help a Lot

During a long debugging session, you are going to spend a lot of time just typing. Not only does this waste precious time, but much more importantly, it is highly distracting; it's easy to lose your train of thought while typing, especially when you must frequently hop around from one part of a file to another, or between files.

I have written some tips for text editing, designed specifically for programmers, at http://heather.cs.ucdavis.edu/~matloff/progedit.html . For example, it mentions that you should make good use of undo/redo operations.1 Consider our binary-search example above, in which we were trying to find an elusive compilation error. The advice given was to delete half the lines in the function, and later restore them. If your text editor includes undo capabilities, then the restoration of those deleted lines will be easy.

It's very important that you use an editor which allows subwindows. This enables you to, for instance, look at the definition of a function in one window while viewing a call to the function in another window.

Often one uses other tools in conjunction with a debugger. For example, the vim editor (an enhanced version of vi) can interface with gdb; see my vim Web page: http://heather.cs.ucdavis.edu/~matloff/vim.html You can initiate a compile from within vim, and then if there are compilation errors, vim will take you to the lines at which they occur.

3.3 Integrated Development Environments

In addition, a debugger will sometimes be part of an overall package, known as an integrated development environment (IDE). An example of the many IDEs available for Unix is Code Crusader: http://heather.cs.ucdavis.edu/~matloff/codecrusader.html

It used to be public-domain, but unfortunately Code Crusader is about to become a commercial product.

One big drawback from the point of view of many people is that one cannot use one's own text editor in most IDEs. Many people would like to use their own editor for everything-programming, composing e-mail, word processing and so on. This is both convenient and also allows them to develop personal alias/macro libraries which save them work.

A big advantage of Code Crusader is that it allows you to use your own text editor. As far as I can tell, this works best with emacs.

3.4 What Do I Use?

Mostly I use gdb, accessed from ddd, along with vim for my editor. I occasionally use Code Crusader.

4 How to Use gdb

4.1 Easy to Learn

In my own debugging, I tend to use just a few gdb commands, only four or five in all. So, you can learn gdb quite quickly. Later, if you wish, you can learn some advanced commands.

4.2 The Basic Strategy

A typical usage of gdb runs as follows: After starting up gdb, we set breakpoints, which are places in the code where we wish execution to pause. Each time gdb encounters a breakpoint, it suspends execution of the program at that point, giving us a chance to check the values of various variables.

In some cases, when we reach a breakpoint, we will single step for a while from that point onward, which means that gdb will pause after every line of source code. This may be important, either to further pinpoint the location at which a certain variable changes value, or in some cases to observe the flow of execution, seeing for example which parts of if-then-else constructs are executed.

As mentioned earlier, another component of the overall strategy concerns segmentation faults. If we receive a ``seg fault'' error message when we exeucte our program (running by itself, not under gdb), we can then run the program under gdb (probably not setting any breakpoints at all). When the seg fault occurs, gdb will tell us exactly where it happened, again pinpointing the location of the error. (In some cases the seg fault itself will occur within a system call, rather than in a function we wrote, in which case gdb's bt command can be used to determine where in our code we made the system call.) At that point you should check the values of all array indices and pointers which are referenced in the line at which the error occurred. Typically you will find that either you have an array index with a value far out of range, or a pointer which is 0 (and thus unrefenceable). Another common error is forgetting an ampersand in a function call, say scanf(). Still another one is that you have made a system call which failed, but you did not check its return value for an error code.

4.3 The Main gdb Commands

Before you start, make sure that when you compiled the program you are debugging, you used the -g option, i.e.

cc -g sourcefile.c

Without the -g option, gdb would essentially be useless, since it will have no information on variable and function names, line numbers, and so on.

Then to start gdb type

gdb filename

where `filename' is the executable file, e.g. a.out for your program.

To quit gdb, type `q'.

4.3.2 The r (Run) Command

This begins execution of your program. Be sure to include any command-line arguments; e.g. if in an ordinary (i.e. nondebugging) run of your program you would type

a.out < z

then within gdb you would type

r < z

If you apply r more than once in the same debugging session, you do not have to type the command-line arguments after the first time; the old ones will be repeated by default.

4.3.3 The l (List) Command

You can use this to list parts of your source file(s). E.g. typing

l 52

will result in display of Line 52 and the few lines following it (to see more lines, hit the carriage return again).

If you have more than one source file, precede the line number by the file name and a colon, e.g.

l X.c:52

You can also specify a function name here, in which case the listing will begin at the first line of the function.

The l command is useful to find places in the file at which you wish to set breakpoints (see below).

4.3.4 The b (Breakpoint) and c (Continue) Commands

This says that you wish execution of the program to pause at the specified line. For example,

b 30

means that you wish to stop every time the program gets to Line 30.

Again, if you have more than one source file, precede the line number by the file name and a colon as shown above.

Once you have paused at the indicated line and wish to continue executing the program, you can type c (for the continue command).

You can also use a function name to specify a breakpoint, meaning the first executable line in the function. For example,

b main

says to stop at the first line of the main program, which is often useful as the first step in debugging.

You can cancel a breakpoint by using the disable command.

You can also make a breakpoint conditional. E.g.

b 3 Z > 92

would tell gdb to stop at breakpoint 3 (which was set previously) only when Z exceeds 92.

4.3.5 The d (Display) and p (Print) Commands:

This prints out the value of the indicated variable or expression every time the program pauses (e.g. at breakpoints and after executions of the n and s commands). E.g. typing

disp NC

once will mean that the current value of the variable NC will automatically be printed to the screen every time the program pauses.

If we have gdb print out a struct variable, the individual fields of the struct will be printed out. If we specify an array name, the entire array will be printed.

After a while, you may find that the displaying a given variable or expression becomes less valuable than it was before. If so, you can cancel a disp command by using the undisplay command.

A related command is p; this prints out the value of the variable or expression just once.

In both cases, keep in mind the difference between global and local variables. If for example, you have a local variable L within the function F, then if you type

disp L

when you are not in F, you will get an error message like ``No variable L in the present context.''

gdb also gives you the option of using nondefault formats for printing out variables with disp or p. For example, suppose you have declared the variable G as

int G;

Then

p G

will result in printing out the variable in integer format, like

printf("%d

",G);

would. But you might want it in hex format, for example, i.e. you may wish to do something like

printf("%x

",G);

gdb allows you to do this by typing

p /x G

4.3.6 The printf Command

This is even better, as it works like C's function of the same name. For example, suppose you have two integer variables, X and Y, which you would like to have printed out. You can give gdb the command:

printf "X = %d, Y = %d

",X,Y

4.3.7 The n (Next) and s (Step) Commands

These tell gdb to execute the next line of the program, and then pause again. If that line happens to be a function call, then n and s will give different results:

If you use s, then the next pause will be at the first line of the function; if you use n, then the next pause will be at the line following the function call (the function will be single-step executed, but there will be no pauses within it). This is very important, and can save you a lot of time: If you think the bug does not lie within the function, then use n, so that you don't waste a lot of time single-stepping within the function itself.

When you use s at a function call, gdb will also tell you the values of the parameters, which is useful for confirmation purposes, as explained at the beginning of this document.

4.3.8 The bt (Backtrace) Command

If you have an execution error with a mysterious message like ``bus error'' or ``segmentation fault,'' the bt command will at least tell you where in your program this occurred, and if in a function, where the function was called from. This can be extremely valuable information.

4.3.9 The set Command

Sometimes it is very useful to use gdb to change the value of a program variable, and this command will do this. For example, if you have an int variable x, the gdb command

set variable x = 12

will change x's value to 12.

4.4 The call Command

You can use this function to call a function in your program during execution. Typically you do this with a function which you've written for debugging purposes, e.g. to print out a linked list.

Example:

(gdb) call x()

Make sure to reemmber to type the parentheses, even if there are no arguments.

4.4.1 The define Command

This saves you typing. You can put together one or more commands into a macro. For instance, recall our example from above,

printf "X = %d, Y = %d

",X,Y

If you wanted to frequently use this command during your debugging session, you could do:

(gdb) define pxy Type commands for definition of "pxy". End with a line saying just "end". >printf "%X = %d, Y = %d

",X,Y >end

Then you could invoke it just by typing ``pxy''.

4.5 Effect of Recompiling the Source Without Exiting gdb

As you know, a debugging session consists of compiling, then debugging, then editing, then recompiling, then debugging, then editing, then recompiling...

A key point is that you should not exit gdb before recompiling. After recompiling, when you issue the r command to rerun your program, gdb will notice that the source file is now newer than the binary executable file which it had been using, and thus will automatically reload the new binary before the rerun. Since it takes time to start gdb from scratch, it's much easier to stay in gdb between compiles, rather than exiting and then starting it up again.

4.6 A gdb Example

In this section we will introduce gdb by showing a script file record of its use on an actual program.In order to distinguish between line numbers in the script file from line numbers within the C source files, I have placed a `g' at the beginning of each of the former. For example, Line g56 means Line 56 within the script file, not Line 56 within one of the C source files.)

First, use cat to show the program source files:

g2 mole.matloff% cat Main.c g3 g4 g5 /* prime-number finding program g6 g7 will (after bugs are fixed) report a list of all primes which are g8 less than or equal to the user-supplied upper bound g9 g10 riddled with errors! */ g11 g12 g13 g14 #include "Defs.h" g15 g16 g17 int Prime[MaxPrimes], /* Prime[I] will be 1 if I is prime, 0 otherwi */ g18 UpperBound; /* we will check all number up through this one for g19 primeness */ g20 g21 g22 main() g23 g24 { int N; g25 g26 printf("enter upper bound

"); g27 scanf("%d",UpperBound); g28 g29 Prime[2] = 1; g30 g31 for (N = 3; N <= UpperBound; N += 2) g32 CheckPrime(); g33 if (Prime[N]) printf("%d is a prime

",N); g34 } g35 g36 mole.matloff% g37 mole.matloff% g38 mole.matloff% g39 mole.matloff% cat CheckPrimes.c g40 cat: CheckPrimes.c: No such file or directory g41 mole.matloff% cat CheckPrime.c g42 g43 g44 #include "Defs.h" g45 #include "Externs.h" g46 g47 g48 CheckPrime(K) g49 int K; g50 g51 { int J; g52 g53 /* the plan: see if J divides K, for all values J which are g54 g55 (a) themselves prime (no need to try J if it is nonprime), and g56 (b) less than or equal to sqrt(K) (if K has a divisor larger g57 than this square root, it must also have a smaller one, g58 so no need to check for larger ones) */ g59 g60 J = 2; g61 while (1) { g62 if (Prime[J] == 1) g63 if (K % J == 0) { g64 Prime[K] = 0; g65 return; g66 } g67 J++; g68 } g69 g70 /* if we get here, then there were no divisors of K, so it is g71 prime */ g72 Prime[K] = 1; g73 } g74 g75 mole.matloff% g76 mole.matloff% g77 mole.matloff% g78 mole.matloff% cat Defs.h g79 g80 #define MaxPrimes 50 g81 mole.matloff% cat Externs.h g82 g83 g84 #include "Defs.h" g85 g86 g87 extern int Prime[MaxPrimes];

The comments in Lines g5-g10 state what the program goal is, i.e. finding prime numbers.

OK, let's get started. First we compile the program:

g92 mole.matloff% make g93 cc -g -c Main.c g94 cc -g -c CheckPrime.c g95 "CheckPrime.c", line 31: warning: statement not reached g96 cc -g -o FindPrimes Main.o CheckPrime.o

The warning concerning Line 31 of CheckPrime.c sounds ominous (and it is), but let's ignore it, and see what happens. Let's run the program:

g100 mole.matloff% FindPrimes g101 enter upper bound g102 20 g103 Segmentation fault

Well, this sounds scary, but actually it usually is the easiest type of bug to fix. The first step is to determine where the error occurred; gdb will do this for us: We enter gdb and then re-run the program, so as to reproduce the error:

g104 mole.matloff% gdb FindPrimes g105 GDB is free software and you are welcome to distribute copies of it g106 under certain conditions; type "show copying" to see the conditions. g107 There is absolutely no warranty for GDB; type "show warranty" for details g108 GDB 4.7, Copyright 1992 Free Software Foundation, Inc...

OK, gdb is now ready for my command (it indicates this by giving me a special prompt, which looks like this:

(gdb)

I now invoke gdb's r command, to run the program (if the program had any command-line arguments, I would have typed them right after the `r'):

g109 (gdb) r g110 Starting program: /tmp_mnt/lion/d/guest/matloff/tmp/FindPrimes g111 enter upper bound g112 20 g113 g114 Program received signal 11, Segmentation fault g115 0xf773cb88 in _doscan ()

So, the error occurred within the function _doscan(). This is not one of my functions, so it must have been called by one of the C library functions which I am using, i.e. printf() or scanf(). Given the name _doscan, it does sound like it must have been the latter, but at any rate, the way to find out is to use gdb's bt (``backtrace'') command, to see where _doscan() was called from:

g116 (gdb) bt g117 #0 0xf773cb88 in _doscan () g118 #1 0xf773c2e8 in _doscan () g119 #2 0xf773b9dc in scanf () g120 #3 0x22dc in main () at Main.c:25

Aha! So it was indeed called from scanf(), which in turn was called from main(), at Line 25.

Now since scanf() is a C library function, it presumably is well debugged already, so the error was probably not in scanf(). So, the error must have been in our call to scanf() on Line 25 of Main.c.

Let's take a look at that latter call. To do so, we use gdb's l (''list'') command, to list some lines of the file Main.c at and arround the line which led to the error:

g121 (gdb) l Main.c:25 g122 20 main() g123 21 g124 22 { int N; g125 23 g126 24 printf("enter upper bound

"); g127 25 scanf("%d",UpperBound); g128 26 g129 27 Prime[2] = 1; g130 28 g131 29 for (N = 3; N <= UpperBound; N += 2)

Yep, a famous ``C-learner's error''-we forgot the ampersand in before UpperBound! Line 25 of Main.c should have been

scanf(``%d'',&UpperBound);

So, in another window (hopefully an X11 window, but if you are just using a text-based terminal, you can use the GNU screen program to get windows), we will fix line 25 of Main.c, and recompile. Note that we do not leave gdb while doing this, since gdb takes a long time to load. In order to do this, though, we must first tell gdb to relinquish our executable file:

(gdb) kill

(Otherwise when we tried to recompile our program, the ld linker would tell us that the executable file is ``busy'' and thus cannot be replaced.)

After fixing and recompiling Main.c, the next time we give gdb the run command

(gdb) r

gdb will automatically load the newly-recompiled executable for our program (it will notice that we recompiled, because it will see that our .c source file is newer than the executable file). Note that we do not have to state the command-line arguments (if any), because gdb remembers them from before. It also remembers our breakpoints, so we do not have to set them again. (And gdb will automatically update the line numbers for those breakpoints, adjusting for whatever line-number changes occurred when we modified the source file.)

Main.c is now the following:

g137 mole.matloff% cat Main.c g138 g139 g140 /* prime-number finding program g141 g142 will (after bugs are fixed) report a list of all primes which are g143 less than or equal to the user-supplied upper bound g144 g145 riddled with errors! */ g146 g147 g148 g149 #include "Defs.h" g150 g151 g152 int Prime[MaxPrimes], /* Prime[I] will be 1 if I is prime, 0 otherwi */ g153 UpperBound; /* we will check all number up through this one for g154 primeness */ g155 g156 g157 main() g158 g159 { int N; g160 g161 printf("enter upper bound

"); g162 scanf("%d",&UpperBound); g163 g164 Prime[2] = 1; g165 g166 for (N = 3; N <= UpperBound; N += 2) g167 CheckPrime(); g168 if (Prime[N]) printf("%d is a prime

",N); g169 }

Now we run the program again (not in gdb, though we do still have gdb open in the other window):

g174 mole.matloff% !F g175 FindPrimes g176 enter upper bound g177 20 g178 Segmentation fault

Don't get discouraged! Let's see where this new seg fault is occurring.

g188 (gdb) r g189 Starting program: /tmp_mnt/lion/d/guest/matloff/tmp/FindPrimes g190 enter upper bound g191 20 g192 g193 Program received signal 11, Segmentation fault g194 0x2388 in CheckPrime (K=1) at CheckPrime.c:21 g195 21 if (Prime[J] == 1)

Now, remember, as mentioned earlier, one of the most common causes of a seg fault is a wildly-erroneous array index. Thus we should be highly suspicious of J in this case, and should check what its value is, using gdb's p (``print'') command:

g207 (gdb) p J g208 $1 = 4024

Wow! Remember, I only had set up the array Prime to contain 50 integers, and yet here we are trying to access Prime[4024]!2

So, gdb has pinpointed the exact source of our error-the value of J is way too large on this line. Now we have to determine why J was so big. Let's take a look at the entire function, using gdb's l command:

g196 (gdb) l CheckPrime.c:12 g53 /* the plan: see if J divides K, for all values J which are g54 g55 (a) themselves prime (no need to try J if it is nonprime), and g56 (b) less than or equal to sqrt(K) (if K has a divisor larger g57 than sqrt(K), it must also have a smaller one, g58 so no need to check for larger ones) */ g59 g200 19 J = 2; g201 20 while (1) { g202 21 if (Prime[J] == 1) g203 22 if (K % J == 0) { g204 23 Prime[K] = 0; g205 24 return; g206 25 }

Look at the comments in Lines g56-g58. We were supposed to stop searching after J got to sqrt(K). Yet you can see in Lines g201-g206 that we never made this check, so J just kept growing and growing, eventually reaching the value 4024 which triggered the seg fault.

After fixing this problem, the new CheckPrime.c looks like this:

g214 mole.matloff% cat CheckPrime.c g215 g216 g217 #include "Defs.h" g218 #include "Externs.h" g219 g220 g221 CheckPrime(K) g222 int K; g223 g224 { int J; g225 g226 /* the plan: see if J divides K, for all values J which are g227 g228 (a) themselves prime (no need to try J if it is nonprime), and g229 (b) less than or equal to sqrt(K) (if K has a divisor larger g230 than this square root, it must also have a smaller one, g231 so no need to check for larger ones) */ g232 g233 for (J = 2; J*J <= K; J++) g234 if (Prime[J] == 1) g235 if (K % J == 0) { g236 Prime[K] = 0; g237 return; g238 } g239 g240 /* if we get here, then there were no divisors of K, so it is g241 prime */ g242 Prime[K] = 1; g243 }

OK, let's give it another try:

g248 mole.matloff% !F g249 FindPrimes g250 enter upper bound g251 20 g252 mole.matloff%

What?! No primes reported up to the number 20? That's not right. Let's use gdb to step through the program. We will pause at the beginning of main(), and take a look around. To do that, we set up a ``breakpoint,'' i.e. a place where gdb will suspend execution of our program, so that we can assess the situation before resuming execution:

g261 (gdb) b main g262 Breakpoint 1 at 0x22b4: file Main.c, line 24.

So, gdb will pause execution of our program whenever it hits Line 24 of the file Main.c. This is Breakpoint 1; we might (and will) set other breakpoints later, so we need numbers to distinguish them, e.g. in order to specify which one we want to cancel.

Now let's run the program, using the r command:

g286 (gdb) r g287 Starting program: /tmp_mnt/lion/d/guest/matloff/tmp/FindPrimes g288 g289 Breakpoint 1, main () at Main.c:24 g290 24 printf("enter upper bound

");

We see that, as planned, gdb did stop at the first line of main() (Line 24). Now we will execute the program one line at a time, using gdb's n (``next'') command:

g291 (gdb) n g292 enter upper bound g293 25 scanf("%d",&UpperBound);

What happened was that gdb executed Line 24 of Main.c as requested-the message from the call to printf() appears on Line g292-and now has paused again, at Line 25 of Main.c, displaying that line for us (Line g293 of the script file).

OK, let's execute Line 25, by typing `n' again:

g294 (gdb) n g295 20 g296 27 Prime[2] = 1;

Since Line 25 was a scanf() call, at Line g295 of the script file, gdb waited for our input, which we typed as 20. Gdb then executed the scanf() call, and paused again, now at Line 27 of Main.c (Line 296 of the script file.

Now let's check to make sure that UpperBound was read in correctly. We think it was, but remember, the basic principle of debugging is to check anyway. To do this, we will use gdb's p (``print'') command:

g297 (gdb) p UpperBound g298 $1 = 20

OK, that's fine. So, let's continue to execute the program one line at a time, by using n:

g299 (gdb) n g300 29 for (N = 3; N <= UpperBound; N += 2)

Also, let's keep track of the value of N, using gdb's disp (``display'') command. The latter is just like the p, except that disp will print out the value of the variable each time the program pauses, as opposed to p, which prints out the value only once .

g301 (gdb) disp N g302 1: N = 0 g303 (gdb) n g304 30 CheckPrime(); g305 1: N = 3 g306 (gdb) n g307 29 for (N = 3; N <= UpperBound; N += 2) g308 1: N = 3

Hey, what's going on here? After executing Line 30, the program then went back to Line 29-skipping Line 31. Here is what the loop looked like:

29 for (N = 3; N <= UpperBound; N += 2) 30 CheckPrime(); 31 if (Prime[N]) printf("%d is a prime

",N);

Oops! We forgot the braces. Thus only Line 30, not Lines 30 and 31, forms the body of the loop. No wonder Line 31 wasn't executed.

After fixing that, Main.c looks like this:

g314 mole.matloff% cat Main.c g315 g316 g317 /* prime-number finding program g318 g319 will (after bugs are fixed) report a list of all primes which are g320 less than or equal to the user-supplied upper bound g321 g322 riddled with errors! */ g323 g324 g325 g326 #include "Defs.h" g327 g328 g329 int Prime[MaxPrimes], /* Prime[I] will be 1 if I is prime, 0 otherwise */ g330 UpperBound; /* we will check all number up through this one for g331 primeness */ g332 g333 g334 main() g335 g336 { int N; g337 g338 printf("enter upper bound

"); g339 scanf("%d",&UpperBound); g340 g341 Prime[2] = 1; g342 g343 for (N = 3; N <= UpperBound; N += 2) { g344 CheckPrime(); g345 if (Prime[N]) printf("%d is a prime

",N); g346 } g347 }

OK, try again:

g352 mole.matloff% !F g353 FindPrimes g354 enter upper bound g355 20 g356 mole.matloff%

Still no output! Well, we will now need to try a more detailed line-by-line execution of the program. Last time, we did not go through the function CheckPrime() line-by-line, so we will need to now:

g586 (gdb) l Main.c:1 g587 1 g588 2 g589 3 /* prime-number finding program g590 4 g591 5 will (after bugs are fixed) report a list of all primes which g592 (gdb) g593 6 are less than or equal to the user-supplied upper bound g594 7 g595 8 riddled with errors! */ g596 9 g597 10 g598 11 g599 12 #include "Defs.h" g600 13 g601 14 g602 15 int Prime[MaxPrimes], /* Prime[I] will be 1 if I is prime, 0 g603 (gdb) g604 16 UpperBound; /* we will check all number up through this one g605 17 primeness */ g606 18 g607 19 g608 20 main() g609 21 g610 22 { int N; g611 23 g612 24 printf("enter upper bound

"); g613 25 scanf("%d",&UpperBound); g614 (gdb) g615 26 g616 27 Prime[2] = 1; g617 28 g618 29 for (N = 3; N <= UpperBound; N += 2) { g619 30 CheckPrime(); g620 31 if (Prime[N]) printf("%d is a prime

",N); g621 32 } g622 33 } g623 34 g624 (gdb) b 30 g625 Breakpoint 1 at 0x2308: file Main.c, line 30.

Here we have placed a breakpoint at the call to CheckPrime.3

Now, let's run the program:

g626 (gdb) r g627 Starting program: /tmp_mnt/lion/d/guest/matloff/tmp/FindPrimes g628 enter upper bound g629 20 g630 g631 Breakpoint 1, main () at Main.c:30 g632 30 CheckPrime();

Gdb has stopped at Line 30, as we requested. Now, instead of using the n command, we will use s (``step''). This latter command is the same as n, except that it will enter the function rather than skipping over the function like n does:

g633 (gdb) s g634 CheckPrime (K=1) at CheckPrime.c:19 g635 19 for (J = 2; J*J <= K; J++)

Sure enough, s has gotten us to the first line within CheckPrime().

Another service gdb provides for us is to tell us what the values of the parameters of the function are, in this case K = 1. But that doesn't sound right-we shouldn't be checking the number 1 for primeness. So gdb has uncovered another bug for us.

In fact, our plan was to check the numbers 3 through UpperBound for primeness: The for loop in main() had the following heading:

for (N = 3; N <= UpperBound; N += 2)

Well, what about the call to CheckPrime()? Here is the whole loop from main():

29 for (N = 3; N <= UpperBound; N += 2) { 30 CheckPrime(); 31 if (Prime[N]) printf("%d is a prime

",N); 32 }

Look at Line 30-we forgot the parameter! This line should have been

30 CheckPrime(N);

After fixing this, try running the program again:

g699 mole.matloff% !F g700 FindPrimes g701 enter upper bound g702 20 g703 3 is a prime g704 5 is a prime g705 7 is a prime g706 11 is a prime g707 13 is a prime g708 17 is a prime g709 19 is a prime

OK, the program now seems to be working.

4.7 Documentation on gdb

There is a bound manual that you can buy, but it is probably not worthwhile, since you can get all the documentation online. First you can get an overview of the categories of commands by simply typing `h' (``help''), and then get information on any category or individual command by typing `h name', where `name' is the name of the category or individual command. For example, to get information on using breakpoints, type

h breakpoints

Footnotes:

1That in turns means that you should make sure to use a good editor which has these operations.

2Note very, very carefully, though: Most C compilers-including the Unix one, which is what we are using here-do not produce checks for violation of array bounds. In fact, a ``moderate'' violation, e.g. trying to access Prime[57], would not have produced a seg fault. The reason that our attempt to access Prime[4024] did produce a seg fault is that it resulted in our trying to access memory which did not belong to us, i.e. belonged to some other user of the machine. The virtual-memory hardware of the machine detected this.

By the way, the `$1' here just means that we can refer to 4024 by this name from now on if we like.

3We could have simply typed ``b CheckPrime'', which would have set a breakpoint at the first line of CheckPrime(), but doing it this way gives as a chance to see how the s command works. By the way, note Lines 592, 603 and 614; here we simply typed the carriage return, which results in gdb listing some further lines for us.



