Using the command line interface of debug

Session 1: why is my program in an infinite loop?

Compile macros:

   $ cc -g -o macros main.c macro.c
   main.c:
   macro.c:

Since you are confident that it will work as you expected, run the program:

   $ cat test1
   define(TEST,this is a test)
   TEST
   define(X,TEST test TEST)
   X
   define(X,x)
   X
   define(A1,1 2 3 4)  A1
   MAC
   $ macros <test1

After a significant amount of time has passed it becomes apparent that the program must be looping somewhere. You can interrupt it and do the following:
- To find out exactly where it is looping, examine the running program. Run the program in the background:
```
   $ macros <test1 &
   27221
```
  Write down the process id (pid).
- Type debug to start the debugger. The
```
   debug>
```
  prompt is displayed.
- grab the running process. This may take some time because macros uses a great deal of memory.
```
   debug> grab 27221
   New program macros (process p1) grabbed
   HALTED p1 [malloc in malloc.c]
   0x800187c1     (malloc+256:)    popl    %ecx
```
The debugger command grab pid suspends process pid and brings it under the debugger's control. From then on, process pid will not do anything until you give debug another command allowing it to run. debug tells you what function was being executed when the process was suspended. In this example it was suspended in malloc.
NOTE: See ``Grabbing processes'' for more information.
Check to see what was calling malloc with the stack (or t) command:
```
   debug> stack
   Stack Trace for p1, Program macros
   *[0] malloc(0xc) [0x800187c1]
    [1] tokenize(buf="(TEST,this is a test)\n") [main.c@62]
    [2] gettok(read_more=1) [main.c@167]
    [3] main(argc=1, argv=0x80479cc, 0x80479d4) [main.c@333]
    [4] _start()    [0x80485a2]
```
The most recently invoked function is always displayed at the top of the stack trace. Here, the process is in malloc(3), which was called by tokenize, called in turn by gettok, and so on. Since the point at which the process was suspended is random, when you try this, the process may be suspended in a different function, and your stack trace would not look exactly like this, but since the program is in a tight loop it should be similar. The program could conceivably be stuck in malloc, but it isn't likely because library functions, particularly frequently used ones like malloc, tend to be well tested. Since macros hasn't been tested at all, you can assume the bug is in the code for macros. Finding the bug is an iterative process and may take several tries.
NOTE: See ``Stack traces'' for more information.

You would like to let the program run until it is finished in malloc but before it does anything else in tokenize. But you are unsure of what option to use with the run command.
Ask debug for help:
```
   debug> help run
```

The run help message is displayed:

run - set thread or process running.

SYNOPSIS
run [-p proclist] [-bfr] [-u location]

DESCRIPTION
Set the specified list of threads and processes
(%thread or %proc by default) running.
The -f option specifies that the debugger will
run the thread or process in the foreground, i.e., it will
wait until the threads or processes stop before returning
control to the user.
The -b option specifies background execution (no waiting).
Control returns immediately to the user, and the thread or
process is started.

The -r option causes the thread or process to run until the
current function returns.
The -u option specifies a "location" to run to.

Note that threads that are not currently running on a kernel
light-weight process cannot be set running (such threads show
up in the "Off LWP" state in the ps command).

EXAMPLES
run -p all -b
run -r
run -u 108

SEE ALSO
%wait, location, proclist, ps, step, thread

NOTE: See ``Getting help'' for more information.

NOTE: Successive runs continue execution from the point where the program stopped, not from the beginning. create re-starts the program.

Type
```
   run -r
```
to get out of malloc (-r stands for run to return). (If you grabbed the process while it was still in tokenize you shouldn't do the next step. If the program is suspended in something malloc calls, you will have to do it more than once.)
```
   debug> run -r
   HALTED p1 [tokenize in main.c]
   62:  token = (struct token *)malloc(sizeof(struct token));
```

Look at what's going on in tokenize:

   debug> list
   62:  token = (struct token *)malloc(sizeof(struct token));
   63:  if (!token)
   64:  {
   65:          (void) fprintf(stderr, "malloc failed!\n");
   66:          exit(1);
   67:  }
   68:  token->string = 0;
   69:  token->next = 0;
   70:
   71:  if (!head)

If the program is really looping forever the odds are good that if you let the program run it will come back to this point.

Stop the program when it gets back to this point by defining a ``stop event'' (also commonly known as setting a breakpoint) on this line:
```
   debug> stop 62
   EVENT [1] assigned
```
To be on the safe side you can also define a stop event at the beginning of the function. This will let you know if it leaves tokenize and comes back, which will suggest that the problem is in not in tokenize, but in gettok instead.
```
   debug> stop tokenize
   EVENT [2] assigned
```

Before you let the program run, look at the local variables:

   debug> symbols -tv
   Symbols for p1, Program macros
   Name        Location  Line  Type           Value
   buf         tokenize   52   const char *   "(TEST,this is a test)\n"
   c           tokenize   52   char           40
   head        tokenize   52   struct token * 0x804a880
   len         tokenize   52   unsigned int   7
   next        tokenize   52   const char *   "(TEST,this is a test)\n"
   save_string tokenize   52   label          0x804881c
   tail        tokenize   52   struct token * 0x81c3784
   token       tokenize   52   struct token * 0x81c3784

NOTE: See ``Printing symbols'' for more information.

Run the program:

   debug> run
   STOP EVENT TRIGGERED: 62  in p1 [tokenize in main.c]
   62:         token = (struct token *)malloc(sizeof(struct token));

The program has stopped at the same place. See what has changed:

   debug> symbols -tv
   Name        Location  Line  Type           Value
   buf         tokenize   52   const char *   "(TEST,this is a test)\n"
   c           tokenize   52   char           40
   head        tokenize   52   struct token * 0x804a880
   len         tokenize   52   unsigned int   7
   next        tokenize   52   const char *   "(TEST,this is a test)\n"
   save_string tokenize   52   label          0x804881c
   tail        tokenize   52   struct token * 0x81c3798
   token       tokenize   52   struct token * 0x81c3798

next and c haven't changed at all. That seems illogical. Look at tokenize to see where they are used:

   debug> list tokenize
   52:   {
   53:           struct token        *head = 0;
   54:           struct token        *tail = 0;
   55:           struct token        *token;
   56:           const char          *next = buf;
   57:           char                c;
   58:           size_t              len;
   59: 
   60:           while ((c = *next) != '\0')
   61:           {

The program is in a while loop that depends on *next. If next doesn't change, the program will never get out of the loop. It is reasonable to assume you have found the bug, so you can quit debug. However, before you leave debug kill the macros process. Since it was a running program that you grabbed, the normal behavior for debug when it exits is to release the process and let it keep on running. You'll have to kill it either at the shell level or in debug, so you might as well do it here to make sure it doesn't do something strange when it resumes running uncontrolled.

   debug> kill
   p1 killed
   No more processes.
   debug> quit
   $

NOTE: You can use either quit, q or exit to get out of debug.

Grabbing processes

In addition to the grab command, you can grab a process directly from the debug command line, or by using its pathname in the /proc file system, so these examples are all equivalent:

   $ debug /proc/27221
   . . .
   $ debug 27221
   . . .
   $ debug
   debug> grab /proc/27221
   . . .
   $ debug
   debug> grab 27221

Of course, the normal security rules apply; you cannot get control over a process with a different effective user id from your own.

The debugger prints a message showing you the name of the program (usually the name of the object file) and a debugger-generated name for the process. The debugger maintains a hierarchy of program, process and thread; programs are at the top and each program is made up of one or more processes, which in turn, are made up of one or more threads (if the program is multi-threaded). For now, you may assume that there is only one of each, and that the program name and process name are synonymous, but the debugger displays its information using the process name where the program name might be ambiguous. The significance of the hierarchy becomes more apparent when debugging pipelines and the effects of fork(S), exec(S) and thr_create(3thread). (See ``Multi-process debugging''.)

By using grab and create (described later) you can debug different programs without having to exit and restart the debugger.

Stack traces

There are several things to note about stack traces:

The bottom of the stack is not main, as you might have expected, but _start. _start is supplied by the compilation system and is the interface between the operating system and main; it will show up in the stack trace but can safely be ignored.
The current frame is marked with an asterisk (``*'') to the left of the frame number.
NOTE: See ``Changing the current context'' for more information on changing the current frames and its effects.
By default, stack frames are numbered from 0 for the most recent frame (the top of the stack) and the bottom frame is the highest numbered. You may change the numbering (by setting the debugger variable %frame_numbers to down) so that the most recent frame has the highest number and the initial frame is numbered 0. This is not generally recommended, as it can cause the stack command to take significantly longer, or even run out of memory without producing any output for processes that have huge stacks, since the debugger must determine the number of frames in the stack before displaying any frames.
debug will print both the parameter names and values wherever it can, but it does not always know the parameter names. If the function was compiled without -g (like the library function malloc) the debugger will not be able to print the parameter names, and it will not always have parameters names even for functions compiled with -g. If you look at the entry for main in the example above, you will see that the first two arguments have names, but the third doesn't. The definition of main specifies only two parameters:
```
   main(int argc, char **argv)
   {   ...
```
but three parameters are actually passed to main [the third is the environment pointer -- see exec(S)], so debug prints the third argument without a name. You will also see this with functions that take a variable number of arguments; the arguments referenced with va_arg (see stdarg(5)) will not have names.
If it has the information, debug will print a file name and line number with each entry (also known as a stack frame) in the stack trace. The line number is the statement that the program was executing when it called the next function or when it was suspended. In the example above, malloc was called from line 62 of tokenize. Even though tokenize calls malloc from two different places, you don't have to guess which statement made the call. If the debugger doesn't know the line number, it will print the value of the program counter instead. You can discern the difference by the format; line numbers are printed in decimal, program counters and other addresses are printed in hexadecimal (starting with 0x).

Stack traces occasionally get to be long, but you can tell stack to print only the top n lines:

   debug> stack -c 3
   Stack Trace for p1, Program ackerman
   *[0] ack(m=0, n=5)        [b1.c@11]
    [1] ack(m=1, n=4)        [b1.c@18]
    [2] ack(m=1, n=5)        [b1.c@18]

If you are debugging code that is not compiled with -g, you will occasionally see a stack trace that looks like this:
```
   debug> stack
   Stack Trace for p1, Program sget
   *[0] main(presumed: 0x1, 0x8047dd4, 0x8047ddc) [0x8300950]
    [1] _start()    [0x8300662]
```
The presumed: indicates that the debugger does not have enough information to know how many arguments were pushed onto the stack for that function. In that case, it prints out three entries on the stack, guessing that they might be arguments.
You will occasionally see a stack trace that looks like this:
```
   debug> stack
   Stack Trace for p1, Program a.out
   *[0] ?()
```
This indicates that debug cannot make any sense out of the stack, and usually means that there is a bad pointer somewhere; either the program scribbled on the stack or it tried to follow a pointer into never-never land. About the best you can do here is start over again and try to find out where it was before it got lost.
NOTE: Users familiar with assembly-level debugging can use the -a and -s options to the stack command to try and recreate the program stack as it was prior to becoming corrupted.

Getting help

Help is always at your fingertips. Typing help command_name will produce a synopsis of the command's syntax, a brief description of what the command does, and a list of other related topics. Examples of two of those other topics are sysnames and format:

   debug> help sysnames

will list all the system call names that the syscall command accepts, and

   debug> help format

will produce a description of the print command's formatting capability. Typing help (or h) by itself will produce a list of all the topics you can get help on, including all the commands.

Printing symbols

The symbols (or syms) command with no options will print out the names of the local variables that are visible from the current scope:

   debug> symbols
   Symbols for p1, Program macros
   Name         Location    Line
   buf          tokenize    52
   c            tokenize    52
   head         tokenize    52
   len          tokenize    52
   next         tokenize    52
   save_string  tokenize    52
   tail         tokenize    52
   token        tokenize    52

You can restrict the symbols displayed by giving symbols a pattern [using sh(C) syntax for pattern matching]:

   debug> symbols [b-h]?*
   Symbols for p1, Program macros
   Name         Location    Line
   buf          tokenize    52
   head         tokenize    52

Adding the -t or -v (or both) options will make debug print the types or values (or both):

   debug> symbols -tv t*
   Symbols for p1, Program macros
   Name         Location    Line   Type              Value
   tail         tokenize    52     struct token *    0x81c3798
   token        tokenize    52     struct token *    0x81c3798

Other options to symbols make it display other groups of symbols. For example, the -f option (for file static) displays static variables visible from the current compilation unit:

   debug> symbols -ft
   Symbols for p1, Program macros
   Name         Location    Line   Type
   Usage        main.c             char *
   add_def      main.c             void(struct token *, int)
   gettok       main.c             struct token *(int)
   lines        main.c             int
   next_tok     main.c             struct token *
   process_def  main.c             void()
   tokenize     main.c             void(const char *)

There are two things to note about the output from symbols:

The value displayed for a function is its address.

   debug> symbols -ftv t*
   Symbols for p1, Program macros
   Name         Location    Line   Type            Value
   tokenize     main.c             void(const char *)    0x80486ec

The Location field displays the symbol's scope. For global symbols this is the name of the program or shared library where they are defined, for static symbols the name of the compilation unit where they are defined, and for local variables, the file, function and line number of the block where they are defined. For example, if you were stopped in the inner loop in main and typed symbols, the list of local symbols would include the symbols from

both the inner and outer blocks. The two blocks are distinguished by different line numbers.
```
   debug> symbols
   Symbols for p1, Program macros
   Name         Location    Line
   . . .
   name         main        300
   parens       main        347
   tail         main        347
   token        main        300
```

Shorthand notations

t, r, l and b are shorthand notations for stack, run, list and stop, respectively. There are several other built-in shorthands (or aliases) available. You can create your own or redefine the built-ins with the alias command:

   debug> alias lm list main
   debug> lm
   300:        {
   301:                struct token        *definition;
   302:                struct token        *name;
   303:                struct token        *token;
   304:                struct arglist      *arguments;
   305:                char                *argptr;
   306:                int                 i;
   307:    
   308:                for (i = 1; i < argc; ++i)
   309:                {

alias with no arguments will list all the aliases, both built-in and user-defined:

   debug> alias
   b => stop
   exit => quit
   h => help
   history => fc -1
   l => list
   lm => list main
   n => step -o
   next => step -o

   ni => step -io
   p => print
   q => quit
   r => run
   rr => fc -e -
   s => step
   si => step -i
   sig => signal
   syms => symbols
   sys => syscall
   t => stack

The commands and their built-in aliases are interchangeable. This tutorial normally uses the complete command names for clarity.

debug also supports ksh-style command history and command-line editing. Both vi and emacs editing modes are supported, and, like ksh, debug determines the default editing modes from the VISUAL and EDITOR environment variables. For information on using this capability see ksh(C).