The debugger (ocamldebug)

Chapter 16 | The debugger (ocamldebug)

This chapter describes the Objective Caml source-level replay debugger `ocamldebug`.
Unix:

> The debugger is available on Unix systems that provide BSD sockets.

Windows:

> The debugger is available under the Cygwin port of Objective Caml, but not under the native Win32 port.

MacOS:

> The debugger is not available.

16.1 | Compiling for debugging

Before the debugger can be used, the program must be compiled and linked with the `-g` option: all `.cmo` and `.cma` files that are part of the program should have been created with `ocamlc -g`, and they must be linked together with `ocamlc -g`.
Compiling with `-g` entails no penalty on the running time of programs: object files and bytecode executable files are bigger and take longer to produce, but the executable files run at exactly the same speed as if they had been compiled without `-g`.

16.2 | Invocation

16.2.1 | Starting the debugger

The Objective Caml debugger is invoked by running the program `ocamldebug` with the name of the bytecode executable file as first argument:

            ocamldebug [options] program [arguments]

The arguments following program are optional, and are passed as command-line arguments to the program being debugged. (See also the `set arguments` command.)
The following command-line options are recognized:

`-I` directory
  Add directory to the list of directories searched for source files and compiled files. (See also the `directory` command.)
`-s` socket
  Use socket for communicating with the debugged program. See the description of the command `set socket` (section 16.8.6) for the format of socket.
`-c` count
  Set the maximum number of simultaneously live checkpoints to count.
`-cd` directory
  Run the debugger program from the working directory directory, instead of the current directory. (See also the `cd` command.)
`-emacs`
  Tell the debugger it is executed under Emacs. (See section 16.10 for information on how to run the debugger under Emacs.)

16.2.2 | Exiting the debugger

The command `quit` exits the debugger. You can also exit the debugger by typing an end-of-file character (usually `ctrl-D`).
Typing an interrupt character (usually `ctrl-C`) will not exit the debugger, but will terminate the action of any debugger command that is in progress and return to the debugger command level.

16.3 | Commands

A debugger command is a single line of input. It starts with a command name, which is followed by arguments depending on this name. Examples:

            run
            goto 1000
            set arguments arg1 arg2

A command name can be truncated as long as there is no ambiguity. For instance, `go 1000` is understood as `goto 1000`, since there are no other commands whose name starts with `go`. For the most frequently used commands, ambiguous abbreviations are allowed. For instance, `r` stands for `run` even though there are others commands starting with `r`. You can test the validity of an abbreviation using the `help` command.
If the previous command has been successful, a blank line (typing just `RET`) will repeat it.

16.3.1 | Getting help

The Objective Caml debugger has a simple on-line help system, which gives a brief description of each command and variable.

`help`
  Print the list of commands.
`help` command
  Give help about the command command.
`help set` variable, `help show` variable
  Give help about the variable variable. The list of all debugger variables can be obtained with `help set`.
`help info` topic
  Give help about topic. Use `help info` to get a list of known topics.

16.3.2 | Accessing the debugger state

`set` variable value
  Set the debugger variable variable to the value value.
`show` variable
  Print the value of the debugger variable variable.
`info` subject
  Give information about the given subject. For instance, `info breakpoints` will print the list of all breakpoints.

16.4 | Executing a program

16.4.1 | Events

Events are ``interesting'' locations in the source code, corresponding to the beginning or end of evaluation of ``interesting'' sub-expressions. Events are the unit of single-stepping (stepping goes to the next or previous event encountered in the program execution). Also, breakpoints can only be set at events. Thus, events play the role of line numbers in debuggers for conventional languages.
During program execution, a counter is incremented at each event encountered. The value of this counter is referred as the current time. Thanks to reverse execution, it is possible to jump back and forth to any time of the execution.
Here is where the debugger events (written §§) are located in the source code:

- Following a function application:

      (f arg)§§

- On entrance to a function:

      fun x y z -> §§ ...

- On each case of a pattern-matching definition (function, `match`...`with` construct, `try`...`with` construct):

      function pat1 -> §§ expr1
             | ...
             | patN -> §§ exprN

- Between subexpressions of a sequence:

      expr1; §§ expr2; §§ ...; §§ exprN

- In the two branches of a conditional expression:

      if cond then §§ expr1 else §§ expr2

- At the beginning of each iteration of a loop:

      while cond do §§ body done
      for i = a to b do §§ body done

Exceptions: A function application followed by a function return is replaced by the compiler by a jump (tail-call optimization). In this case, no event is put after the function application.

16.4.2 | Starting the debugged program

The debugger starts executing the debugged program only when needed. This allows setting breapoints or assigning debugger variables before execution starts. There are several ways to start execution:

`run`
  Run the program until a breakpoint is hit, or the program terminates.
`step 0`
  Load the program and stop on the first event.
`goto` time
  Load the program and execute it until the given time. Useful when you already know approximately at what time the problem appears. Also useful to set breakpoints on function values that have not been computed at time 0 (see section 16.5).

The execution of a program is affected by certain information it receives when the debugger starts it, such as the command-line arguments to the program and its working directory. The debugger provides commands to specify this information (`set arguments` and `cd`). These commands must be used before program execution starts. If you try to change the arguments or the working directory after starting your program, the debugger will kill the program (after asking for confirmation).

16.4.3 | Running the program

The following commands execute the program forward or backward, starting at the current time. The execution will stop either when specified by the command or when a breakpoint is encountered.

`run`
  Execute the program forward from current time. Stops at next breakpoint or when the program terminates.
`reverse`
  Execute the program backward from current time. Mostly useful to go to the last breakpoint encountered before the current time.
`step` [count]
  Run the program and stop at the next event. With an argument, do it count times.
`backstep` [count]
  Run the program backward and stop at the previous event. With an argument, do it count times.
`next` [count]
  Run the program and stop at the next event, skipping over function calls. With an argument, do it count times.
`previous` [count]
  Run the program backward and stop at the previous event, skipping over function calls. With an argument, do it count times.
`finish`
  Run the program until the current function returns.
`start`
  Run the program backward and stop at the first event before the current function invocation.

16.4.4 | Time travel

You can jump directly to a given time, without stopping on breakpoints, using the `goto` command.
As you move through the program, the debugger maintains an history of the successive times you stop at. The `last` command can be used to revisit these times: each `last` command moves one step back through the history. That is useful mainly to undo commands such as `step` and `next`.

`goto` time
  Jump to the given time.
`last` [count]
  Go back to the latest time recorded in the execution history. With an argument, do it count times.
`set history` size
  Set the size of the execution history.

16.4.5 | Killing the program

`kill`
  Kill the program being executed. This command is mainly useful if you wish to recompile the program without leaving the debugger.

16.5 | Breakpoints

A breakpoint causes the program to stop whenever a certain point in the program is reached. It can be set in several ways using the `break` command. Breakpoints are assigned numbers when set, for further reference. The most comfortable way to set breakpoints is through the Emacs interface (see section 16.10).

`break`
  Set a breakpoint at the current position in the program execution. The current position must be on an event (i.e., neither at the beginning, nor at the end of the program).
`break` function
  Set a breakpoint at the beginning of function. This works only when the functional value of the identifier function has been computed and assigned to the identifier. Hence this command cannot be used at the very beginning of the program execution, when all identifiers are still undefined; use `goto` time to advance execution until the functional value is available.
`break @` [module] line
  Set a breakpoint in module module (or in the current module if module is not given), at the first event of line line.
`break @` [module] line column
  Set a breakpoint in module module (or in the current module if module is not given), at the event closest to line line, column column.
`break @` [module] `#` character
  Set a breakpoint in module module at the event closest to character number character.
`break` address
  Set a breakpoint at the code address address.
`delete` [breakpoint-numbers]
  Delete the specified breakpoints. Without argument, all breakpoints are deleted (after asking for confirmation).
`info breakpoints`
  Print the list of all breakpoints.

16.6 | The call stack

Each time the program performs a function application, it saves the location of the application (the return address) in a block of data called a stack frame. The frame also contains the local variables of the caller function. All the frames are allocated in a region of memory called the call stack. The command `backtrace` (or `bt`) displays parts of the call stack.
At any time, one of the stack frames is ``selected'' by the debugger; several debugger commands refer implicitly to the selected frame. In particular, whenever you ask the debugger for the value of a local variable, the value is found in the selected frame. The commands `frame`, `up` and `down` select whichever frame you are interested in.
When the program stops, the debugger automatically selects the currently executing frame and describes it briefly as the `frame` command does.

`frame`
  Describe the currently selected stack frame.
`frame` frame-number
  Select a stack frame by number and describe it. The frame currently executing when the program stopped has number 0; its caller has number 1; and so on up the call stack.
`backtrace` [count], `bt` [count]
  Print the call stack. This is useful to see which sequence of function calls led to the currently executing frame. With a positive argument, print only the innermost count frames. With a negative argument, print only the outermost -count frames.
`up` [count]
  Select and display the stack frame just ``above'' the selected frame, that is, the frame that called the selected frame. An argument says how many frames to go up.
`down` [count]
  Select and display the stack frame just ``below'' the selected frame, that is, the frame that was called by the selected frame. An argument says how many frames to go down.

16.7 | Examining variable values

The debugger can print the current value of simple expressions. The expressions can involve program variables: all the identifiers that are in scope at the selected program point can be accessed.
Expressions that can be printed are a subset of Objective Caml expressions, as described by the following grammar:

`expr` | ::= | `lowercase-ident`
| | | { `capitalized-ident` `.` } `lowercase-ident`
| | | `*`
| | | `$` `integer`
| | | `expr` `.` `lowercase-ident`
| | | `expr` `.(` `integer` `)`
| | | `expr` `.[` `integer` `]`
| | | `!` `expr`
| | | `(` `expr` `)`

The first two cases refer to a value identifier, either unqualified or qualified by the path to the structure that define it. `*` refers to the result just computed (typically, the value of a function application), and is valid only if the selected event is an ``after'' event (typically, a function application). `$` `integer` refer to a previously printed value. The remaining four forms select part of an expression: respectively, a record field, an array element, a string element, and the current contents of a reference.

`print` variables
  Print the values of the given variables. `print` can be abbreviated as `p`.
`display` variables
  Same as `print`, but limit the depth of printing to 1. Useful to browse large data structures without printing them in full. `display` can be abbreviated as `d`.

When printing a complex expression, a name of the form `$`integer is automatically assigned to its value. Such names are also assigned to parts of the value that cannot be printed because the maximal printing depth is exceeded. Named values can be printed later on with the commands `p $`integer or `d $`integer. Named values are valid only as long as the program is stopped. They are forgotten as soon as the program resumes execution.

`set print_depth` d
  Limit the printing of values to a maximal depth of d.
`set print_length` l
  Limit the printing of values to at most l nodes printed.

16.8 | Controlling the debugger

16.8.1 | Setting the program name and arguments

`set program` file
  Set the program name to file.
`set arguments` arguments
  Give arguments as command-line arguments for the program.

A shell is used to pass the arguments to the debugged program. You can therefore use wildcards, shell variables, and file redirections inside the arguments. To debug programs that read from standard input, it is recommended to redirect their input from a file (using `set arguments < input-file`), otherwise input to the program and input to the debugger are not properly separated, and inputs are not properly replayed when running the program backwards.

16.8.2 | How programs are loaded

The `loadingmode` variable controls how the program is executed.

`set loadingmode direct`
  The program is run directly by the debugger. This is the default mode.
`set loadingmode runtime`
  The debugger execute the Objective Caml runtime `ocamlrun` on the program. Rarely useful; moreover it prevents the debugging of programs compiled in ``custom runtime'' mode.
`set loadingmode manual`
  The user starts manually the program, when asked by the debugger. Allows remote debugging (see section 16.8.6).

16.8.3 | Search path for files

The debugger searches for source files and compiled interface files in a list of directories, the search path. The search path initially contains the current directory `.` and the standard library directory. The `directory` command adds directories to the path.
Whenever the search path is modified, the debugger will clear any information it may have cached about the files.

`directory` directorynames
  Add the given directories to the search path. These directories are added at the front, and will therefore be searched first.
`directory`
  Reset the search path. This requires confirmation.

16.8.4 | Working directory

Each time a program is started in the debugger, it inherits its working directory from the current working directory of the debugger. This working directory is initially whatever it inherited from its parent process (typically the shell), but you can specify a new working directory in the debugger with the `cd` command or the `-cd` command-line option.

`cd` directory
  Set the working directory for `camldebug` to directory.
`pwd`
  Print the working directory for `camldebug`.

16.8.5 | Turning reverse execution on and off

In some cases, you may want to turn reverse execution off. This speeds up the program execution, and is also sometimes useful for interactive programs.
Normally, the debugger takes checkpoints of the program state from time to time. That is, it makes a copy of the current state of the program (using the Unix system call `fork`). If the variable checkpoints is set to `off`, the debugger will not take any checkpoints.

`set checkpoints` on/off
  Select whether the debugger makes checkpoints or not.

16.8.6 | Communication between the debugger and the program

The debugger communicate with the program being debugged through a Unix socket. You may need to change the socket name, for example if you need to run the debugger on a machine and your program on another.

`set socket` socket
  Use socket for communication with the program. socket can be either a file name, or an Internet port specification host:port, where host is a host name or an Internet address in dot notation, and port is a port number on the host.

On the debugged program side, the socket name is passed through the `CAML_DEBUG_SOCKET` environment variable.

16.8.7 | Fine-tuning the debugger

Several variables enables to fine-tune the debugger. Reasonable defaults are provided, and you should normally not have to change them.

`set processcount` count
  Set the maximum number of checkpoints to count. More checkpoints facilitate going far back in time, but use more memory and create more Unix processes.

As checkpointing is quite expensive, it must not be done too often. On the other hand, backward execution is faster when checkpoints are taken more often. In particular, backward single-stepping is more responsive when many checkpoints have been taken just before the current time. To fine-tune the checkpointing strategy, the debugger does not take checkpoints at the same frequency for long displacements (e.g. `run`) and small ones (e.g. `step`). The two variables `bigstep` and `smallstep` contain the number of events between two checkpoints in each case.

`set bigstep` count
  Set the number of events between two checkpoints for long displacements.
`set smallstep` count
  Set the number of events between two checkpoints for small displacements.

The following commands display information on checkpoints and events:

`info checkpoints`
  Print a list of checkpoints.
`info events` [module]
  Print the list of events in the given module (the current module, by default).

16.8.8 | User-defined printers

Just as in the toplevel system (section 9.2), the user can register functions for printing values of certain types. For technical reasons, the debugger cannot call printing functions that reside in the program being debugged. The code for the printing functions must therefore be loaded explicitly in the debugger.

`load_printer "`file-name`"`
  Load in the debugger the indicated `.cmo` or `.cma` object file. The file is loaded in an environment consisting only of the Objective Caml standard library plus the definitions provided by object files previously loaded using `load_printer`. If this file depends on other object files not yet loaded, the debugger automatically loads them if it is able to find them in the search path. The loaded file does not have direct access to the modules of the program being debugged.
`install_printer` printer-name
  Register the function named printer-name (a value path) as a printer for objects whose types match the argument type of the function. That is, the debugger will call printer-name when it has such an object to print. The printing function printer-name must use the `Format` library module to produce its output, otherwise its output will not be correctly located in the values printed by the toplevel loop.
  The value path printer-name must refer to one of the functions defined by the object files loaded using `load_printer`. It cannot reference the functions of the program being debugged.
`remove_printer` printer-name
  Remove the named function from the table of value printers.

16.9 | Miscellaneous commands

`list` [module] [beginning] [end]
  List the source of module module, from line number beginning to line number end. By default, 20 lines of the current module are displayed, starting 10 lines before the current position.
`source` filename
  Read debugger commands from the script filename.

16.10 | Running the debugger under Emacs

The most user-friendly way to use the debugger is to run it under Emacs. See the file `emacs/README` in the distribution for information on how to load the Emacs Lisp files for Caml support.
The Caml debugger is started under Emacs by the command `M-x camldebug`, with argument the name of the executable file progname to debug. Communication with the debugger takes place in an Emacs buffer named `*camldebug-`progname`*`. The editing and history facilities of Shell mode are available for interacting with the debugger.
In addition, Emacs displays the source files containing the current event (the current position in the program execution) and highlights the location of the event. This display is updated synchronously with the debugger action.
The following bindings for the most common debugger commands are available in the `*camldebug-`progname`*` buffer:

`C-c C-s`
  (command `step`): execute the program one step forward.
`C-c C-k`
  (command `backstep`): execute the program one step backward.
`C-c C-n`
  (command `next`): execute the program one step forward, skipping over function calls.
Middle mouse button
  (command `display`): display named value. `$`n under mouse cursor (support incremental browsing of large data structures).
`C-c C-p`
  (command `print`): print value of identifier at point.
`C-c C-d`
  (command `display`): display value of identifier at point.
`C-c C-r`
  (command `run`): execute the program forward to next breakpoint.
`C-c C-v`
  (command `reverse`): execute the program backward to latest breakpoint.
`C-c C-l`
  (command `last`): go back one step in the command history.
`C-c C-t`
  (command `backtrace`): display backtrace of function calls.
`C-c C-f`
  (command `finish`): run forward till the current function returns.
`C-c <`
  (command `up`): select the stack frame below the current frame.
`C-c >`
  (command `down`): select the stack frame above the current frame.

In all buffers in Caml editing mode, the following debugger commands are also available:

`C-x C-a C-b`
  (command `break`): set a breakpoint at event closest to point
`C-x C-a C-p`
  (command `print`): print value of identifier at point
`C-x C-a C-d`
  (command `display`): display value of identifier at point