Chapter 20 The debugger (ocamldebug)

This chapter describes the OCaml 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 OCaml, but not under the native Win32 ports.

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.

2 Invocation

2.1 Starting the debugger

The OCaml 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:

-c count
Set the maximum number of simultaneously live checkpoints to count.
-cd dir
Run the debugger program from the working directory dir, instead of the current directory. (See also the cd command.)
Tell the debugger it is executed under Emacs. (See section ‍20.10 for information on how to run the debugger under Emacs.)
-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 ‍20.8.8) for the format of socket.
Print version string and exit.
Print short version number and exit.
-help or --help
Display a short usage summary and exit.

2.2 Initialization file

On start-up, the debugger will read commands from an initialization file before giving control to the user. The default file is .ocamldebug in the current directory if it exists, otherwise .ocamldebug in the user’s home directory.

2.3 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.

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:

        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.

3.1 Getting help

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

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.

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.

4 Executing a program

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:

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.

4.2 Starting the debugged program

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

Run the program until a breakpoint is hit, or the program terminates.
goto 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 ‍20.5).

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).

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.

Execute the program forward from current time. Stops at next breakpoint or when the program terminates.
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. If count is 0, run until the program terminates or a breakpoint is hit.
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.
Run the program until the current function returns.
Run the program backward and stop at the first event before the current function invocation.

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.

4.5 Killing the program

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

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 ‍20.10).

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 frag:pc, break pc
Set a breakpoint at code address frag:pc. The integer frag is the identifier of a code fragment, a set of modules that have been loaded at once, either initially or with the Dynlink module. The integer pc is the instruction counter within this code fragment. If frag is omitted, it defaults to 0, which is the code fragment of the program loaded initially.
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.

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.

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.

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 OCaml expressions, as described by the following grammar:

simple-expr::= lowercase-ident
  { capitalized-ident. } lowercase-ident

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.

8 Controlling the debugger

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.

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 OCaml 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 ‍20.8.8).

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 directorynames for modulename
Same as directory directorynames, but the given directories will be searched only when looking for the source file of a module that has been packed into modulename.
Reset the search path. This requires confirmation.

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 ocamldebug to directory.
Print the working directory for ocamldebug.

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.

8.6 Behavior of the debugger with respect to fork

When the program issues a call to fork, the debugger can either follow the child or the parent. By default, the debugger follows the parent process. The variable follow_fork_mode controls this behavior:

set follow_fork_mode child/parent
Select whether to follow the child or the parent in case of a call to fork.

8.7 Stopping execution when new code is loaded

The debugger is compatible with the Dynlink module. However, when an external module is not yet loaded, it is impossible to set a breakpoint in its code. In order to facilitate setting breakpoints in dynamically loaded code, the debugger stops the program each time new modules are loaded. This behavior can be disabled using the break_on_load variable:

set break_on_load on/off
Select whether to stop after loading new code.

8.8 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.

8.9 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).

8.10 User-defined printers

Just as in the toplevel system (section ‍14.2), 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 OCaml 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.

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.

10 Running the debugger under Emacs

The most user-friendly way to use the debugger is to run it under Emacs with the OCaml mode available through MELPA and also at

The OCaml 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 OCaml 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