The debugger (camldebug)

This chapter describes the Caml Light source-level replay debugger camldebug.

The debugger resides in the directory contrib/debugger in the distribution. It requires a Unix system that provides BSD sockets.

The debugger is not available.

The debugger is not available.

Compiling for debugging

Before the debugger can be used, the program must be compiled and linked with the -g option: all .zo files that are part of the program should have been created with camlc -g, and they must be linked together with camlc -g.

Compiling with -g entails no penalty on the running time of programs: .zo files and bytecode executable files are bigger and take slightly longer to produce, but the executable files run at exactly the same speed as if they had been compiled without -g. It is therefore perfectly acceptable to compile always in -g mode.


Starting the debugger

The Caml Light debugger is invoked by running the program camldebug with the name of the bytecode executable file as argument:

        camldebug program
The following command-line options are recognized:

-stdlib directory
Look for the standard library files in directory instead of in the default directory.

-s socket
Use socket for communicating with the debugged program. See the description of the command set socket (section 10.8) for the format of socket.

-c count
Set the maximum number of checkpoints to count.

-cd directory
Run the debugger program from the working directory directory, instead of the current directory.

Tell the debugger it is executing under Emacs. (See section 12.4 for information on how to run the debugger under Emacs.)

Quitting 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.


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.

Getting help

The Caml Light 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.

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.

Executing a program


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. Also, no event is put after a function application when the function is a primitive function (written in C). Finally, several events may correspond to the same location in the compiled program. Then, the debugger cannot distinguish them, and selects one of the events to associate with the given code location. The event chosen is a ``function application'' event if there is one at that location, or otherwise the event which appears last in the source. This heuristic generally picks the ``most interesting'' event associated with the code location.

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 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 10.5).

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).

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.
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.
Run the program until the current function returns.

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.

Killing the program

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


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.

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. Moreover, C functions are not recognized by the debugger.

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.

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.

Examining variable values

The debugger can print the current value of a program variable (either a global variable or a local variable relative to the selected stack frame). It can also print selected parts of a value by matching it against a pattern.

Variable names can be specified either fully qualified (module-name__var-name) or unqualified (var-name). Unqualified names either correspond to local variables, or are completed into fully qualified global names by looking at a list of ``opened'' modules that define the same name (see section 10.8 for how to open modules in the debugger.) The completion follows the same rules as in the Caml Light language (see section 3.3).

print variables
Print the values of the given variables.
match variable pattern
Match the value of the given variable against a pattern, and print the values bound to the identifiers in the pattern.

The syntax of patterns for the match command extends the one for Caml Light patterns:

   |  _
   |  ( pattern )
   |  ncconstr pattern
   |  pattern , pattern {, pattern}
   |  { label = pattern {; label = pattern} }
   |  [ ]
   |  [ pattern {; pattern} ]
   |  pattern :: pattern
   |  # integer-literal pattern
   |  > pattern

The pattern ident, where ident is an identifier, matches any value, and binds the identifier to this value. The pattern # n pattern matches a list, a vector or a tuple whose n-th element matches pattern. The pattern > pattern matches any constructed value whose argument matches pattern, regardless of the constructor; it is a shortcut for skipping a constructor.

Example: assuming the value of a is Constr{x = [1;2;3;4]}, the command match a > {x = # 2 k} prints k = 3.

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.

Controlling the debugger

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.

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 Caml Light runtime camlrun 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 10.8).

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.

Reset the search path. This requires confirmation.

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.

Print the working directory for camldebug.

Module management

Like the Caml Light compiler, the debugger maintains a list of opened modules in order to resolves variable name ambiguities. The opened modules also affect the printing of values: whether fully qualified names or short names are used for constructors and record labels.

When a program is executed, the debugger automatically opens the modules of the standard library it uses.

open modules
Open the given modules.

close modules
Close the given modules.

info modules
List the modules used by the program, and the open modules.

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.

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 either by the -D command line option to camlrun, or through the CAML_DEBUG_SOCKET environment variable.

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).

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.