[[bbv2.util]]
= Utilities

[[bbv2.util.debugger]]
== Debugger

[[bbv2.util.debugger.overview]]
=== Overview

Boost.Build comes with a debugger for Jamfiles. To run the debugger,
start Boost.Build with `b2 -dconsole`.

----
$ b2 -dconsole
(b2db) break gcc.init
Breakpoint 1 set at gcc.init
(b2db) run
Starting program: /usr/bin/b2
Breakpoint 1, gcc.init ( ) at /usr/share/boost-build/tools/gcc.jam:74
74      local tool-command = ;
(b2db) quit
----

[[bbv2.util.debugger.running]]
=== Running the Program

The `run` command is used to start a new `b2` subprocess for debugging.
The arguments to `run` are passed on the command line. If a child
process is already running, it will be terminated before the new child
is launched.

When the program is paused `continue` will resume execution. The `step`
command will advance the program by a single statement, stopping on
entry to another function or return from the current function. `next` is
like `step` except that it skips over function calls. `finish` executes
until the current function returns.

The `kill` command terminates the current child immediately.

[[bbv2.util.debugger.break]]
=== Breakpoints

Breakpoints are set using the `break` command. The location of the
breakpoint can be specified as either the name of a function (including
the module name) or or a file name and line number of the form
`file:line`. When a breakpoint is created it is given a unique id which
is used to identify it for other commands.

----
(b2db) break Jamfile:10
Breakpoint 1 set at Jamfile:10
(b2db) break msvc.init
Breakpoint 2 set at msvc.init
----

A breakpoint can be temporarily disabled using the `disable` command.
While a breakpoint is disabled, the child will not stop when it is hit.
A disabled breakpoint can be activated again with `enable`.

----
(b2db) disable 1
(b2db) enable 1
----

Breakpoints can be removed permanently with `delete` or `clear`. The
difference between them is that `delete` takes the breakpoint id while
`clear` takes the location of the breakpoint as originally specified to
break.

----
(b2db) clear Jamfile:10
Deleted breakpoint 1
(b2db) delete 2
----

[[bbv2.util.debugger.stack]]
=== Examining the Stack

The `backtrace` command will print a summary of every frame on the
stack.

The `print` command can be used to show the value of an expression.

----
(b2db) print [ modules.peek : ARGV ]
/usr/bin/b2 toolset=msvc install
(b2db) print $(__file__)
Jamfile.jam
----

[[bbv2.util.debugger.misc]]
=== Miscellaneous Commands

`quit` exits the debugger. `help` describes the available commands.
