docs/debugger_usage.md - garnet - Git at Google

 # zxdb: Fuchsia native debugger user guide

 This is the command usage guide for zxdb. Please see the [setup and
 troubleshooting guide](debugger.md) to get running.

 ## Quick start

 ```
 connect 192.168.3.1:2345
 break main
 run /system/bin/cowsay moo
 next
 print argv[1]
 continue
 quit
 ```

 There is an extensive built-in help system. Type `help` and the command prompt
 for a list of commands, or `help <command>` for details about that command
 (much more than is in this document) as well as options and examples.

 ## Interaction model

 Most command-line debuggers use an exclusive model for input: you’re either
 interacting with the debugged process’ stdin and stdout, or you’re interacting
 with the debugger. In contrast, zxdb has an asynchronous model similar to most
 GUI debuggers. In this model, the user is exclusively interacting with the
 debugger while arbitrary processes or threads are running or stopped.

 Currently there is no way to see or interact with a process’ stdin and stdout
 from the debugger. See [DX-595](https://fuchsia.atlassian.net/browse/DX-595) and
 [DX-596](https://fuchsia.atlassian.net/browse/DX-596).

 zxdb has a regular noun/verb model for typed commands. The rest of this section
 gives an overview of the syntax that applies to all commands. Specific commands
 will be covered in the “Task guide” section below.

 ### Nouns

 The possible nouns (and their abbreviations) are:

   * `process` (`pr`)
   * `thread` (`t`)
   * `frame` (`f`)
   * `breakpoint` (`bp`)

 #### Listing nouns

 If you type a noun by itself, it lists the available objects of that type:

 List attached processes

 ```
 [zxdb] process
   # State       Koid Name
 ▶ 1 Not running 3471 /pkgfs/packages/debug_agent_tests/0/test/zxdb_test_app
 ```

 List threads in the current process:

 ```
 [zxdb] thread
   # State   Koid Name
 ▶ 1 Blocked 1348 initial-thread
   2 Blocked 1356 some-other-thread
 ```

 List stack frames in the current thread (the thread must be stopped):

 ```
 [zxdb] frame
 ▶ 0 fxl::CommandLineFromIterators<const char *const *>() • command_line.h:203
   1 fxl::CommandLineFromArgcArgv() • command_line.h:224
   2 main() • main.cc:174
 ```

 #### Selecting defaults

 If you type a noun and its index, you select that as the default for subsequent
 commands. It also tells you the stats about the new default.

 Select thread 3 to be the default for future commands:

 ```
 [zxdb] thread 3
 Thread 3 Blocked koid=9940 worker-thread
 ```

 Select breakpoint 2 to be the default:

 ```
 [zxdb] breakpoint 2
 Breakpoint 2 (Software) on Global, Enabled, stop=All, @ MyFunction
 ```

 ### Verbs

 By default, a verb (`run`, `next`, `print`, etc.) applies to the current
 defaults. So to evaluate an expression in the context of the current stack
 frame, just type `print` by itself:

 ```
 [zxdb] print argv[1]
 "--foo=bar"
 ```

 You can override the default context by prefixing the verb with a noun and its
 index. So to evaluate an expression in the context of a specific stack frame
 (in this case, frame 2 of the current thread):

 ```
 [zxdb] frame 2 print argv[1]
 "--foo=bar"
 ```

 You can keep adding different types of context. This specifies the process,
 thread, and frame for the print command:

 ```
 [zxdb] process 1 thread 1 frame 2 print argv[1]
 "--foo=bar"
 ```

 # Attaching and running

 ### Connecting to the target system

 zxdb currently runs only in remote mode. This means that the debugger fromtend
 (zxdb) runs on your development Linux or Mac workstation, while the target
 Fuchsia system is running connected over a network or in QEMU.

 The set up guide provides more information on how to do this. In short, first
 run the debug agent on the target system with a port number:

 ```
 $ debug_agent --port=2345
 ```

 Then on the host use the `-c` command-line option or the connect command to
 connect to the target’s IP and the port you specified above:

 ```
 [zxdb] connect 192.168.3.1:2345
 ```

 Use `disconnect` to close the connection, or `quit` to disconnect and close the
 frontend. Note that currently you may need to also exit the debug agent before
 reconnecting ([DX-517](https://fuchsia.atlassian.net/browse/DX-517)).

 ### Debugging modules

 Debugging processes started by the application manager is currently only
 possible by attaching to them once they’re started (see “Attaching to an
 existing process” below). Work on a better experience is ongoing
 [DX-322](https://fuchsia.atlassian.net/browse/DX-322).

 If necessary you can hack something by busy-looping for 30 seconds or something
 at the beginning of your application to give you time to attach. Unfortunately
 zxdb can not currently set variables
 ([DX-597](https://fuchsia.atlassian.net/browse/DX-597)) which makes it
 impossible to busy-loop on a variable.

 ### Debugging drivers

 Debugging driver start-up is not currently possible
 ([DX-598](https://fuchsia.atlassian.net/browse/DX-598)).

 You can debug running drivers by attaching like any other process (see
 “Attaching to an existing process” below). The caveat is that since zxdb uses
 the network to communicate between the zxdb client and the debug agent, you can
 not debug any network-related drivers.

 ### Debugging crash dumps

 Work on this capability is ongoing
 ([DX-603](https://fuchsia.atlassian.net/browse/DX-603)).

 ### Directly running a new process

 You can start a new process from the debugger. Most applications in Fuchsia are
 launched in a specific context from the application manager or the dev manager
 and these won’t work when started from the debugger directly. But certain
 processes like command line utilities and most tests can be.

 To start a process, provide the full path and any command line arguments
 (optional):

 ```
 [zxdb] run /path/to/process --command --line=args go here
 ```

 Most tests are in `/pkgfs/<complicated_path>` while some legacy utilities are
 in `/system/bin`. It can be hard to find the full path in some cases, so
 Fuchsia’s `find` utility is your friend. On the target Fuchsia system:

 ```
 $ find . -name my_test
 ```

 The run command will immediately start running the process. Many utilities run
 and exit right away, so you’ll see:

 ```
 [zxdb] run /system/bin/cowsay moo
 Process 1 Running koid=10734 /system/bin/cowsay
 Exited with code 1: Process 1 Not running /system/bin/cowsay
 ```

 This is expected because the process did its work and exited. Currently stdout
 and stdin from the running process are inaccessible so you won’t see any
 printed output. In many cases you’ll want to set a breakpoint before running to
 catch it at some point (see below for more on breakpoints):

 ```
 [zxdb] break main
 Breakpoint 1 (Software) on Global, Enabled, stop=All, @ main
 Pending: No matches for location, it will be pending library loads.
 [zxdb] run
 ```

 In this this example we didn’t need to supply any parameters to `run` because
 the process and command line switches carry over from the previous invocation.
 It gives a warning that there are no matching symbols because the process is
 not started yet. This is expected: the symbols will be resolved when the
 process (and therefore symbols) are loaded.

 To terminate the process:

 ```
 [zxdb] kill
 ```

 Or keep the process running outside of the debugger:

 ```
 [zxdb] detach
 ```

 ### Attaching to an existing process

 You can attach to most running processes given the process’ KOID. You can get
 the KOID by running `ps` on the target Fuchsia system. zxdb also has a built-in
 `ps` command:

 ```
 [zxdb] ps
 j: 1030 root
   j: 1079 zircon-drivers
     p: 1926 devhost:sys
 ...
 ```

 Then to attach:

 ```
 [zxdb] attach 3517
 Process 1 Running koid=1249 pwrbtn-monitor
 ```

 When you’re done, you can choose to `detach` (keep running) or `kill`
 (terminate) the process.

 ### Debugging multiple processes

 You can debug many arbitrary processes at the same time. When you start, one
 “process context” (the container that may or may not have a running process)
  is created for you to use. When you run or attach, that process because
 associated with that context.

 To debug a second program, create a new context with:

 ```
 [zxdb] process new
 ```

 This will clone the current process’ settings into a new context but not run
 anything yet. You can then run or attach as normal.

 Recall from the “Interaction model” section you can list the current processes
 with:

 ```
 [zxdb] process
   # State       Koid Name
 ▶ 1 Running     1249 pwrbtn-monitor
   2 Not running 7235 pwrbtn-monitor
 ```

 Select one of those as the default by providing its index (not KOID):

 ```
 [zxdb] process 2
 ```

 Or apply commands to a specific process (even if it’s not the default) with:

 ```
 [zxdb] process 2 pause
 ```

 # Running

 ### Working with breakpoints

 Breakpoints stop execution when some code is executed. To create a breakpoint,
 use the `break` command (`b` for short) and give it a location:

 ```
 [zxdb] break main
 Breakpoint 3 (Software) on Global, Enabled, stop=All, @ main
    180
  ◉ 181 int main(int argc, char**argv) {
    182     fbl::unique_fd dirfd;
 ```

 A location can be expressed in many different ways.

 Plain function name:

 ```
 break main
 ```

 Member function or functions inside namespaces:

 ```
 break my_namespace::MyClass::MyFunction
 ```

 Source file + line number (separate with a colon):

 ```
 break mymain.cc:22
 ```

 Line number within the current frame’s current source file (useful when
 stepping):

 ```
 break 23
 ```

 Memory address:

 ```
 break 0xf72419a01
 ```

 To list all breakpoints:

 ```
 [zxdb] breakpoint
 ```

 _Note: this is the “breakpoint” noun (a noun by itself lists the things
 associated with it). It is not plural._

 To clear a specific breakpoint, give that breakpoint index as the context for
 the clear command (see “Interaction model” above). Here’s we’re using the
 abbreviation for `breakpoint` (`bp`):

 ```
 [zxdb] bp 2 clear
 ```

 Or you can clear the current breakpoint:

 ```
 [zxdb] clear
 ```

 Whenever you create or stop on a breakpoint, that breakpoint becomes the
 default automatically so clear always clears the one you just hit. Note that
 unlike GDB, “clear” takes a breakpoint context before the verb and there are
 never any arguments after it. Support for GDB-like “clear <location>” is
 [DX-594](https://fuchsia.atlassian.net/browse/DX-594).

 ### Working with threads

 To list the current process’ threads (see “Interaction model” above for more):

 ```
 [zxdb] thread
   # State   Koid Name
 ▶ 1 Blocked 1323 initial-thread
   2 Running 3462 worker-thread
 ```

 Often when you attach to a process the thread will be “blocked”, meaning it is
 stopped on a system call. For asynchronous programs this will typically be some
 kind of wait.

 Most thread control and introspection commands only work when a thread is
 suspended (not blocked or running). A thread will be suspended when it is
 stopped at a breakpoint or crashes. You can explicitly suspend a thread with
 the `pause` command:

 ```
 [zxdb] thread 2 pause
 ```

 Running `pause` by itself with no context will pause all threads of all
 processes currently attached:

 ```
 [zxdb] pause
 ```

 Unpause a thread with `continue`. As before, `continue` with no context will
 resume all threads:

 ```
 [zxdb] continue
 ```

 Or continue a specific thread:

 ```
 [zxdb] thread 1 continue
 ```

 ### Working with stack frames

 A stack frame is a function call. When a function calls another function, a new
 nested frame is created. So listing the frames of a thread tells you the call
 stack. You can only see the stack frames when a thread is suspended (see
 “Working with threads” above).

 To list the current thread’s stack frames (the `f` abbreviation will also
 work).

 ```
 [zxdb] frame
 ▶ 0 fxl::CommandLineFromIterators<const char *const *>() • command_line.h:203
   1 fxl::CommandLineFromArgcArgv() • command_line.h:224
   2 main() • main.cc:174
 ```

 And to select a given frame as the default:

 ```
 [zxdb] frame 2
 ```

 Frames are numbered with “0” being the top of the stack. Increasing numbers go
 backwards in time.

 For more context, you can use the `backtrace` command. This is identical
 to `frame` but gives more detailed address information as well as function
 parameters. This command can be abbreviated `bt`:

 ```
 [zxdb] bt
 ▶ 0 fxl::CommandLineFromIteratorsFindFirstPositionalArg<const char *const *>() • command_line.h:185
       IP = 0x10f982cf2ad0, BP = 0x66b45a01af50, SP = 0x66b45a01af38
       first = (const char* const*) 0x59f4e1268dc0
       last = (const char* const*) 0x59f4e1268dc8
       first_positional_arg = (const char* const**) 0x0
   1 fxl::CommandLineFromIterators<const char *const *>() • command_line.h:204
       IP = 0x10f982cf2ac0, BP = 0x66b45a01af50, SP = 0x66b45a01af40
       first = <'first' is not available at this address. >
       last = <'last' is not available at this address. >
 ...
 ```

 Each stack frame has a code location. Use the `list` command to look at source
 code. By itself, it lists the source code around the current stack frame’s
 instruction pointer:

 ```
 [zxdb] list
    183 inline CommandLine CommandLineFromIteratorsFindFirstPositionalArg(
    184     InputIterator first, InputIterator last,
  ▶ 185     InputIterator* first_positional_arg) {
    186   if (first_positional_arg)
    187     *first_positional_arg = last;
 ```

 You can list code around the current instruction pointer of other stack frames,
 too:

 ```
 [zxdb] frame 3 list
 ```

 Or you can list specific things like functions:

 ```
 [zxdb] list MyClass::MyFunc
 ```

 File/line numbers:

 ```
 [zxdb] list foo.cc:43
 ```

 Or whole files:

 ```
 [zxdb] list --all myfile.cc:1
 ```

 ### Printing values

 The `print` command can evaluate simple C/C++ expressions in the context of a
 stack frame. When a thread is suspended (see “Working with threads” above) just
 type:

 ```
 [zxdb] print i
 34
 ```

 Expressions can use most simple C/C++ syntax:

 ```
 [zxdb] print &foo->bar[baz]
 (const MyStruct*) 0x59f4e1268f70

 ```

 You can also evaluate expressions in the context of other stack frames without
 switching to them (see “Interaction model” above for more):

 ```
 [zxdb] frame 2 print argv[0]
 "/system/bin/cowsay"
 ```

 Often you will want to see all local variables:

 ```
 [zxdb] locals
 argc = 1
 argv = (const char* const*) 0x59999ec02dc0
 ```

 Things that don’t currently work are:

   * Math ([DX-600](https://fuchsia.atlassian.net/browse/DX-600))
   * Function calls ([DX-599](https://fuchsia.atlassian.net/browse/DX-599))
   * Casting ([DX-479](https://fuchsia.atlassian.net/browse/DX-479))
   * Global and static variables ([DX-481](https://fuchsia.atlassian.net/browse/DX-481))
   * Pretty-printing (especially for STL) ([DX-601](https://fuchsia.atlassian.net/browse/DX-601))

 ### Controlling execution (stepping, etc.)

 When a thread is suspended (see “Working with threads” above) you can control
 its execution:

 `next` / `n`: Advances to the next line, stepping over function calls.

 ```
 [zxdb] n
 ```

 `step` / `s`: Advances to the next line. If a function call happens before the
 next line, that function will be stepped into and execution will stop at the
 beginning of it.

 ```
 [zxdb] s
 ```

 `finish` / `fi`: Exits the function and stops right after the call.

 ```
 [zxdb] finish
 ```

 `until` / `u`: Given a location (the same as breakpoints, see above), continues
 the thread until execution gets there. For example, to run until line 45 of the
 current file:

 ```
 [zxdb] u 45
 ```

 There different things you can do with context. For example, to run until
 execution gets back to a given stack frame:

 ```
 [zxdb] frame 2 until
 ```

 ### Assembly language

 There are commands that deal with assembly language:

   * `disassemble` / `di`: Disassemble at the current location (or a given
     location)

   * `nexti` / `ni`: Step to the next instruction, stepping over function calls.

   * `stepi` / `si`: Step the the next instruction, following function calls.

   * `regs`: Get the CPU registers.

 zxdb maintains information about whether the last command was an assembly
 command or a source-code and will show that information on stepping or
 breakpoint hits. To switch to assembly-language mode, type `disassemble`, and
 to switch back to source-code mode, type `list`.

 ### Low-level memory

   * `mem-read` / `x`: Dumps memory

   * `stack`: Provides a low-level analysis of the stack. This is a handy
     command for low-level debugging.
	# zxdb: Fuchsia native debugger user guide

	This is the command usage guide for zxdb. Please see the [setup and
	troubleshooting guide](debugger.md) to get running.

	## Quick start

	```
	connect 192.168.3.1:2345
	break main
	run /system/bin/cowsay moo
	next
	print argv[1]
	continue
	quit
	```

	There is an extensive built-in help system. Type `help` and the command prompt
	for a list of commands, or `help <command>` for details about that command
	(much more than is in this document) as well as options and examples.

	## Interaction model

	Most command-line debuggers use an exclusive model for input: you’re either
	interacting with the debugged process’ stdin and stdout, or you’re interacting
	with the debugger. In contrast, zxdb has an asynchronous model similar to most
	GUI debuggers. In this model, the user is exclusively interacting with the
	debugger while arbitrary processes or threads are running or stopped.

	Currently there is no way to see or interact with a process’ stdin and stdout
	from the debugger. See [DX-595](https://fuchsia.atlassian.net/browse/DX-595) and
	[DX-596](https://fuchsia.atlassian.net/browse/DX-596).

	zxdb has a regular noun/verb model for typed commands. The rest of this section
	gives an overview of the syntax that applies to all commands. Specific commands
	will be covered in the “Task guide” section below.

	### Nouns

	The possible nouns (and their abbreviations) are:

	* `process` (`pr`)
	* `thread` (`t`)
	* `frame` (`f`)
	* `breakpoint` (`bp`)

	#### Listing nouns

	If you type a noun by itself, it lists the available objects of that type:

	List attached processes

	```
	[zxdb] process
	# State Koid Name
	▶ 1 Not running 3471 /pkgfs/packages/debug_agent_tests/0/test/zxdb_test_app
	```

	List threads in the current process:

	```
	[zxdb] thread
	# State Koid Name
	▶ 1 Blocked 1348 initial-thread
	2 Blocked 1356 some-other-thread
	```

	List stack frames in the current thread (the thread must be stopped):

	```
	[zxdb] frame
	▶ 0 fxl::CommandLineFromIterators<const char const >() • command_line.h:203
	1 fxl::CommandLineFromArgcArgv() • command_line.h:224
	2 main() • main.cc:174
	```

	#### Selecting defaults

	If you type a noun and its index, you select that as the default for subsequent
	commands. It also tells you the stats about the new default.

	Select thread 3 to be the default for future commands:

	```
	[zxdb] thread 3
	Thread 3 Blocked koid=9940 worker-thread
	```

	Select breakpoint 2 to be the default:

	```
	[zxdb] breakpoint 2
	Breakpoint 2 (Software) on Global, Enabled, stop=All, @ MyFunction
	```

	### Verbs

	By default, a verb (`run`, `next`, `print`, etc.) applies to the current
	defaults. So to evaluate an expression in the context of the current stack
	frame, just type `print` by itself:

	```
	[zxdb] print argv[1]
	"--foo=bar"
	```

	You can override the default context by prefixing the verb with a noun and its
	index. So to evaluate an expression in the context of a specific stack frame
	(in this case, frame 2 of the current thread):

	```
	[zxdb] frame 2 print argv[1]
	"--foo=bar"
	```

	You can keep adding different types of context. This specifies the process,
	thread, and frame for the print command:

	```
	[zxdb] process 1 thread 1 frame 2 print argv[1]
	"--foo=bar"
	```

	# Attaching and running

	### Connecting to the target system

	zxdb currently runs only in remote mode. This means that the debugger fromtend
	(zxdb) runs on your development Linux or Mac workstation, while the target
	Fuchsia system is running connected over a network or in QEMU.

	The set up guide provides more information on how to do this. In short, first
	run the debug agent on the target system with a port number:

	```
	$ debug_agent --port=2345
	```

	Then on the host use the `-c` command-line option or the connect command to
	connect to the target’s IP and the port you specified above:

	```
	[zxdb] connect 192.168.3.1:2345
	```

	Use `disconnect` to close the connection, or `quit` to disconnect and close the
	frontend. Note that currently you may need to also exit the debug agent before
	reconnecting ([DX-517](https://fuchsia.atlassian.net/browse/DX-517)).

	### Debugging modules

	Debugging processes started by the application manager is currently only
	possible by attaching to them once they’re started (see “Attaching to an
	existing process” below). Work on a better experience is ongoing
	[DX-322](https://fuchsia.atlassian.net/browse/DX-322).

	If necessary you can hack something by busy-looping for 30 seconds or something
	at the beginning of your application to give you time to attach. Unfortunately
	zxdb can not currently set variables
	([DX-597](https://fuchsia.atlassian.net/browse/DX-597)) which makes it
	impossible to busy-loop on a variable.

	### Debugging drivers

	Debugging driver start-up is not currently possible
	([DX-598](https://fuchsia.atlassian.net/browse/DX-598)).

	You can debug running drivers by attaching like any other process (see
	“Attaching to an existing process” below). The caveat is that since zxdb uses
	the network to communicate between the zxdb client and the debug agent, you can
	not debug any network-related drivers.

	### Debugging crash dumps

	Work on this capability is ongoing
	([DX-603](https://fuchsia.atlassian.net/browse/DX-603)).

	### Directly running a new process

	You can start a new process from the debugger. Most applications in Fuchsia are
	launched in a specific context from the application manager or the dev manager
	and these won’t work when started from the debugger directly. But certain
	processes like command line utilities and most tests can be.

	To start a process, provide the full path and any command line arguments
	(optional):

	```
	[zxdb] run /path/to/process --command --line=args go here
	```

	Most tests are in `/pkgfs/<complicated_path>` while some legacy utilities are
	in `/system/bin`. It can be hard to find the full path in some cases, so
	Fuchsia’s `find` utility is your friend. On the target Fuchsia system:

	```
	$ find . -name my_test
	```

	The run command will immediately start running the process. Many utilities run
	and exit right away, so you’ll see:

	```
	[zxdb] run /system/bin/cowsay moo
	Process 1 Running koid=10734 /system/bin/cowsay
	Exited with code 1: Process 1 Not running /system/bin/cowsay
	```

	This is expected because the process did its work and exited. Currently stdout
	and stdin from the running process are inaccessible so you won’t see any
	printed output. In many cases you’ll want to set a breakpoint before running to
	catch it at some point (see below for more on breakpoints):

	```
	[zxdb] break main
	Breakpoint 1 (Software) on Global, Enabled, stop=All, @ main
	Pending: No matches for location, it will be pending library loads.
	[zxdb] run
	```

	In this this example we didn’t need to supply any parameters to `run` because
	the process and command line switches carry over from the previous invocation.
	It gives a warning that there are no matching symbols because the process is
	not started yet. This is expected: the symbols will be resolved when the
	process (and therefore symbols) are loaded.

	To terminate the process:

	```
	[zxdb] kill
	```

	Or keep the process running outside of the debugger:

	```
	[zxdb] detach
	```

	### Attaching to an existing process

	You can attach to most running processes given the process’ KOID. You can get
	the KOID by running `ps` on the target Fuchsia system. zxdb also has a built-in
	`ps` command:

	```
	[zxdb] ps
	j: 1030 root
	j: 1079 zircon-drivers
	p: 1926 devhost:sys
	...
	```

	Then to attach:

	```
	[zxdb] attach 3517
	Process 1 Running koid=1249 pwrbtn-monitor
	```

	When you’re done, you can choose to `detach` (keep running) or `kill`
	(terminate) the process.

	### Debugging multiple processes

	You can debug many arbitrary processes at the same time. When you start, one
	“process context” (the container that may or may not have a running process)
	is created for you to use. When you run or attach, that process because
	associated with that context.

	To debug a second program, create a new context with:

	```
	[zxdb] process new
	```

	This will clone the current process’ settings into a new context but not run
	anything yet. You can then run or attach as normal.

	Recall from the “Interaction model” section you can list the current processes
	with:

	```
	[zxdb] process
	# State Koid Name
	▶ 1 Running 1249 pwrbtn-monitor
	2 Not running 7235 pwrbtn-monitor
	```

	Select one of those as the default by providing its index (not KOID):

	```
	[zxdb] process 2
	```

	Or apply commands to a specific process (even if it’s not the default) with:

	```
	[zxdb] process 2 pause
	```

	# Running

	### Working with breakpoints

	Breakpoints stop execution when some code is executed. To create a breakpoint,
	use the `break` command (`b` for short) and give it a location:

	```
	[zxdb] break main
	Breakpoint 3 (Software) on Global, Enabled, stop=All, @ main
	180
	◉ 181 int main(int argc, char**argv) {
	182 fbl::unique_fd dirfd;
	```

	A location can be expressed in many different ways.

	Plain function name:

	```
	break main
	```

	Member function or functions inside namespaces:

	```
	break my_namespace::MyClass::MyFunction
	```

	Source file + line number (separate with a colon):

	```
	break mymain.cc:22
	```

	Line number within the current frame’s current source file (useful when
	stepping):

	```
	break 23
	```

	Memory address:

	```
	break 0xf72419a01
	```

	To list all breakpoints:

	```
	[zxdb] breakpoint
	```

	_Note: this is the “breakpoint” noun (a noun by itself lists the things
	associated with it). It is not plural._

	To clear a specific breakpoint, give that breakpoint index as the context for
	the clear command (see “Interaction model” above). Here’s we’re using the
	abbreviation for `breakpoint` (`bp`):

	```
	[zxdb] bp 2 clear
	```

	Or you can clear the current breakpoint:

	```
	[zxdb] clear
	```

	Whenever you create or stop on a breakpoint, that breakpoint becomes the
	default automatically so clear always clears the one you just hit. Note that
	unlike GDB, “clear” takes a breakpoint context before the verb and there are
	never any arguments after it. Support for GDB-like “clear <location>” is
	[DX-594](https://fuchsia.atlassian.net/browse/DX-594).

	### Working with threads

	To list the current process’ threads (see “Interaction model” above for more):

	```
	[zxdb] thread
	# State Koid Name
	▶ 1 Blocked 1323 initial-thread
	2 Running 3462 worker-thread
	```

	Often when you attach to a process the thread will be “blocked”, meaning it is
	stopped on a system call. For asynchronous programs this will typically be some
	kind of wait.

	Most thread control and introspection commands only work when a thread is
	suspended (not blocked or running). A thread will be suspended when it is
	stopped at a breakpoint or crashes. You can explicitly suspend a thread with
	the `pause` command:

	```
	[zxdb] thread 2 pause
	```

	Running `pause` by itself with no context will pause all threads of all
	processes currently attached:

	```
	[zxdb] pause
	```

	Unpause a thread with `continue`. As before, `continue` with no context will
	resume all threads:

	```
	[zxdb] continue
	```

	Or continue a specific thread:

	```
	[zxdb] thread 1 continue
	```

	### Working with stack frames

	A stack frame is a function call. When a function calls another function, a new
	nested frame is created. So listing the frames of a thread tells you the call
	stack. You can only see the stack frames when a thread is suspended (see
	“Working with threads” above).

	To list the current thread’s stack frames (the `f` abbreviation will also
	work).

	```
	[zxdb] frame
	▶ 0 fxl::CommandLineFromIterators<const char const >() • command_line.h:203
	1 fxl::CommandLineFromArgcArgv() • command_line.h:224
	2 main() • main.cc:174
	```

	And to select a given frame as the default:

	```
	[zxdb] frame 2
	```

	Frames are numbered with “0” being the top of the stack. Increasing numbers go
	backwards in time.

	For more context, you can use the `backtrace` command. This is identical
	to `frame` but gives more detailed address information as well as function
	parameters. This command can be abbreviated `bt`:

	```
	[zxdb] bt
	▶ 0 fxl::CommandLineFromIteratorsFindFirstPositionalArg<const char const >() • command_line.h:185
	IP = 0x10f982cf2ad0, BP = 0x66b45a01af50, SP = 0x66b45a01af38
	first = (const char* const*) 0x59f4e1268dc0
	last = (const char* const*) 0x59f4e1268dc8
	first_positional_arg = (const char* const**) 0x0
	1 fxl::CommandLineFromIterators<const char const >() • command_line.h:204
	IP = 0x10f982cf2ac0, BP = 0x66b45a01af50, SP = 0x66b45a01af40
	first = <'first' is not available at this address. >
	last = <'last' is not available at this address. >
	...
	```

	Each stack frame has a code location. Use the `list` command to look at source
	code. By itself, it lists the source code around the current stack frame’s
	instruction pointer:

	```
	[zxdb] list
	183 inline CommandLine CommandLineFromIteratorsFindFirstPositionalArg(
	184 InputIterator first, InputIterator last,
	▶ 185 InputIterator* first_positional_arg) {
	186 if (first_positional_arg)
	187 *first_positional_arg = last;
	```

	You can list code around the current instruction pointer of other stack frames,
	too:

	```
	[zxdb] frame 3 list
	```

	Or you can list specific things like functions:

	```
	[zxdb] list MyClass::MyFunc
	```

	File/line numbers:

	```
	[zxdb] list foo.cc:43
	```

	Or whole files:

	```
	[zxdb] list --all myfile.cc:1
	```

	### Printing values

	The `print` command can evaluate simple C/C++ expressions in the context of a
	stack frame. When a thread is suspended (see “Working with threads” above) just
	type:

	```
	[zxdb] print i
	34
	```

	Expressions can use most simple C/C++ syntax:

	```
	[zxdb] print &foo->bar[baz]
	(const MyStruct*) 0x59f4e1268f70

	```

	You can also evaluate expressions in the context of other stack frames without
	switching to them (see “Interaction model” above for more):

	```
	[zxdb] frame 2 print argv[0]
	"/system/bin/cowsay"
	```

	Often you will want to see all local variables:

	```
	[zxdb] locals
	argc = 1
	argv = (const char* const*) 0x59999ec02dc0
	```

	Things that don’t currently work are:

	* Math ([DX-600](https://fuchsia.atlassian.net/browse/DX-600))
	* Function calls ([DX-599](https://fuchsia.atlassian.net/browse/DX-599))
	* Casting ([DX-479](https://fuchsia.atlassian.net/browse/DX-479))
	* Global and static variables ([DX-481](https://fuchsia.atlassian.net/browse/DX-481))
	* Pretty-printing (especially for STL) ([DX-601](https://fuchsia.atlassian.net/browse/DX-601))

	### Controlling execution (stepping, etc.)

	When a thread is suspended (see “Working with threads” above) you can control
	its execution:

	`next` / `n`: Advances to the next line, stepping over function calls.

	```
	[zxdb] n
	```

	`step` / `s`: Advances to the next line. If a function call happens before the
	next line, that function will be stepped into and execution will stop at the
	beginning of it.

	```
	[zxdb] s
	```

	`finish` / `fi`: Exits the function and stops right after the call.

	```
	[zxdb] finish
	```

	`until` / `u`: Given a location (the same as breakpoints, see above), continues
	the thread until execution gets there. For example, to run until line 45 of the
	current file:

	```
	[zxdb] u 45
	```

	There different things you can do with context. For example, to run until
	execution gets back to a given stack frame:

	```
	[zxdb] frame 2 until
	```

	### Assembly language

	There are commands that deal with assembly language:

	* `disassemble` / `di`: Disassemble at the current location (or a given
	location)

	* `nexti` / `ni`: Step to the next instruction, stepping over function calls.

	* `stepi` / `si`: Step the the next instruction, following function calls.

	* `regs`: Get the CPU registers.

	zxdb maintains information about whether the last command was an assembly
	command or a source-code and will show that information on stepping or
	breakpoint hits. To switch to assembly-language mode, type `disassemble`, and
	to switch back to source-code mode, type `list`.

	### Low-level memory

	* `mem-read` / `x`: Dumps memory

	* `stack`: Provides a low-level analysis of the stack. This is a handy
	command for low-level debugging.