zxdb: Fuchsia native debugger user guide

This is the command usage guide for zxdb. Please see also:

Quick start

Connecting in-tree

In-tree developers should use the fx debug command to start the debugger. The system must already be running and reachable via networking from your computer:

$ scripts/fx debug
Attempting to start the Debug Agent.
Waiting for the Debug Agent to start.
Connecting (use "disconnect" to cancel)...
Connected successfully.

Debugging a process or component.

Running a process on Fuchsia is more complicated than in other systems because there are different loader environments (see “A note about launcher environments” below).

The only want to reliably debug all types of processes is to create a filter on the process name and start it the normal way you would start that process. This example sets a pending breakpoint on main to stop at the beginning of execution, and waits for a process called “my_app” to start:

[zxdb] set filters my_app
Set value(s) for job 1:
• my_app

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

Then run the process the way you would in normal use (direcly on the command line, via fx run-test, via the shell's run fuchsia-pkg://..., or another way. The debugger should then immediately break on main (it may take some time to load symbols so you may see a delay before showing the source code):

Attached Process 1 [Running] koid=51590 my_app.cmx
🛑 on bp 1 main(…) • main.cc:222
   220 }
 â–¶ 222 int main(int argc, const char* argv[]) {
   223   foo::CommandLineOptions options;
   224   cmdline::Status status = ParseCommandLine(argc, argv, &options);

You can then do basic commands that are similar to GDB:

print argv[1]

A note about launcher environments

The following loader environments all have different capabilities (in order from least capable to most capable):

  • The debugger's run <file name> command (base system process stuff).
  • The system console or fx shell (adds some libraries).
  • The base component environment via the shell‘s run and the debugger’s run -c <package url> (adds component capabilities).
  • The test environment via fx run-test.
  • The user environment when launched from a “story” (adds high-level services like scenic).

This panoply of environments is why the debugger can't have a simple “run” command that always works.

Launching simple command-line processes

Minimal console apps including some unit tests can be launched directly from within the debugger which avoids the “set filters” dance:

[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 /bin/cowsay

If you get a shared library load error or errors about files or services not being found, it means the app can‘t be run from within the debugger’s launcher environment. This is true even for things that may seem relatively simple.

Directly launching components

Components that can be executed with the console comand run fuchsia-pkg://... can be loaded in the debugger with the following command, substituting your component's URL:

[zxdb] run -c fuchsia-pkg://fuchsia.com/your_app#meta/your_app.cmx

Not all components can be launched this way since most higher-level services won‘t be accessible: if you can’t do run ... from the system console, it won‘t work from the debugger either. Note also that fx run-test is a different environment. According to your test’s dependencies, it may or may not work from the debugger's run command.

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.

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 and 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.


The possible nouns (and their abbreviations) are:

  • process (pr)
  • job (j)
  • 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
â–¶ 1 Not running 3471 /pkgfs/packages/debug_agent_tests/0/test/zxdb_test_app

List attached jobs

[zxdb] job
  # State   Koid Name
â–¶ 1 running 3471 sys

List threads in the current process:

[zxdb] thread
  # State   Koid Name
â–¶ 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


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]

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]

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]

Attaching and running

Debugging drivers

It's not currently possible to set up the debugger early enough in system startup to debug most driver initialization. And since zxdb itself uses the network, no drivers associated with network communication can be debugged.

Driver debugging support is tracked in DX-598.

You can debug running drivers by attaching like any other process (see “Attaching to an existing process” below). You can delay initialization to allow time to attach by adding a busyloop at the beginning of your code:

volatile bool stop = false;
while (!stop) {}

To break out of the loop after attaching, either set the variable to true:

[zxdb] print stop = true
[zxdb] continue

Or jump to the line after the loop:

[zxdb] jump <line #>
[zxdb] continue

Debugging crash dumps

You can load a minidump generated by a crash report. Use the “opendump” verb and supply the local file name of the dump. The debugger must not be attached to another dump or a running system (use “disconnect” first if so).

[zxdb] opendump upload_file_minidump-e71256ba30163a0.dmp
Opening dump file
Dump loaded successfully.

Now the thread, stack, and memory commands can be used to inspect the state of the program. Use “disconnect” to close the dump.

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
â–¶ 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


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
 â—‰ 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 ” is DX-594.

Programatic breakpoints

You can insert a hardcoded breakpoint in your code if you want to catch some specific condition. Clang has a builtin (it won't work in GCC Zircon builds):


If the debugger is already attached to the process, it will stop as if a normal breakpoint was hit. You can step or continue from there. If the debugger is not already attached, this will cause a crash.

Working with threads

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

[zxdb] thread
  # State   Koid Name
â–¶ 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,
 â–¶ 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

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]

Often you will want to see all local variables:

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

You can also set variables to integer and boolean values (as long as those variables are in memory and not registers):

[zxdb] print done_flag = true
[zddb] print i = 56

Things that don’t currently work are:

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. You can also supply an argument which is a substring to match of a specific function call. Function names not containing this substring will be skipped and only matching ones will be stepped into:

[zxdb] s
[zxdb] s MyFunction

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

jump: Move the instruction pointer to a new address.

[zxdb] jump 22  // Line number
[zxdb] jump 0x87534123  // Address

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 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.