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Zxdb console commands and interaction model


Zxdb has a built-in help system:

[zxdb] help

To get help on a specific command or topic (in this case, the step command):

[zxdb] help step

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.

When the debugger itself launches a program it will print the program's stdout and stderr to the console. When you attach (either with a filter or with the attach command) they will go to the original place. Currently there is no way to interact with a process’ stdin.

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
    ▶ 1 Not running 3471 debug_agent_unit_tests.cmx
  • List attached jobs

    [zxdb] job
      # State   Koid Name
    ▶ 1 running 3471 sys
  • 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—see pause below):

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

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]

Attributes and settings

Debugger objects have settings associated with them. Use the "get" verb to list the settings for a given object:

[zxdb] breakpoint 1 get
  enabled  true
  location main
  one-shot false
  scope    global
  stop     all
  type     software

The "get" command with a specific attribute will list the attribute and help associated with it:

[zxdb] breakpoint 1 get scope

  ... help text here ...

scope = global

The "set" command sets a value:

[zxdb] breakpoint 1 set scope="process 1 thread 2"
[zxdb] breakpoint 1 set enabled=false

Some settings are hierarchical. A thread inherits settings from its process which in turn inherits settings from the global scope. The "get" command with no context or parameters will list the global settings and the ones for the current process and thread. You can set a global setting to apply to all threads and processes without specific overrides, or override a specific context:

[zxdb] set show-stdout = false            # Applies to all processes with no override.
[zxdb] process 2 set show-stdout = true   # Overrides a specific process.

Some settings are lists. You can use += to append, or specify a new value with "=". List elements are space-separated (quote strings with spaces).

[zxdb] set symbol-paths = /foo/bar/baz "/home/Dr. Strangelove/cache"
[zxdb] set symbol-paths += /tmp
[zxdb] get symbol-paths
  ... help text ...

symbol-paths =
  • /foo/bar/baz
  • "/home/Dr. Strangelove/cache"
  • /tmp