Control thread execution

When you use zxdb, you can control the execution of threads to help you debug. As you debug you can control execution through the use of the following:

  • Thread

    A thread is a unit of execution within a process. It represents a single sequence of instructions that can be executed independently.

    To control execution through threads in zxdb, see Threads.

  • Stack frame

    A stack frame is a section of the call stack that is allocated when a function is called. It stores information needed for the function's execution, such as:

    • Local variables: Variables declared within the function.
    • Parameters: Values passed to the function.
    • Return address: The location in the code to return to after the function completes.

    To control execution through stack frames in zxdb, see Stack Frames.


In zxdb, a thread is a noun that you can use with zxdb verbs.

To list the threads in the current process:

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

In some cases, you may notice that a thread is marked as Blocked which means that the thread is stopped on a system call. Typically, when you are debugging an asynchronous application this may also indicate a wait time.

Thread control commands only work on a suspended thread, not blocked or running threads. There are several ways to suspend a thread:

pause a thread

For example, to suspend thread 2 with the pause command:

[zxdb] thread 2 pause
🛑 syscalls-x86-64.S:67
   65 m_syscall zx_port_create 60 2 1
   66 m_syscall zx_port_queue 61 2 1
 ▶ 67 m_syscall zx_port_wait 62 3 0
   68 m_syscall zx_port_cancel 63 3 1
   69 m_syscall zx_timer_create 64 3 1

When a thread is paused zxdb shows the current source code location. If a thread is in a system call, like the example above, the source code location resolves to the location in the assembly-language macro file that generated the system call.

If you run pause without any additional context, zxdb pauses all threads of all processes that are currently attached.

For example:

[zxdb] pause
   508                 const zx_port_packet_t* packet))
 ▶ 510 BLOCKING_SYSCALL(port_wait, zx_status_t, /* no attributes */, 3, (handle, deadline, packet),
   511                  (_ZX_SYSCALL_ANNO(use_handle("Fuchsia")) zx_handle_t handle, zx_time_t deadline,
   512                   zx_port_packet_t* packet))
🛑 $elf(SYSCALL_zx_port_wait) + 0x7 •

continue a thread

After you have paused a thread and started debugging an issue, you may want to continue the thread. Continuing means resuming execution until your program completes normally.

For example, to continue thread 1:

[zxdb] thread 1 continue

If you run continue without any additional context, zxdb continues all the threads of all attached processes.

For example:

[zxdb] continue

Stepping a thread

When a thread is paused you can control its execution. You can use any of these commands:

  • finish (fi)

    Exits the function and stops right after the call.

    [zxdb] finish
  • next (n)

    Advances to the next line, stepping over function calls.

    [zxdb] next
  • nexti

    Advances to the next instruction, but steps over call instructions for the target architecture.

    [zxdb] nexti
  • ss

    List function calls on the current line and step in to the call selected. This automatically completes any of the other calls that happen to occur first.

    [zxdb] ss
      1 std::string::string
      2 MyClass::MyClass
      3 HelperFunctionCall
      4 MyClass::~MyClass
      5 std::string::~string
  • step (s)

    Advances to the next code line. If a function call happens before the next line, that function is stepped into and execution stops at the beginning of that function.

    You can also supply an argument substring to match a specific function call. Function names that do not contain the argument substring are skipped and only matching functions are stepped into.

    [zxdb] step
    [zxdb] step MyFunction
  • stepi

    Advances exactly one machine instruction.

    [zxdb] stepi
  • until (u)

    Given a line location, continues the thread until execution gets there. For example, to run until line 45 of the current file:

    [zxdb] until 45

    You can also run until execution gets back to a given stack frame:

    [zxdb] frame 2 until

Stack frames

A stack frame is a function call. When a function calls another function, a new frame is created. Listing the frames of a thread returns the call stack.

To list the stack frames in the current thread:

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

When you work with stack frames, 0 indicates the top of the stack, which indicates the end of the execution. The bottom of the stack, which is the highest stack frame number, indicates the start of the execution.

You can use the up and down commands to navigate the frame list.

For example, use up to navigate from the current frame 0 to frame 1:

[zxdb] up
  1 fxl::CommandLineFromIterators<const char *const *>() • command_line.h:204

For example, use down to navigate from the current frame 1 to frame 0:

[zxdb] down
  0 fxl::CommandLineFromIteratorsFindFirstPositionalArg<const char *const *>() • command_line.h:185

You can also navigate to a specific frame by using the frame command with a frame number:

[zxdb] frame 1

Use backtrace for additional details

In some cases, you may want to see additional address information that stack frames don't provide. The backtrace command is identical to frame but gives you more detailed address information as well as function parameters.

To list the stack frames in the current thread, but with more detailed information, use backtrace:

[zxdb] backtrace
▶ 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. >

Use list to look at source code

Each stack frame has a code location. Use the list command to look at the source code.

You can list code around the instruction pointer of specific stack frames.

For example, to list the source code around the instruction pointer of stack frame 3:

[zxdb] frame 3 list

When you use list without context, zxdb lists the source code around the instruction pointer of the current stack frame:

[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;

Additional use cases for list

Additionally, you can use list to list specific things:


Use list to list functions:

[zxdb] list MyClass::MyFunc


Use list to list specific files:

[zxdb] list --all

File with line numbers

Use list to list specific files with specific line numbers:

[zxdb] list