Using breakpoints in zxdb

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. This will match functions with the name in any namespace:

    [zxdb] break main
    
  • Member function or functions inside a specific namespace or class:

    [zxdb] break my_namespace::MyClass::MyFunction
    [zxdb] break ::OtherFunction
    
  • Source file + line number (separate with a colon):

    [zxdb] break mymain.cc:22
    
  • Line number within the current frame’s current source file (useful when stepping):

    [zxdb] break 23
    
  • Memory address:

    [zxdb] break 0xf72419a01
    
  • Expression: Prefixing with "*" will treat the following input as an expression that evaluates to an address. This is most often used with hardware breakpoints.

    [zxdb] break --type=write *&foo
    

To list all breakpoints:

[zxdb] breakpoint

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.

clear can also take an optional location just like a break command. In this way, it will try to clear all breakpoints at that location and ignore the default breakpoint context.

Note for GDB users: delete <index> is mapped to bp <index> clear, while clear <number> behaves the same in GDB and zxdb.

Breakpoints can also be enabled or disabled:

[zxdb] disable
[zxdb] bp 4 enable

Other properties can be modified via the "get" and "set" commands.

[zxdb] bp 1 set location = Frobulator::GetThing

Conditional breakpoints

A breakpoint can optionally have a condition, which is an expression that evaluates to either true or false. The breakpoint will not trigger a stop unless the condition is true. For example, given a source file

    7 void do_loop(int n) {
    8   for (int i = 0; i < n; i++) {
 ▶  9     std::cout << "Hello world!" << std::endl;
   10   }
   11 }

It's possible to set a breakpoint that only stops on the last iteration.

b 9 if i == n - 1

Hardware data breakpoints ("watchpoints")

The processor can be set to break execution when it reads or writes certain addresses. This can be particularly useful to track down memory corruption. Create a hardware breakpoint by specifying "write", "execute" or "read-write" in the "type" for a break command (unlike in some other debuggers, hardware breakpoints are exposed as a type of breakpoint rather than as a separate "watchpoint" concept).

[zxdb] break --type=read-write --size=4 0x12345670

As a shortcut, the "watch" command will take the contents of a variable or the result of an expression and set a data write breakpoint over its range:

[zxdb] watch i
[zxdb] watch foo[5]->bar

Notes:

  • CPUs only support a limited number of hardware watchpoints, typically around 4.

  • The size of a watchpoint range is limited to 1, 2, 4, or 8 bytes and the address must be an even multiple of the size.

  • Unlike GDB, "watch" will evaluate the expression once and set a breakpoint on the result. It won't re-evaluate the expression. In the above example, it will trigger when "bar" changes but not if "foo[5]" changes to point to a different "bar".

  • If you watch a variable on the stack and nobody touches it, you will often see it hit in another part of the program when the stack memory is re-used. If you get a surprising breakpoint hit, check that execution is still in the frame you expect.

Programmatic 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):

__builtin_debugtrap();

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.