Breaking into the debugger on iPhone

Question

Hi everyone,

For assert macros in my iPhone project, I'm looking for a way to programmatically break into the debugger. On Windows (MSVC++), I can use __debugbreak() for this purpose. Invoking this function will stop my program, launch the debugger, and display a callstack of the line that called __debugbreak().

Is there anything similar to __debugbreak() for the iPhone? I've tried Debugger(), but that gives me a linker error.

Thanks, Claus

Answer 1

I just set a breakpoint at the place I want to stop. Xcode remembers breakpoints persistently, so any time I run the app with gdb, it'll stop at that point.

If you want to break on assertion failures, a good place to set a breakpoint is on the function objc_exception_throw, in the Objective-C runtime, which is what actually throws an exception. Use the Run > Show > Breakpoints window and double-click the "Double-click for symbol" row, then type the name.

Answer 2

Check out conditional breakpoints:

http://www.cocoabuilder.com/archive/message/xcode/2008/10/22/25358

Answer 3

First Add -DDEBUG to OTHER_CFLAGS on your debug target; this will define the DEBUG symbol when building a debug build.

Then add a simple assert macro to your prefix header:

#ifdef DEBUG
#define MyAssert(val) _MyAssert(val)
#else
#define MyAssert(val) do { } while(0)
#endif

Next create a _MyAssert function in a module somewhere:

#ifdef DEBUG
void _MyAssert(int expression)
{
    if (expression == 0) {
       NSLog(@"Assertion failed!"); // Place breakpoint here
    }
}
#endif

Finally create a breakpoint on the NSLog line.

Answer 4

Is there something wrong with the simple assert() macro? Something like

assert(pointerToTest != nil);

will halt your process at that point if the condition is not true. If running under the debugger, you'll be presented with a stack trace of calls that led to the failed assertion. If you want to trigger it every time you hit a certain code path, you could do

assert(false);

I find these assertions extremely useful for verifying that all IBOutlets are non-nil when a window or view is brought up from a NIB.

Answer 5

A helpful person on Apple's developer forum gave me the tip to use asm("trap") when running on the device and asm("int3") when running on the simulator. This makes the program break into the debugger if you started your programm in debug mode (Option-Command-Y).

(__builtin_trap() also breaks into the debugger, but you can't continue afterwards. assert(false) terminates the program with a message, but doesn't break into the debugger.)

Answer 6

I use this:

#define Debugger() { kill( getpid(), SIGINT ) ; }

I think it works in the simulator and on the device.. no assembly required!