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What's the instruction to cause a hard-break in Xcode? For example under Visual Studio I could do '_asm int 3' or 'DebugBreak()'. Under some GCC implementations it's asm("break 0") or asm("trap").

I've tried various combos under Xcode without any luck. (inline assembler works fine so it's not a syntax issue).

For reference this is for an assert macro. I don't want to use the definitions in assert.h both for portability, and because they appear to do an abort() in the version XCode provides.

John - Super, cheers. For reference the int 3 syntax is the one required for Intel Macs and iPhone.

Chris - Thanks for your comment but there are many reasons to avoid the standard assert() function for codebases ported to different platforms. If you've gone to the trouble of rolling your own assert it's usually because you have additional functionality (logging, stack unwinding, user-interaction) that you wish to retain.

Your suggestion of attempting to replace the hander via an implementation of '__assert" or similar is not going to be portable. The standard 'assert' is usually a macro and while it may map to __assert on the Mac it doesn't on other platforms.

gcamp
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Andrew Grant
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7 Answers7

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http://developer.apple.com/documentation/DeveloperTools/Conceptual/XcodeProjectManagement/090_Running_Programs/chapter_11_section_3.html

asm {trap}            ; Halts a program running on PPC32 or PPC64.

__asm {int 3}         ; Halts a program running on IA-32.
John Millikin
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12

You can just insert a call to Debugger() — that will stop your app in the debugger (if it's being run under the debugger), or halt it with an exception if it's not.

Also, do not avoid assert() for "portability reasons" — portability is why it exists! It's part of Standard C, and you'll find it wherever you find a C compiler. What you really want to do is define a new assertion handler that does a debugger break instead of calling abort(); virtually all C compilers offer a mechanism by which you can do this.

Typically this is done by simply implementing a function or macro that follows this prototype:

void __assert(const char *expression, const char *file, int line);

It's called when an assertion expression fails. Usually it, not assert() itself, is what performs "the printf() followed by abort()" that is the default documented behavior. By customizing this function or macro, you can change its behavior.

Chris Hanson
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    What is a portable way to drop into the debugger on the failed line? assert() drops you into library code... – JBRWilkinson Oct 05 '11 at 17:41
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    Thanks Chris, but first - Debugger() is marked "deprecated" in the headers, without any direction for a replacement, Second - I don't know what to link against, in order to have it - normal "Cocoa" and "CoreFoundation" frameworks don't have it - I have a link error for a missing symbol on _Debugger – Motti Shneor Jan 14 '16 at 08:29
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__builtin_trap();

Since Debugger() is depreciated now this should work instead.

https://developer.apple.com/library/mac/technotes/tn2124/_index.html#//apple_ref/doc/uid/DTS10003391-CH1-SECCONTROLLEDCRASH

Tod
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    `__builtin_trap` crashes your program with a `SIGILL`, though (instead of barely interrupt it). – zneak Oct 28 '12 at 21:58
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For posterity: I have some code for generating halts at the correct stack frame in the debugger and (optionally) pausing the app so you can attach the debugger just-in-time. Works for simulator and device (and possibly desktop, if you should ever need it). Exhaustively detailed post at http://iphone.m20.nl/wp/2010/10/xcode-iphone-debugger-halt-assertions/

Steven Kramer
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2

I found the following in an Apple Forum:

Xcode doesn't come with any symbolic breaks built in - but they're quick to add. Go to the breakpoints window and add:

-[NSException raise]

GateKiller
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kill(getpid(), SIGINT);

Works in the simulator and the device.

Sonny Saluja
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  • First of all, = `raise(SIGINT)`. Second, these have pesky tendency to get forwarded into the main thread, so if you do it on another thread, you don't actually stop where you raised it. – Ilya Jul 26 '15 at 11:26
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There is also the following function that is available as cross platform straight Halt() alternative:

#include <stdlib.h>

void abort(void);

We use it in our cross platform engine for the iPhone implementation in case of fatal asserts. Cross platform across Nintendo DS/Wii/XBOX 360/iOS etc...

logancautrell
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