Debug Break

debugbreak.h allows you to put breakpoints in your C/C++ code with a call to debug_break():

#include <stdio.h>
#include "debugbreak.h"

int main()
{
	debug_break();
	printf("hello world\n");
	return 0;
}

This works like the DebugBreak() intrinsic provided by Windows and QNX. Currently it supports gcc and Clang, works well on ARM, AArch64, i686, x86-64 and has a fallback code path for other architectures.

Just include the header in your C/C++ code.

License: 2-Clause BSD.

Implementation Notes

The requirements for the debug_break() function are:

• Act as a compiler code motion barrier
• Don't cause the compiler optimizers to think that the code following it can be removed
• Trigger a software breakpoint hit when executed (i.e. SIGTRAP on Linux)
• continue, next, step, stepi GDB commands work after a debug_break() hit

Ideally, both gcc and Clang would provide a __builtin_debugger() that satisfies the above on all supported architectures and operating systems. Unfortunately, we're not there yet. gcc's __builtin_trap() causes the optimizers to think the code follwing can be removed (test/trap.c):

#include <stdio.h>

int main()
{
	__builtin_trap();
	printf("hello world\n");
	return 0;
}

main
0x0000000000400390 <+0>:     0f 0b	ud2    

__builtin_trap() also generates an ud2 instruction which triggers SIGILL instead of SIGTRAP on i386 / x86-64. This makes it necessary to change GDB's default behavior on SIGILL to not terminate the inferior:

(gdb) handle SIGILL stop nopass

Even after this, continuing execution in GDB doesn't work well on some GCC, GDB combinations.

On ARM, __builtin_trap() generates a call to abort().

debug_break() generates an int3 instruction on i386 / x86-64 (test/break.c):

#include <stdio.h>
#include "debugbreak.h"
   
int main()
{
	debug_break();
	printf("hello world\n");
	return 0;
}

main
0x00000000004003d0 <+0>:     50	push   %rax
0x00000000004003d1 <+1>:     cc	int3   
0x00000000004003d2 <+2>:     bf a0 05 40 00	mov    $0x4005a0,%edi
0x00000000004003d7 <+7>:     e8 d4 ff ff ff	callq  0x4003b0 <puts@plt>
0x00000000004003dc <+12>:    31 c0	xor    %eax,%eax
0x00000000004003de <+14>:    5a	pop    %rdx
0x00000000004003df <+15>:    c3	retq   

Which correctly trigges SIGTRAP and stepping in GDB after a debug_break() hit works well. Clang / LLVM also has a __builtin_trap() that generates ud2 but further provides __builtin_debugger() that generates int3 on i386 / x86-64.

On ARM, debug_break() generates .inst 0xe7f001f0 in ARM mode and .inst 0xde01 in Thumb mode which correctly triggers SIGTRAP on Linux. Unfortunately, stepping in GDB after a debug_break() hit doesn't work and requires a workaround like:

(gdb) set $l = 2
(gdb) tbreak *($pc + $l)
(gdb) jump   *($pc + $l)
(gdb) # Change $l from 2 to 4 for ARM mode

to jump over the instruction. A new GDB command, debugbreak-step, is defined in debugbreak-gdb.py to automate the above.

$ arm-none-linux-gnueabi-gdb -x debugbreak-gdb.py test/break-c++
<...>
(gdb) run
Program received signal SIGTRAP, Trace/breakpoint trap.
main () at test/break-c++.cc:6
6		debug_break();

(gdb) debugbreak-step

7		std::cout << "hello, world\n";

On AArch64, debug_break() generates .inst 0xd4200000.

On other architectures, debug_break() generates a call to raise(SIGTRAP).