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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(); /* will break into debugger */
	printf("hello world\n");
	return 0;
}
  • Include one header file and insert calls to debug_break() in the code where you wish to break into the debugger.
  • Supports GCC, Clang and MSVC.
  • Works well on ARM, AArch64, i686, x86-64, POWER and has a fallback code path for other architectures.
  • Works like the DebugBreak() fuction provided by Windows and QNX.

License: the very permissive 2-Clause BSD.

Known Problem: if continuing execution after a debugbreak breakpoint hit doesn't work (e.g. on ARM or POWER), see HOW-TO-USE-DEBUGBREAK-GDB-PY.md for a workaround.

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 the code following it can be removed
  • Trigger a software breakpoint hit when executed (e.g. SIGTRAP on Linux)
  • GDB commands like continue, next, step, stepi must work after a debug_break() hit

Ideally, both GCC and Clang would provide a __builtin_debugtrap() that satisfies the above on all architectures and operating systems. Unfortunately, that is not the case (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;
}

compiles to:

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

Notice how the call to printf() is not present in the assembly output.

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

(gdb) handle SIGILL stop nopass

Even after this, continuing execution in GDB doesn't work well on some GCC, GDB combinations. See GCC Bugzilla 84595.

On ARM, __builtin_trap() generates a call to abort(), making it even less suitable.

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;
}

compiles to:

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 single-stepping in GDB after a debug_break() hit works well.

Clang / LLVM also has a __builtin_trap() that generates ud2 but further provides __builtin_debugtrap() that generates int3 on i386 / x86-64 (original LLVM intrinsic, further fixes, Clang builtin support).

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. See HOW-TO-USE-DEBUGBREAK-GDB-PY.md for sample usage.

$ 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).

Behavior on Different Architectures

Architecture debug_break()
x86/x86-64 int3
ARM mode, 32-bit .inst 0xe7f001f0
Thumb mode, 32-bit .inst 0xde01
AArch64, ARMv8 .inst 0xd4200000
POWER .4byte 0x7d821008
MSVC compiler __debugbreak
Apple compiler on AArch64 __builtin_trap()
Otherwise raise(SIGTRAP)