What is the difference between int3 instruction and call __debugbreak?

Question

What is __debugbreak? Is it used for triggering SIGTRAP? What is the difference between int3 and __debugbreak?

Answer 1

int3 is an x86 instruction.

__debugbreak() is an intrinsic supported by MSVC that will get the compiler to emit that instruction when compiling for x86, or whatever software-breakpoint instruction is appropriate for the target ISA (e.g. ARM, AArch64, etc.)

ICC also supports it, but other compilers (like gcc) don't..

You wouldn't do call __debugbreak in asm, you'd just write int3. e.g. if you compile a function that uses it, like

void foo() {
    __debugbreak();
}

MSVC on the Godbolt compiler explorer produces this asm:

void foo(void) PROC                                        ; foo
    npad    2
    int     3
    ret     0

Notice the lack of a call instruction anywhere. It's an intrinsic that "inlines" even with optimization disabled. It's not "just" a function.

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This is the same as how _mm_mfence() is an intrinsic for the mfence instruction, or _mm_popcnt_u64 for 64-bit operand-size popcnt.

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Related: Is there a portable equivalent to DebugBreak()/__debugbreak?
says clang has a __builtin_debugtrap().

Another answer there says the more widely available GNU C __builtin_trap() is assumed to stop / abort the program, not act like a breakpoint. (So gcc won't emit any code after an unconditional __builtin_trap.)

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update: apparently MSVC does let you take its address, so I guess there is a library version of it somewhere. So you could write call __debugbreak in asm, but you still wouldn't because it's pointless.

GCC does not let you take the address of builtins, for example trying to compile:

int (*getbuiltin(void))(unsigned) {    return &__builtin_popcount;  }

gives you this error:

error: built-in function '__builtin_popcount' must be directly called

But MSVC and ICC compile void (*getFunc(void))(void) { return &__debugbreak; } into this (on Godbolt)

void (__cdecl*getFunc(void))(void) PROC                         ; getFunc
    lea     rax, OFFSET FLAT:__debugbreak
    ret     0