`BUG()` and `BUG_ON(condition)` are used as a debugging help when something in the kernel goes terribly wrong. When a `BUG_ON()` assertion fails, or the code takes a branch with `BUG()` in it, the kernel will print out the contents of the registers and a stack trace. After that the current process will die. The following are examples of how `BUG()` and `BUG_ON()` are used, from a piece of code that is not supposed to run in interrupt context. The explicit `if` with `BUG()` is the coding style used in older kernels. In the 2.6 kernel, generally `BUG_ON()` is preferred. {{{ if (in_interrupt()) BUG(); BUG_ON(in_interrupt()); }}} == How it works == {{{ #define BUG() __asm__ __volatile__("ud2\n") }}} `BUG()` is defined as an invalid instruction, which means the CPU will throw an invalid opcode exception. This is caught in `arch/i386/kernel/entry.S`, in the `invalid_op` entry point, which calls the generated function `do_invalid_op` from arch/i386/kernel/traps.c. The following macros generate the `do_invalid_op()` function: {{{ #define DO_ERROR_INFO(trapnr, signr, str, name, sicode, siaddr) \ fastcall void do_##name(struct pt_regs * regs, long error_code) \ { \ siginfo_t info; \ info.si_signo = signr; \ info.si_errno = 0; \ info.si_code = sicode; \ info.si_addr = (void __user *)siaddr; \ if (notify_die(DIE_TRAP, str, regs, error_code, trapnr, signr) \ == NOTIFY_STOP) \ return; \ do_trap(trapnr, signr, str, 0, regs, error_code, &info); \ } DO_ERROR_INFO( 6, SIGILL, "invalid opcode", invalid_op, ILL_ILLOPN, regs->eip) }}} The `do_trap()` function will discover that the trap happened while running in kernel mode, and that there is no fixup for exceptions that happen while running at this address. See [[FAQ/TestWpBit]] to learn about exception fixups. {{{ kernel_trap: { if (!fixup_exception(regs)) die(str, regs, error_code); return; } }}} That in turn means that the current thread dies, printing a register dump and stack trace before it goes. The `die()` function has some magic of its own, which I won't go into here. ---- [[CategoryFAQ]]