Microsoft Specific

Immediately terminates the calling process with minimum overhead.


void __fastfail(unsigned int code);


[in] A FAST_FAIL_<description> symbolic constant from winnt.h or wdm.h that indicates the reason for process termination.

Return value

The __fastfail intrinsic does not return.


The __fastfail intrinsic provides a mechanism for a fast fail request—a way for a potentially corrupted process to request immediate process termination. Critical failures that may have corrupted program state and stack beyond recovery cannot be handled by the regular exception handling facility. Use __fastfail to terminate the process using minimal overhead.

Internally, __fastfail is implemented by using several architecture-specific mechanisms:

Architecture Instruction Location of code argument
x86 int 0x29 ecx
x64 int 0x29 rcx
ARM Opcode 0xDEFB r0
ARM64 Opcode 0xF003 x0

A fast fail request is self-contained and typically requires just two instructions to execute. After a fast fail request has been executed, the kernel then takes the appropriate action. In user-mode code, there are no memory dependencies beyond the instruction pointer itself when a fast fail event is raised. That maximizes its reliability, even in cases of severe memory corruption.

The code argument, one of the FAST_FAIL_<description> symbolic constants from winnt.h or wdm.h, describes the type of failure condition. It's incorporated into failure reports in an environment-specific manner.

User-mode fast fail requests appear as a second chance non-continuable exception with exception code 0xC0000409, and with at least one exception parameter. The first exception parameter is the code value. This exception code indicates to the Windows Error Reporting (WER) and debugging infrastructure that the process is corrupted, and that minimal in-process actions should be taken in response to the failure. Kernel-mode fast fail requests are implemented by using a dedicated bugcheck code, KERNEL_SECURITY_CHECK_FAILURE (0x139). In both cases, no exception handlers are invoked because the program is expected to be in a corrupted state. If a debugger is present, it's given an opportunity to examine the state of the program before termination.

Support for the native fast fail mechanism began in Windows 8. Windows operating systems that don't support the fast fail instruction natively will typically treat a fast fail request as an access violation, or as an UNEXPECTED_KERNEL_MODE_TRAP bugcheck. In these cases, the program is still terminated, but not necessarily as quickly.

__fastfail is only available as an intrinsic.


Intrinsic Architecture
__fastfail x86, x64, ARM, ARM64

Header file <intrin.h>

END Microsoft Specific

See also

Compiler intrinsics