cuda-samples/cpp/0_Introduction/simpleAssert

Sample: Device-Side assert (simpleAssert)

Description

Use the standard C assert macro inside a CUDA kernel. The sample launches threads that each assert gtid < N; threads whose global index reaches N trip the assertion, print a diagnostic message to the host, and cause the kernel to fail. The host detects this through the cudaErrorAssert status returned by cudaDeviceSynchronize and reports it as the expected outcome.

This sample demonstrates device-side assert, a debugging aid for catching invalid conditions in kernel code, and shows how an assertion failure surfaces on the host as an asynchronous CUDA error.

What You'll Learn

• Calling assert directly from device code (__global__ kernel)
• How a failed device assertion is reported to the host as cudaErrorAssert
• Detecting that error after the launch via cudaDeviceSynchronize
• Computing a global thread index from blockIdx, blockDim, and threadIdx
• Turning an error code into a human-readable message with cudaGetErrorString

Key Concepts

• Device-side assert — assert(condition) in a kernel; a false condition halts the kernel, prints file:line: function: block: ... Assertion ... failed, and flags the launch as failed
• Asynchronous error reporting — the assertion failure is not seen at launch time; it is surfaced at the next synchronization point as cudaErrorAssert
• Global thread indexing — blockIdx.x * blockDim.x + threadIdx.x

Key APIs

CUDA Runtime

• cudaSetDevice — select the active GPU
• cudaDeviceGetAttribute — query compute capability (major, minor) and SM count
• cudaDeviceSynchronize — block the host until the kernel finishes; flushes assert output and returns cudaErrorAssert if an assertion failed
• cudaGetErrorString — convert a cudaError_t into a readable description

Requirements

Hardware

• NVIDIA GPU with Compute Capability 7.5 or higher

Software

• CMake 3.20 or newer
• A C++17-capable host compiler

How to Build

See the top-level README for full build instructions, including how to build all samples or a single sample standalone.

How to Run

./simpleAssert

Expected Output

The kernel launches 2 blocks × 32 threads = 64 threads and asserts gtid < 60, so the 4 threads with global indices 60–63 fail the assertion. The assertion failures are expected — the sample reports OK because it successfully detected cudaErrorAssert:

simpleAssert starting...

GPU Device 0: with compute capability X.Y and Number of SMs <smCount>

Launch kernel to generate assertion failures

-- Begin assert output

simpleAssert.cu:50: void simpleAssertKernel(int): block: [1,0,0], thread: [28,0,0] Assertion `gtid < N` failed.
...

-- End assert output

Device assert failed as expected, CUDA error message is: device-side assert triggered

simpleAssert completed, returned OK

The order and exact set of failing-thread lines may vary between runs.

Files

• simpleAssert.cu — device-side assert kernel + host driver
• README.md — this file
• CMakeLists.txt — build configuration

See Also

• CUDA C++ Programming Guide - Assertion