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Release 13.4 of the CUDA Samples supported by CUDA Toolkit 13.4. See Changelog for more information.
84 lines
3.3 KiB
Markdown
84 lines
3.3 KiB
Markdown
# Sample: Device-Side assert (simpleAssert)
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## Description
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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.
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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.
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## What You'll Learn
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- Calling `assert` directly from device code (`__global__` kernel)
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- How a failed device assertion is reported to the host as `cudaErrorAssert`
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- Detecting that error after the launch via `cudaDeviceSynchronize`
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- Computing a global thread index from `blockIdx`, `blockDim`, and `threadIdx`
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- Turning an error code into a human-readable message with `cudaGetErrorString`
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## Key Concepts
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- **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
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- **Asynchronous error reporting** — the assertion failure is not seen at launch time; it is surfaced at the next synchronization point as `cudaErrorAssert`
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- **Global thread indexing** — `blockIdx.x * blockDim.x + threadIdx.x`
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## Key APIs
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### CUDA Runtime
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- `cudaSetDevice` — select the active GPU
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- `cudaDeviceGetAttribute` — query compute capability (major, minor) and SM count
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- `cudaDeviceSynchronize` — block the host until the kernel finishes; flushes assert output and returns `cudaErrorAssert` if an assertion failed
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- `cudaGetErrorString` — convert a `cudaError_t` into a readable description
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## Requirements
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### Hardware
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- NVIDIA GPU with Compute Capability 7.5 or higher
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### Software
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- CMake 3.20 or newer
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- A C++17-capable host compiler
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## How to Build
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See the [top-level README](../../../README.md#building-cuda-samples) for full build instructions, including how to build all samples or a single sample standalone.
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## How to Run
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```bash
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./simpleAssert
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```
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## Expected Output
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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`:
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```text
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simpleAssert starting...
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GPU Device 0: with compute capability X.Y and Number of SMs <smCount>
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Launch kernel to generate assertion failures
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-- Begin assert output
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simpleAssert.cu:50: void simpleAssertKernel(int): block: [1,0,0], thread: [28,0,0] Assertion `gtid < N` failed.
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...
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-- End assert output
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Device assert failed as expected, CUDA error message is: device-side assert triggered
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simpleAssert completed, returned OK
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```
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The order and exact set of failing-thread lines may vary between runs.
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## Files
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- `simpleAssert.cu` — device-side `assert` kernel + host driver
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- `README.md` — this file
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- `CMakeLists.txt` — build configuration
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## See Also
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- [CUDA C++ Programming Guide - Assertion](https://docs.nvidia.com/cuda/cuda-programming-guide/05-appendices/cpp-language-extensions.html#assertion)
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