Hi there, the function clock() is a CUDA function, and there’s a CUDA sample demonstrating how to use it properly in the CUDA SDK samples named 0_Simple/clock_nvrtc. Documentation is here: https://docs.nvidia.com/cuda/cuda-c-programming-guide/index.html#time-function
The docs mention that cudaDevAttrClockRate() returns the peak clock rate in kilohertz, which might not the be your current clock rate. You can lock your clock rate to a specific value for profiling and timings using nvidia-smi.
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David.