how to use printf inside a CUDA kernel?
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Daniel Castaño Díez
on 28 Jun 2024
Commented: Daniel Castaño Díez
on 11 Jul 2024
Hi,
I wonder why I cannot use printf in cuda kernels. The code inside my file test.cu (adapted from the Mathworks help)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <iostream>
__global__ void add2( double * v1, const double * v2)
{
int idx = threadIdx.x;
v1[idx] += v2[idx];
printf("identity: %d \n",idx);
}
compiles nicely with mexcuda with
mexcuda -ptx test.cu
but trying to runt it from the command line as
k = parallel.gpu.CUDAKernel("test.ptx","test.cu");
N = 8;
k.ThreadBlockSize = N;
in1 = ones(N,1,"gpuArray");
in2 = ones(N,1,"gpuArray");
result = feval(k,in1,in2);
gather(result);
does not put any result on screen.
this link suggests some operations with the header, as #undef printf to avoid conflicts with mex.h... but it didn't work for me.
5 Comments
Umar
on 4 Jul 2024
Hi Daniel,
In CUDA C/C++, snprintf is allowed in device code because it is a host function that can be used in device code without any special modifications and facilitating easier code development. Also, when you use snprintf in device code, CUDA automatically handles the necessary translations and optimizations for device execution. Therefore, you do not need to specify snprintf as a special version for host or device; it can be used directly in device code as you would in host code.
Accepted Answer
Joss Knight
on 6 Jul 2024
Edited: Joss Knight
on 6 Jul 2024
Just use it, and launch MATLAB from a terminal. On Linux, the output will appear in the terminal window. On Windows you will need to launch MATLAB with the undocumented options -wait -log.
3 Comments
Joss Knight
on 8 Jul 2024
If you want to redirect the output into the MATLAB command window you are going to need to use one of the tricks off the internet or mentioned here to redirect the kernel stdout into a buffer so you can invoke mexPrintf with it on the host. For debugging purposes, this is more trouble than it's worth.
More Answers (1)
Udit06
on 1 Jul 2024
Hi Daniel,
One more suggestion that I found in the following discussion is to use "cudaDeviceSynchronize" to ensure that the kernel finishes and the driver flushes the output buffer.
If the issue still persists, you can refer to the solution given in the following discussion:
I hope this helps.
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