CUDA GPU __global__ function does not complete - c++

Question

__global__ void functionA()
{
printf("functionA");
}
int main()
{
printf("main1");
functionA<<<1,1>>>();
printf("main2");
}
I'm trying to run a simple test with the above. But the program only outputs "main1". The program should output "functionA" and "main2" too.

Answer 1

This seems to have two reasons:
First of all you need to add
cudaDeviceSynchronize();
after the CUDA routine in order to block the main until the device has completed all tasks.
Furthermore this might happen if you set the wrong GPU architecture/compute capability XX when compiling the code
$ nvcc -gencode=arch=compute_XX,code=sm_XX -o my_app my_app.cu
In this case only the host code is run while the parts on the accelerator will be omitted it seems. You can find an overview of the corresponding number XX for the different hardware generations over here. The K20m you are running is 35. So it should be
$ nvcc -gencode=arch=compute_35,code=sm_35 -o my_app my_app.cu
in your case.
This might also occur if you have multiple graphic accelerators in your system and the code is executed on the wrong one. Each graphics card/accelerator is assigned a particular device id. The device with number 0 should be assigned automatically to the most powerful device and will be used by default. Therefore the first time I compiled the code on my system containing a powerful Tesla K80 (architecture 37) and a low power Quadro P620 (architecture 60) I selected 37 and had the same error as you have while when selecting 60 the code would run. I then used then the Querying Device Properties example to give me a list of the CUDA-capable devices and their corresponding device id, just to find out that on my system the Tesla K80 is set as 1 and 2 while the simple Quadro P620 graphics card is set as 0. I assume this is the case as the K80 is deprecated in CUDA 11!
You can select the device inside your code with cudaSetDevice or change it when launching the program with
$ CUDA_VISIBLE_DEVICES="1" ./my_app
where 1 has to be replaced by the device id you wish to use. Doing so should make your code run without any problems.
You can also test if this really is the issue this by cloning the Github repository of "Learn CUDA Programming", then browsing Chapter01/01_cuda_introduction/01_hello_world/, compile the make file with $ make and finally run it with $ ./hello_world. It automatically compiles for multiple architectures/compute capabilities and should therefore run without any issue!