Square brackets after float4 vector type

Question

I'm studying a CUDA tutorial and at a some point the shared memory is dynamically allocated like so:

extern __shared__ float4[] buffer;

It seems to be quite simple to understand except for the square brackets [] after the vector type float4. What do they mean? why should I write them?

Answer 1

This is used for a dynamic shared memory and the proper way is :

extern __shared__ float4 buffer[]

, not

extern __shared__ float4[] buffer

When you use this ,you must also use the size of the allocated memory in the kernel call:

size_t blockSize = theBlockSize * theBlockSize
myfunc<<< NbBlocks, NbThreadsPerBlock , sizeof(float4) * blockSize>>>