cudaMemcpy invalid argument

My program runs 2 threads - Thread A (for input) and B (for processing). I also have a pair of pointers to 2 buffers, so that when Thread A has finished copying data into Buffer 1, Thread B starts processing Buffer 1 and Thread A starts copying data into Buffer 2. Then when Buffer 2 is full, Thread A copies data into Buffer 1 and Thread B processes Buffer 2, and so on.

My problem comes when I try to cudaMemcpy Buffer[] into d_Buffer (which was previously cudaMalloc'd by the main thread, i.e. before thread creation. Buffer[] were also malloc'd by the main thread). I get a "invalid argument" error, but have no idea which is the invalid argument.

I've reduced my program to a single-threaded program, but still using 2 buffers. That is, the copying and processing takes place one after another, instead of simultaneously. The cudaMemcpy line is exactly the same as the double-threaded one. The single-threaded program works fine.

I'm not sure where the error lies.

Thank you.

Regards, Rayne