How do I multiply two sparse matrices in C?

How do I multiply two sparse matrices in C?

I have a sparse matrix D, and I want to multiply D_transpose and D to get
L as follows:
L = D'*D;
I am using sparseBLAS to deal with sparse matrices, but the documentation
says there's nothing to multiply two sparse matrices.
I am completely stuck and have no idea how to proceed.
The dimensions of D are typically around 500,000 by 250,000. I cannot
assign that much memory at all, so it just has to be done using sparse
matrices.
I did this using MATLAB, and I don't understand how MATLAB does it if it
also uses sparseBLAS underneath the interface - or does it? If not, what
does it use? I could use that too.
Thank you for reading!