Rather later for an Answer, but here goes anyway:
You should never use literal integers for API calls, because you can't be sure the default literal integer size is the same as the integer size that the API routines expect. Always use typed variables that match the API doc exactly. E.g., here is the signaure from the doc:
mwPointer mxCreateDoubleMatrix(m, n, ComplexFlag)
mwSize m, n
integer*4 ComplexFlag
so the code sould be something like
mwSize m, n
integer*4 ComplexFlag
m = 100
n = 1
ComplexFlag = 0
plhs(1) = mxCreateDoubleMatrix(m,n,ComplexFlag)
