Why is the function name in the FORTRAN program not the same? I would use the same name in both programs but perhaps this is FORTRAN-related. I will use the C function name you wrote for this answer and yes, I agree that this is quite a task for a beginner. This will not be a complete solution but should steer you in the right direction (hopefully). First... arrays in C have fixed dimensions determined at compile time. So your function declaration will not compile. If your intention is to dereference the dimension pointers in the array dimention parameters, this will not work because it's not executable code; those are parameter declarations used only for compilation.
Second, it seems like you want to pass arrays that can have different dimensions from one call to another or that you don't know in advance how many elements will be in your arrays. Assuming this, you need only pass the address of your arrays (assuming your dimensions are always the right ones). Your function prototype could then be:
void cuda_(int *idim, int *jdim, int *kdim, float *U, float* V, float* W);
Your function could be implemented as follow (so to show you the pointer arithmetic):
void cuda_(int *idim, int *jdim, int *kdim, float* U, float* V, float* W)
{
int i, j, k;
// Assuming you want to process each individual array
// items one after the other; this to show pointer arithmetic
for (i=0; i<*idim; i++) {
for (j=0; j<*jdim; j++) {
for (k=0; k<*kdim; k++) {
// Do whatever with the i,j,kth element;
// We assign unique values so you can check
// that this example accesses the right array elements
*(U + i * sizeof(float) + j * sizeof(float) + k) = i + j + k;
*(V + i * sizeof(float) + j * sizeof(float) + k) = 2*i + 2*j + 2*k;
*(W + i * sizeof(float) + j * sizeof(float) + k) = 3*i + 3*j + 3*k;
}
}
}
return;
}
Of course, you need to test this example thoroughly to make sure that this is what you really intend on doing and that I did map the 3D array properly (I mapped it over a one-dimentional array assuming that this is how it is implemented in your FORTRAN program; otherwise, you will have to come up with the proper pointer arithmitic but the principle will be the same as in the examples). I did very superficial testing of this code and be warned that pointer manipulation is tricky and error prone! Good luck.
Now, and this is of secondary importance but I feel the point should be made. I don't see why you want to pass the array dimensions as pointers; perhaps it's a requirement for FORTRAN (that is maybe you cannot pass parameters by value/copy). If you can pass parameters by copy (as opposed to by reference), then I suggest the following change:
// Prototype:
void cuda_(int idim, int jdim, int kdim, float* U, float* V, float* W);
// Implementation:
void cuda_(int idim, int jdim, int kdim, float* U, float* V, float* W)
{
int i, j, k;
for (i=0; i<idim; i++) {
for (j=0; j<jdim; j++) {
for (k=0; k<kdim; k++) {
*(U + i * sizeof(float) + j * sizeof(float) + k) = i + j + k;
*(V + i * sizeof(float) + j * sizeof(float) + k) = 2*i + 2*j + 2*k;
*(W + i * sizeof(float) + j * sizeof(float) + k) = 3*i + 3*j + 3*k;
}
}
}
}
