What does ** mean in C?

In this case, double means a variable of type double.

double* means a pointer to a double variable.

double** means a pointer to a pointer to a double variable.

In the case of the function you posted, it is used to create a sort of two-dimensional array of doubles. That is, a pointer to an array of double pointers, and each of those pointers points to an array of pointers.

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