My main function generates a matrix as an array of values "m", and another array of pointers to the start of rows "M". I want to pass this matrix to a subroutine such that no values can be modified, and such that no row pointers can be modified. I.e., the subroutine must not alter the matrix. As such, I pass a pointer to a constant pointer to a constant value. This works fine. The expected error messages are generated by the example below.
#include<stdio.h>
#include<stdlib.h>
void fun(double const * V, double const * const * M)
{
V = V; // allowed but pointless
V[0] = V[0]; // not allowed
M = M; // allowed but pointless
M[0] = M[0]; // not allowed
M[0][0] = M[0][0]; // not allowed
}
int main()
{
double *V = (double *)malloc(2*sizeof(double));
double *m = (double *)malloc(4*sizeof(double));
double **M = (double **)malloc(2*sizeof(double *));
M[0] = &m[0];
M[1] = &m[2];
fun(V,M);
return 0;
}
Error messages:
test.c: In function ‘fun’:
test.c:7:2: error: assignment of read-only location ‘*V’
test.c:9:2: error: assignment of read-only location ‘*M’
test.c:10:2: error: assignment of read-only location ‘**M’
These are as expected. All good so far.
The problem is that passing a non-constant matrix also generates the following warning. I'm using gcc v4.5 with no options.
test.c: In function ‘main’:
test.c:22:2: warning: passing argument 2 of ‘fun’ from incompatible pointer type
test.c:4:6: note: expected ‘const double * const*’ but argument is of type ‘double **’
Note that passing the vector "V" generates no such warnings.
My question is this: Can I pass a fully mutable matrix to a subroutine in such a way that it cannot be modified, without casting, and without compiler warnings?
