const arrays in prototypes

[Peter Montgomery]

/*
	When declaring a pointer to a function, careful use of const 
lets us distinguish whether the function is allowed to modify the 
pointer and/or the data.  For example
*/
void proc1 (  int       *p1,	/* non-const pointer to non-const int */
 	const int       *p2,	/* non-const pointer to     const int */
              int *const p3,	/*     const pointer to non-const int */
	const int *const p4)    /*     const pointer to     const int */
{
    p1++;
    p2++;
    p3++;	/* illegal modification of pointer */
    p4++;	/* illegal modification of pointer */
    *p1 = 1;
    *p2 = 2;	/* illegal modification of data    */
    *p3 = 3;
    *p4 = 4;	/* illegal modification of data    */
}
/*
	As desired, gcc 1.37.1 issues warnings for the four lines
marked "illegal".  

	Suppose instead that I am passing an array argument.  If I 
put "const" before the declaration, then I am stating that the data 
will not be modified.  How do I specify that the pointer to the array
will not be altered within the subroutine?  For example,
*/

void DISALLOW(const int mat[2][2])
{
    mat++;		/* I want to forbid this usage.		       */	
    mat[0][1] = 4;	/* Gcc 1.37.1 properly warns about this line.  */
}

--
--------
        Peter Montgomery
        pmontgom at MATH.UCLA.EDU 
	Department of Mathematics, UCLA, Los Angeles, CA 90024