I'm getting “incompatible pointer type” and I don't understand why

Question

I'm getting two types of errors:

Compiler's complaints

pr.c: In function ‘main’:

pr.c:20:2: warning: passing argument 1 of ‘printMatrix’ from incompatible pointer type [enabled by default]

pr.c:9:6: note: expected ‘const int (*)[80]’ but argument is of type ‘int (*)[80]’

pr.c:22:2: warning: passing argument 1 of ‘lights’ from incompatible pointer type [enabled by default]

pr.c:10:6: note: expected ‘const int (*)[80]’ but argument is of type ‘int (*)[80]’

It seems the compiler complains about receiving a non-const in a function that takes a const, but I was told this was the correct way of using const...

#include <stdio.h>
#include <stdlib.h>

#define MAXCOL  80
#define MAXROW  20
#define randNormalize() srand(time(0))

void fillMatrix(int m[][MAXCOL], size_t rows, size_t cols);
void printMatrix(const int m[][MAXCOL], size_t rows, size_t cols);
void lights(const int m[][MAXCOL], size_t rows, size_t cols);
int star(const int m[][MAXCOL], int row, int col);

int main()
{
    int m[MAXROW][MAXCOL];

    randNormalize();

    fillMatrix(m, 5, 5);
    printMatrix(m, 5, 5);

    lights(m, 5, 5);

    return 0;
}

void fillMatrix(int m[][MAXCOL], size_t rows, size_t cols)
{
    int i, j;

    for(i = 0; i < rows; i++)
        for(j = 0; j < cols; j++)
            m[i][j] = rand()%21;

}

void printMatrix(const int m[][MAXCOL], size_t rows, size_t cols)
{
    int i, j;

    for(i = 0; i < rows; i++)
    {
        printf("\n");

        for(j = 0; j < cols; j++)
            printf("%d ", m[i][j]);
    }

    printf("\n");
}


void lights(const int m[][MAXCOL], size_t rows, size_t cols)
{
    int i, j;

    for(i = 1; i < rows - 1; i++)
    {
        printf("\n");

        for(j = 1; j < cols - 1; j++)
        {
            if( star(m, i, j) )
                printf("*");
            else
                printf(" ");
        }
    }

    printf("\n");
}



int star(const int m[][MAXCOL], int row, int col)
{
    int i, j;
    int sum = 0;

    for(i = row - 1; i <= row + 1; i++)
        for(j = col - 1 ; j <= col + 1; j++ )
            sum += m[i][j];

    return (sum/9 > 10);
}

I'm looking for the best solution that doesn't use pointers, as this was from an exercise of a course in which we have not yet covered them (although I have studied them).

The const-thing is fine, but fillMatrix expects a pointer to array of 80 ints, while you pass a pointer to an array of 5. Those are not compatible. This question might explain more. — Kninnug
FYI, the messages are not two seperate warnings. They are part of the same warning. — kaylum
Oh, ok I now see you have provided half the message for two warnings. What I meant was that each warning should tell you the warning type and then the warning note. Which is what you have shown respectively but for different warning lines. So you should actually post all 4 lines from the warnings output or at least post consistently the same part of the warning messages for each line. — kaylum

M.M M.M · Accepted Answer · 2015-11-16T00:28:11

Unfortunately, in C there is no implicit conversion from int[X][Y] to const int[X][Y]. Nor is there implicit conversion from int (*)[Y] to const int (*)[Y].

This is a defect in the language; there's no technical reason why such a conversion should not be allowed. (C++ does allow this conversion).

You have two options, both unappealing:

Have the functions accept int instead of const int
Write a cast when calling the const int functions, e.g. printMatrix((const int (*)[MAXCOL])m, 5, 5);

Normally option 1 would be used and we just have to do without const-correctness for multi-dimensional arrays.

I'm getting “incompatible pointer type” and I don't understand why

2 Answers