c Functions with arrays

Yes, if you know the number of columns in the 2D array at the time of passing it to the function. You do not have to define COL beforehand.

void foo(int col, int arr[][col]) {
//Do something
}

You can try this:

void func(void *data, int row, int col)
{
    int (*a)[col] = (int(*)[col])data;
    //now you can access a[i][j] with i<row and j<col
    //data must be an continous array
    //replace int with your data type
}

Working code:

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

int main()
{
    int n = 5;
    int (*a)[n];
    int b[5][5];

    a = (int(*)[n])b;
    b[0][0]=0;
    b[0][1]=1;
    b[2][1]=111;
    b[1][2]=666;
    b[4][3]=222;

    printf("%d\n", a[0][0]);
    printf("%d\n", a[0][1]);
    printf("%d\n", a[2][1]);
    printf("%d\n", a[1][2]);
    printf("%d\n", a[4][3]);

    return 0;
}

may be you can add a new parameter row to your function fillMatrix(double mat[][COLS]), that is, fillMatrix(double mat[][col], int row)

another way:

double** matrix = (double**) malloc(sizeof(double*)*n);
for(int i = 0; i<n; i++){
    matrix[i] = (double*) malloc(sizeof(double)*n);
}

then change the function to: fillMatrix(double** matrix, int n)

Is it possible to make a function that works with arrays of undefined length?

You'll better know the dimension of the array. If it is a function argument, you should in general pass the dimension in some other argument.

Remember that when passed as argument, an array is decayed into a pointer. Look also into some C reference site and later refer to the C11 standard n1570.

In your case, you want to define then have an abstract data type (probably some opaque pointer) for your matrixes. Using flexible array members could be useful. See this answer for details.

yes, you can do it using dinamic memory.

you will something like:

void fillMatrix(double** mat, int rows, int cols)    {
    int i,j;    

    printf("Enter the %d matrix elements:\n",rows*cols);
    for(i=0;i<rows;i++)
    {
        for(j=0;j<cols;j++)
        {
            scanf("%lf",&mat[i][j]);
        }                   
    }
    printf("\n");   
 }

where double** mat is a vector of vectors that you will have to ask for memory using malloc for the rows and the malloc for the columns.