8 Queens diagonal checking

Question

At the moment I am creating a program where the user is asked to place the queens for the 8 Queens Problem. Right now I have nearly created the program, but I'm stuck on how to make the program check diagonally.

This is the (unfinished)code:

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int check_r_c(int**chess,int*p,int N,int M)
{
    int times=0;
    for(int i=0; i<N; i++)
    {
        times=0;
        for(int j=0; j<M; j++)
        {
            if(chess[i][j] == 1)
                times++;

        }
        if( times != 1)
        {
            *p=1;
            return 1;
        }
    }
    for(int j=0; j<M; j++)
    {
        times=0;
        for(int i=0; i<N; i++)
        {
            if(chess[i][j] == 1)
                times++;
        }
        if( times != 1)
        {
            *p=1;
            return 1;
        }
    }
    *p=0;
    return 0;
}
int main()
{
    int N,M;
    printf("Give the number of rows: \n");
    scanf("%d",&N);
    printf("Give the number of columns: \n");
    scanf("%d",&M);
    int**chess = malloc(N*sizeof(int));
    int i,j,ch;
    int*ptr;
    ptr=&ch;
    if(chess==NULL)
    {
        printf("\nMemory cannot be allocated\n");
        return 1;
    }
    for(i=0; i<N; i++)
    {
        if((chess[i] = malloc(M*sizeof(int)))==NULL)
        {
            printf("\nMemory cannot be allocated\n");
            return 1;
        }
    }
    for(int i=0; i<N; i++)
        for(int j=0; j<M; j++)
            chess[i][j]= 0;
    for(int k=0; k<N; k++)
    {
        printf("Give the position of the %d queen\n",k+1);
        scanf("%d",&i);
        scanf("%d",&j);
        if(chess[i][j] == 1)
        {
            printf("You cant put 2 queens in the same place!!!\n" );
            return 0;
        }
        chess[i][j] = 1;
    }
    check_r_c(chess,ptr,N,M);
    if(ch == 0)
        printf("Solution is correct!");
    else
        printf("Solution is incorrect!");
    for(int i=0; i<N; i++)
        free(chess[i]);
    free(chess);
}

Answer 1

you subtract the rows of 2 queens you subtract the cols of 2 queens you compare 2 result (absolute value) If they are equal => same diagonal line

Answer 2

The logic for checking diagonal for a queen placed is (p,q) is to check for all the places located at (p+i,q+i) for all i, where both p+i and q+i are within the board. For negative side, use (p-i,q-i).