Problem when passing multidimensional array to a function in C

Your code has two problems:

1. sum function receive 10x10 array. Your code can receive matrix in any size. What happens if you ask for 2x2 matrix, then sum treat it as 10x10 array and try to access to [2][2]? that is undefined behavior. You should not declare size of array in sum arguments.

2. You can not receive array size as input and then declare an array that way. You should allocate dynamic memory. I encourage you to read more about dynamic memory allocation

Look at the following changes:

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

int sum(int a[][],int d);

int main(){

    int r,c;
    puts("Enter the dimension of square matrix");
    scanf("%d %d",&r,&c);
    if (r != c) {puts("Not a square matrix");exit(0);}//check if square matrix

    //allocate memory
    int* a = (int*)malloc(sizeof(*a)*r);
    for(int i=0;i<r;i++){
        a[i] = (int*)malloc(sizeof(int)*c)
    }
    puts("Enter the elements of the matrix");
    for(int i=0; i<r; ++i){
        for(int j=0; j<c; ++j){
            printf("Enter a[%d][%d] element = ",i+1,j+1);
            scanf("%d",&a[i][j]);
        }
    }

    int result = sum(a,r);

    printf("Sum of diagonal elements = %d \n",result);

    //Free memory
    for(int i=0;i<r;i++)
        free(a[i])
    free(a)
    return 0;
}

int sum(int a[][d],int d){
//for a square matrix no of diagonal element = row/col of matrix
    int result=0;
    for(int i=0; i<d; ++i){
        result=result+a[i][i];
    }

    return result;
}

When you create a true 2D array, all dimensions but the first one are used in actual index computation. You have two options here:

1. Use a static maximum size for the matrix (say 10):

   int sum(int a[][10],int d);
   
   int main(){
   
       int r,c;
       puts("Enter the dimension of square matrix");
       scanf("%d %d",&r,&c);
       scanf("%d %d",&r,&c);
       if (r != c) {puts("Not a square matrix");exit(0);}//check if square matrix
       if (r > 10) {puts("Size must be at most 10"); exit(0);} // check size
       int a[r][10];             // Force a well know row size
       ...
   int sum(int a[][10],int d){
   //for a square matrix no of diagonal element = row/col of matrix
       ...
   

2. Pass the dynamic dimension in the function call before the array:

   int sum(int d, int a[][d]);
   
   int main(){
   
       int r,c;
       puts("Enter the dimension of square matrix");
       scanf("%d %d",&r,&c);
       if (r != c) {puts("Not a square matrix");exit(0);}//check if square matrix
       int a[r][c];
       ...
       int result = sum(r, a);
       ...
   int sum(int d, int a[][d]){
   //for a square matrix no of diagonal element = row/col of matrix
   

As noted, you have a problem with feeding data into a wrongly sized static array for any values of r and c other than 10 and 10.
There are several ways this can be refactored to address this problem, one being dynamic memory memory allocation:

Change This:

int sum(int a[10][10],int d);

int main(){

    int r,c;
    puts("Enter the dimension of square matrix");
    scanf("%d %d",&r,&c);
    if (r != c) {puts("Not a square matrix");exit(0);}//check if square matrix
    int a[r][c];  

To this:

int sum(int **a,int d);

int main(){

    int r,c;
    puts("Enter the dimension of square matrix");
    scanf("%d %d",&r,&c);
    if (r != c) {puts("Not a square matrix");exit(0);}//check if square matrix
    int **a = calloc(c, sizeof(int *));
    if(a)
    {
        for(int i = 0;i<c;i++)
        {
            a[i] = calloc(r, sizeof(int));
        }
    }

Then change the function prototype below:

int sum(int a[10][10],int d){

To:

int sum(int **a,int d){

Each element created using calloc must be freed once it is no longer needed.

eg :

        for(int i = 0;i<c;i++)
        {
            free(a[i]);
        }
        free(a);

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

int sum(int *a,int r);

int main(){

    int r,c;
    puts("Enter the dimension of square matrix");
    scanf("%d %d",&r,&c);
    if (r != c) {puts("Not a square matrix");exit(0);}//check if square matrix
    int a[r][c];
    puts("Enter the elements of the matrix");
    for(int i=0; i<r; ++i){
        for(int j=0; j<c; ++j){
            printf("Enter a[%d][%d] element = ",i+1,j+1);
            scanf("%d",&a[i][j]);
        }
    }

    int result = sum((int *)a,r);

    printf("Sum of diagonal elements = %d \n",result);
    return 0;
}

int sum(int *a,int r){
//for a square matrix no of diagonal element = row/col of matrix
    int result=0;
    for(int i=0; i<r; i++){
        result=result+(*(a+ i*r) + i);
    }
return result;
}

The basic idea is to flatten the 2D array. For instance, one 2D array of 3 rows and 5 columns has its elements in the same order as a 1D array of 15 elements.

int a2[3][5] = {{0, 1, 2, 3, 4}, {1, 2, 3, 4, 5}, {4, 5, 6, 7, 8}};

has the exact same sequence as

int a1[15] = {0, 1, 2, 3, 4, 1, 2, 3, 4, 5, 4, 5, 6, 7, 8};

though a1 and a2 have incompatible types.

So... how do you flatten an array?

You can simply use the address of a[0][0] (of type int*) and assume all elements follow sequentially. Also pass dimension info so you can calculate indexes inside the function.

#include <stdio.h>

void foo(int *flattened, int rows, int cols) {
    printf("original[2][3] is %d\n", flattened[2*cols + 3]);
}

int main(void) {
    int one[14][14]; one[2][3] = 42;
    foo(&one[0][0], 14, 14);

    int two[4][100]; two[2][3] = 42;
    foo(&two[0][0], 4, 100);

    return 0;
}