segmentation fault when trying to pass two array pointers to a function that swaps them randomly

Question

I am trying to swap two pointers in the same array 52 times using a function within a for-loop. I am getting a segmentation fault and i'm not sure where the issue is.

Here's my code:

#include <limits.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "shuffle.h"

extern void shuffle(int** tempPtr1 , int** tempPtr2);
extern void deal(int numPlayers, int numHands, int** tempPtr2);

int main(void) {
    int numPlayers;
    int numHands;
    int randomNum;

    printf("Enter number of players: ");
    scanf("%d", &numPlayers);

    printf("Enter number of hands per player: ");
    scanf("%d", &numHands);

    int card[52] = {0};
    char faces[] = {'A', '2', '3', '4', '5', '6', '7', '8', '9','X', 'J', 'Q', 'K'};
    char suit[] = {'S', 'D', 'H', 'C'};

    srand((unsigned)time(NULL));

    for(int i = 0; i < 52; i++) {

        card[i] = suit[i/13];
        card[i] = card[i] << CHAR_BIT;
        card[i] = card[i] | faces[i%13];
    }
    int *firstIndxPtr;
    firstIndxPtr = &card[0];
    int *randIndxPtr1;

    for(int i = 0; i<52;i++){
        randomNum = rand() % 52 + 1;
        randIndxPtr1 = &card[randomNum];
        shuffle(&firstIndxPtr, &randIndxPtr1);
    }
    deal(numPlayers, numHands, &firstIndxPtr);
}

and then my function shuffle:

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

void shuffle(int** tempPtr1 , int** tempPtr2);

void shuffle(int** tempPtr1 , int** tempPtr2)
{       
    int* tempPtr = *tempPtr2;
    *tempPtr2 = *tempPtr1;
    *tempPtr1 = tempPtr;
}

the function should swap the pointer address of the 0 index in the card array with the pointer address of a random index in the card array. This would then be done 52 times in the for loop, creating a fully shuffled array. Instead I get a segmentation fault.

Are you sure that the segmetation fault happens in shuffle and not in another point (deal() for example) ? — Panos Antoniadis
@P.Antoniadis I have completely removed deal() from main and still get the fault. — MCkea444
don't think it's the problem, but use caution when shifting signed types,, usually best to stick to unsigned types for bitwise operations: stackoverflow.com/a/44859950/3476780 — yano

Panos Antoniadis Panos Antoniadis · Accepted Answer · 2019-01-31T22:03:31

The problem is:

 randomNum = rand() % 52 + 1;

Because rand()%52 is a number between 0 and 51. So, after adding 1 you have a number between 1 and 52. In case, rand()%52 is 51 randomNum is 52 and here:

randIndxPtr1 = &card[randomNum];

you access index 52 in an array with size 52. So, don't add 1 to randomNum.

segmentation fault when trying to pass two array pointers to a function that swaps them randomly

2 Answers