Segmentation fault while passing integer pointer into function

Question

Background context: I was doing leetcode twoSum and trying to understand one of the solutions. So, I decided to implement the solutions in the code block and use the debugger.

#include <stdio.h>
#include <stdlib.h>
#define SIZE 50000
int hash(int key);
void insert(int *keys, int *values, int key, int value);
int search(int *keys, int *values, int key);
int* twoSum(int* nums, int numsSize, int target, int* returnSize);

int main()
{
    int ar[4]={2,7,11,15};
    int *ans;
    int *returnSize;
    ans=malloc(2*sizeof(int));
    ans=twoSum(ar,4,9,returnSize);
    printf("d d ",ans[0],ans[1]);
    free(ans);
    return 0;
}


int hash(int key) {
    int r = key % SIZE;
    return r < 0 ? r + SIZE : r;
}

void insert(int *keys, int *values, int key, int value) {
    int index = hash(key);
    while (values[index]) {
        index = (index + 1) % SIZE;
    }
    keys[index] = key;
    values[index] = value;
}

int search(int *keys, int *values, int key) {
    int index = hash(key);
    while (values[index]) {
        if (keys[index] == key) {
            return values[index];
        }
        index = (index + 1) % SIZE;
    }
    return 0;
}

int* twoSum(int* nums, int numsSize, int target, int* returnSize){

    *returnSize = 2;
    int keys[SIZE]; //new array
    int values[SIZE] = {0}; //new array
    for (int i = 0; i < numsSize; i++) {
        int complements = target - nums[i];
        // check for complements in the hash table
        int value = search(keys, values, complements);
        if (value) {
            //return an array
            int *indices = (int *) malloc(sizeof(int) * 2);
            indices[0] = value - 1;
            indices[1] = i;
            return indices;
        }
        //if not insert the current values
        insert(keys, values, nums[i], i +1);
    }
    return NULL;
}

When I use the debugger, the error SEGMENTATION fault appears at line *returnSize=2? What is the problem?
I was trying to understand why i+1 in insert(keys, values, nums[i], i +1) instead of i?

score 1 · Accepted Answer · answered Apr 29 '20 at 07:30

You need to initialize returnSize before you can dereference it. You've got UB because you dereference a pointer that is not initialized. But I suspect what you really wanted was to have returnSize as an output parameter, which would look like this:

int main()
{
    int ar[4]={2,7,11,15};
    int *ans;
    int returnSize;
    ans=malloc(2*sizeof(int));
    ans=twoSum(ar,4,9, &returnSize);
    printf("d d ",ans[0],ans[1]);
    free(ans);
    return 0;
}

Note that returnSize in main() is now of type int (not int*). Its address is passed to the function twoSum. The difference is that the pointer passed to the function points to an existing variable.

Segmentation fault while passing integer pointer into function

1 Answers1