Dynamic Array in C input via terminal

Question

I am new to C and I was trying to programm a dynamic int Array which gets its values via terminal. I got it to work but it only works for a few numbers before i get the following error: realloc(): invalid next size Aborted. I would like to know why that is Here is my code so far:

void dynamicIntArray(){
    int *dynamicArray = malloc(0);
    int length = 0;

    printf("Please put in your numbers: \n");
    while(1){
        int x;
        scanf("%d", &x);

        if(x == -1){ break; }
        else{
            dynamicArray = realloc(dynamicArray, sizeof(int));
            if(dynamicArray == NULL){ return EXIT_FAILURE; }
            dynamicArray[length++] = x;
        }
    }
}


int main(){
    dynamicIntArray();
    return 0;
}

The second argument of realloc is the new size of the array in bytes — ForceBru
– ForceBru, Commented May 9, 2021 at 21:11
Oh ok thanks I thought it expends by this value ok makes sense. — NoCodeNoBlunder
– NoCodeNoBlunder, Commented May 9, 2021 at 21:18
You did not check the return value of scanf(), meaning any use of x after that line may invoke undefined behavior. — DevSolar
– DevSolar, Commented May 9, 2021 at 22:16

DevSolar · Accepted Answer · 2021-05-09 22:14:58Z

2

Replace

realloc(dynamicArray, sizeof(int))

With

realloc(dynamicArray, length * sizeof(int))

The former option will allocate memory for a single int. Accessing element at index other than zero invokes undefined behaviour, likely a crash.

edited May 9, 2021 at 22:14

DevSolar

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answered May 9, 2021 at 21:50

tstanisl

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Comments

marc_s · Accepted Answer · 2021-07-04 12:36:34Z

0

This is my solution now. I also had to change the index when assigning a new value to dynamicArray otherwise it would always hold the default int value at index 0 which makes me think malloc(0) is exactly the same as malloc(1).

void dynamicIntArray() {
    int *dynamicArray = malloc(0);
    int length = 0;

    printf("Please put in your numbers: \n");

    while(1) {
        int x;
        scanf("%d", &x);

        if(x == -1) { break; }
        else {
            dynamicArray = realloc(dynamicArray, ++length*sizeof(int));

            if(dynamicArray == NULL) { return EXIT_FAILURE; }

            dynamicArray[length-1] = x;
        }
    }
}

int main() {
    dynamicIntArray();
    return 0;
}

edited Jul 4, 2021 at 12:36

marc_s

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answered May 9, 2021 at 22:10

NoCodeNoBlunder

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1 Comment

hookenz Over a year ago

Please don't answer your question with a "here is my solution now" response. It's not really a clear answer. Think about how others who come to the site are seeing a similar problem and want to know where it is failing. tstanisl's answer points you to the right place.

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