Question

I am reading integers from a file, and when I try to grow my array, I am getting a segmentation fault on the second call to growMyArray(struct myArray), specifically at int *grownArray = malloc(arrayToGrow.maxCount * sizeof(int));:

struct myArray growMyArray(struct myArray arrayToGrow) {

    arrayToGrow.maxCount *= 2;

    int *grownArray = malloc(arrayToGrow.maxCount * sizeof(int));

    int i;
    for (i = 0; i < arrayToGrow.count; i++)
        grownArray[i] = arrayToGrow.numbers[i];

    free(arrayToGrow.numbers);

    arrayToGrow.numbers = grownArray;

    return arrayToGrow;
}

My structure:

typedef struct myArray {
    int count;
    int maxCount;
    int *numbers;
} myArray;

Reading from input redirection:

struct myArray getRandomNumbers() {

    struct myArray randomNumbers;
    randomNumbers.count = 0;
    randomNumbers.maxCount = DEFAULT_SIZE;
    randomNumbers.numbers = malloc(randomNumbers.maxCount * sizeof(int));

    while (scanf("%d", &randomNumbers.numbers[randomNumbers.count]) == 1) {

        randomNumbers.count++;

        if (randomNumbers.count > randomNumbers.maxCount)
            randomNumbers = growMyArray(randomNumbers);
    }

    return randomNumbers;
}

I find this particularly odd because growing the array always works the first time but never works the second time. I have used multiple values for DEFAULT_SIZE, ranging from 2 to 20000 on a set of test data of size 200000.

Is there an apparent reason why I am getting a segmentation fault on the second call to growMyArray, specifically at int *grownArray = malloc(arrayToGrow.maxCount * sizeof(int));?

Answer 1

You wrote past the end of the array.

while (scanf("%d", &randomNumbers.numbers[randomNumbers.count]) == 1) {

    randomNumbers.count++;

    if (randomNumbers.count > randomNumbers.maxCount)
        randomNumbers = growMyArray(randomNumbers);
}

Because you use > in the test, the if only fires once randomNumbers.count = randomNumbers.maxCount + 1, i.e. the scanf writes to randomNumbers.numbers[randomNumbers.maxCount] which is past the end of the array.

Therefore, change > to >= in the if statement there.