2d dynamic array in C fault

Question

I have to write a program with dynamic 2d array but I've got a problem. Everything works fine until I try to print the array with more than 4 rows. Up to 4 rows all looks good, but with 5th row the length of last row expands to 7631472 which cause a segmentation fault.

Edit:

Thank you guys for response, I did what you'd suggested me,. However I discovered that problem is somewhere else, it's in "lengths" array. Each time when I add mroe than these 4 items it starts to fill up with some random values from memory.

Code:

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

int addRow(int i, int **array2d, int * lengths) {
    int len, value;
    int counter;
    i++;
    scanf("%d", &len); /*write the length of each row*/
        *(array2d + i-1) = malloc(sizeof (int) * len);
        *(lengths + i-1) = len;
    for(counter = 0; counter < len; counter++) {
        scanf("%d", &value); /*write each element in row*/
        *(*(array2d+i-1)+counter) = value; 
    }
    return i;
}

void print(int *i, int **array2d, int * lengths) {
    int counter, counter2;
    for(counter = 0; counter < *i; counter++) {
        for(counter2 = 0; counter2 < *(lengths+counter); counter2++) {
            printf("%d ",*(*(array2d+counter)+counter2));
        }
        printf("\n");
    } 
}

void end(int *i, int **array2d, int * lengths, char * word) {
    int counter;
    for(counter = 0; counter < *i; counter++)
        free(*(array2d + counter));
    free(array2d);
    free(lengths);
    free(word);
}

int main() {
    int ** array2d = (int**) malloc (0*sizeof(int*));
    int * lengths = (int*) malloc (0*sizeof(int)); /*length of each row*/
    char * word = (char*) malloc (3*sizeof(char));
    int i, isEqual; /* i - number of rows */
    i = 0;

    while(1) {
        scanf("%s", word);
        isEqual = strcmp(word, "add");
        if(!isEqual)
        i = addRow(i, array2d, lengths);
        isEqual = strcmp(word, "print");
        if(!isEqual) 
            print(&i, array2d, lengths);
        isEqual = strcmp(word, "end");
        if(!isEqual) {
            end(&i, array2d, lengths, word);
            return 0;
        } 
    }
    return 0;
}

Answer 1

In the main, you allocate arrays of size 0, if we read the man of malloc:

The malloc() function allocates size bytes and returns a pointer to the allocated memory. The memory is not initialized. If size is 0, then malloc() returns either NULL, or a unique pointer value that can later be successfully passed to free().

This mean you create an undefined behavior, so it can work on every cases, as it can never work.

Instead of this, you should allocate arrays of size 1, and then increase there sizes with realloc when you need it.

Answer 2

You should allocate array2d first to allocate *array2d.

The second pointer ,*array2d, points to the first new row cell that will contains your typed data. But first, you have to allocate a new item of your typed data rows list:

array2d = (int**) realloc(array2d, i)

Try this:

int addRow(int i, int **array2d, int * lengths) {
    int len, value;
    int counter;
    i++;
    array2d = (int**) realloc(array2d, sizeof (int*) * i);  //<<== realloc adding a new list item
    scanf("%d", &len); /*write the length of each row*/
        *(array2d + i-1) = malloc(sizeof (int) * len);
        *(lengths + i-1) = len;
    for(counter = 0; counter < len; counter++) {
        scanf("%d", &value); /*write each element in row*/
        *(*(array2d+i-1)+counter) = value; 
    }
    return i;
}