My goal is to dynamically reallocate memory for a two dimensional int array in C. I know there are already several questions about that topic, but unfortunately my code does not run properly and i don't know what is going wrong.
First i am allocating memory:
int n = 10;
int m = 4;
int** twoDimArray;
twoDimArray = (int**)malloc(n * sizeof(int*));
for(int i = 0; i < n; i++) {
twoDimArray[i] = (int*)malloc(m * sizeof(int));
}
And initializing the array with integer numbers:
for(int i = 0; i < n; i++) {
for(j = 0; j < 4; j++) {
twoDimArray[i][j] = i * j;
}
}
Then i use realloc()
to reallocate memory dynamically:
int plus = 10;
int newArraySize = n + plus;
twoDimArray = (int**)realloc(twoDimArray, newArraySize * sizeof(int));
I am expecting my aray twoDimArray
to be accessible at [10][0] now, but when running
printf("twoDimArray[10][0] = %d\n", twoDimArray[10][0]);
i get an "EXC_BAD_ACCESS" runtime error.
Probably i am missing something rather simple, but since i am new to C and can't figure out my mistake. Any help is appreciated.
reallocating the array of pointers is necessary, but then you have only n
values that point to something valid. You need to allocate the rest of the sub-arrays because the newly allocated memory points to unallocated/invalid areas. The error is not from accessing the pointer, but from dereferencing it.
You need to add something like:
for(int i = n; i < n+plus; i++) {
twoDimArray[i] = malloc(m * sizeof(int));
}
(same goes for deallocation: first deallocate the arrays in a loop, then deallocate the array of pointers)
Aside:
realloc
directly to the original variable can be a problem if realloc
returns NULL
. Even if it's rare in those resizing cases (Under what circumstances can malloc return NULL?), you should copy the result in a temp variable, check for NULL
, and free
the old pointer if reallocation failed.