How to use realloc in a function in C

Question

Building on what I learned here: Manipulating dynamic array through functions in C.

void test(int data[])
{
    data[0] = 1;    
}    

int main(void)
{    
    int *data = malloc(4 * sizeof *data);

    test(data);

    return 0;
}

This works fine. However, I am also trying to using realloc in a function.

void increase(int data[])
{
    data = realloc(data, 5 * sizeof *data);    
}

This complies but the program crashes when run.

Question

How should I be using realloc in a function?

I understand that I should assign the result of realloc to a variable and check if it is NULL first. This is just a simplified example.

Answer 1

You want to modify the value of an int* (your array) so need to pass a pointer to it into your increase function:

void increase(int** data)
{
    *data = realloc(*data, 5 * sizeof int);
}

Calling code would then look like:

int *data = malloc(4 * sizeof *data);
/* do stuff with data */
increase(&data);
/* more stuff */
free(data);