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); 

Answer 2

Keep in mind the difference between a pointer and an array.
An array is a chuck of memory in the stack, and that's all.If you have an array:

int arr[100]; 

Then arr is an address of memory, but also &arr is an adress of memory, and that address of memory is constant, not stored in any location.So you cannot say arr=NULL, since arr is not a variable that points to something.It's just a symbolic address: the address of where the array starts.Instead a pointer has it's own memory and can point to memory addresses.

It's enough that you change int[] to int*.
Also, variables are passed by copy so you need to pass an int** to the function.

About how using realloc, all the didactic examples include this:

1. Use realloc;
2. Check if it's NULL.In this case use perror and exit the program;
3. If it's not NULL use the memory allocated;
4. Free the memory when you don't need it anymore.

So that would be a nice example:

int* chuck= (int*) realloc (NULL, 10*sizeof(int)); // Acts like malloc,               // casting is optional but I'd suggest it for readability assert(chuck); for(unsigned int i=0; i<10; i++) {     chunk[i]=i*10;     printf("%d",chunk[i]); } free(chunk);