What function is used to deallocate memory that was previously allocated by malloc?

The function used to deallocate memory that was previously allocated by malloc is free().

Here is a step by step guide on how to use it:

1. First, you allocate memory using malloc(). For example:

int *ptr = (int*) malloc(sizeof(int));

This will allocate enough memory to store an integer and the pointer ptr points to this memory.

2. You can use the allocated memory to store an integer:

*ptr = 10;

3. Once you are done with the memory (i.e., you don't need to use the stored integer anymore), you can deallocate the memory using free(). This is done as follows:

free(ptr);

After this step, the memory previously pointed to by ptr is deallocated and can be used for other purposes.

4. It's a good practice to set the pointer to NULL after freeing the memory to avoid dangling pointer:

ptr = NULL;

Remember, every malloc() should have a corresponding free(). If you forget to free the memory, it can lead to memory leaks where the memory is not available for other processes until your program ends.