C: Expanding an array with malloc

Question

I'm a bit new to malloc and C in general. I wanted to know how I can, if needed, extend the size of an otherwise fixed-size array with malloc.

Example:

#define SIZE 1000
struct mystruct
{
  int a;
  int b;
  char c;
};
mystruct myarray[ SIZE ];
int myarrayMaxSize = SIZE;
....
if ( i > myarrayMaxSize )
{
   // malloc another SIZE (1000) elements
   myarrayMaxSize += SIZE;
}

• The above example should make clear what I want to accomplish.

(By the way: I need this for an interpreter I write: Work with a fixed amount of variables and in case more are needed, just allocate them dynamically)

Answer 1

Use realloc, but you have to allocate the array with malloc first. You're allocating it on the stack in the above example.

   size_t myarray_size = 1000;
   mystruct* myarray = malloc(myarray_size * sizeof(mystruct));
   myarray_size += 1000;
   mystruct* myrealloced_array = realloc(myarray, myarray_size);
   if (myrealloced_array) {
     myarray = myrealloced_array;
   } else {
     // deal with realloc failing because memory could not be allocated.
   }

Answer 2

No, you can't. You can't change the size of an array on the stack once it's defined: that's kind of what fixed-size means. Or a global array, either: it's not clear from your code sample where myarray is defined.

You could malloc a 1000-element array, and later resize it with realloc. This can return you a new array, containing a copy of the data from the old one, but with extra space at the end.

Answer 3

a) you did not use malloc to create it so you cannot expand with malloc. Do:

  mystruct *myarray = (mystruct*)malloc(sizeof( mystruct) *SIZE);

b) use realloc (RTM) to make it bigger

Answer 4

You want to use realloc (as other posters have already pointed out). But unfortunately, the other posters have not shown you how to correctly use it:

POINTER *tmp_ptr = realloc(orig_ptr, new_size);
if (tmp_ptr == NULL)
{
    // realloc failed, orig_ptr still valid so you can clean up
}
else
{
    // Only overwrite orig_ptr once you know the call was successful
    orig_ptr = tmp_ptr;
}

You need to use tmp_ptr so that if realloc fails, you don't lose the original pointer.