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

Answer 2

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.