Is it possible to set the size of an array within structure during runtime?

Question

I have a structure as below:

struct Query {  
    int     pages[];
    int     currentpage;
};

I was wondering if it was possible to set the size of this array after creating the structure.

Query new = malloc(sizeof(struct Query));

After this, I will perform some calculations which will then tell me the size that pages[] needs to be. If pages[] needed to be of size 4, how can I set it as such?

Answer 1

In C99 you can use Flexible array members:

struct Query {  
    int currentpage;
    int pages[]; /* Must be the last member */
};

struct Query *new = malloc(sizeof(struct Query) + sizeof(int) * 4);

Answer 2

Change the type of pages member to pointer.

struct Query {  
    int *pages;
    int currentpage;
};

struct Query *test = malloc(sizeof(struct Query));

if (test != NULL)
{
   //your calculations

   test->pages = malloc(result_of_your_calcs);
   if (test->pages != NULL)
   {
      // YOUR STUFF
   }
   else
   {
      // ERROR
   }
}
else
{
   // ERROR
}

When you'll free your struct, you have to do that on the contrary.

free(test->pages);
free(test);

Answer 3

You can use a Flexible array member (details in @AlterMann's answer) (C99+), or a Zero length array (GNU C).

Quoting from https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html,

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure that is really a header for a variable-length object:

struct line {
   int length;
   char contents[0];
};

struct line *thisline = (struct line *)
   malloc (sizeof (struct line) + this_length);
thisline->length = this_length;

For standard C90, the linked site mentions

In ISO C90, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

This means that for the code to work in standard C90/C89, char contents[0]; should be char contents[1];.

Answer 4

Declare it as pointer and use malloc afterwards

struct Query {  
    int * pages;
    int   currentpage;
};

. . .

struct Query obj;
obj.pages = malloc(n * sizeof(int));   // n is the length you want