How to increase the size of an array in C

Question

I have an array inside a structure as below

struct st
{
    ....

    int index[1];

    ....
}

How can I increase size of the array present inside structure to say 6, when I want to use the array inside a function.

Do you want to set different sizes at compile time or do you need to resize the array dynamically at run time? — moooeeeep, Apr 29 '14 at 07:36
during compile time it self. My scenario is I got one structure from the existing code and I need to increase capacity of array present in the structure from 1 to 6 for my use. — Phalgun Sirga, Apr 29 '14 at 08:39
Then, why don't you just change the array size in the declaration, say `int index[6];`? — moooeeeep, Apr 29 '14 at 11:41

score 2 · Answer 1 · edited May 23 '17 at 10:32

2

Probably you looking for struct hack. Struct hack is a technique which allows you allocate additional memory for an array inside struct. Here is an example

struct str {
    int value;
    char ar[0];
};


int main()
{

   struct str *s = malloc( sizeof(struct str) + 20 );
   strncpy( s->ar,"abcd", 5);
   printf("%s",s->ar);
   return 0;
 }

As array defined at the end of the struct, s->ar will get those additional 20 bytes added to sizeof(struct str) in malloc.

Edit as Daan Timmer noted, this technique can be applied only to last member of a struct.

edited May 23 '17 at 10:32

Community

1
1

answered Apr 29 '14 at 07:27

Dabo

2,371
2
18
26

1

Do note: the `char ar[0]` has to be at the end (bottom) of the structure. – Daan Timmer Apr 29 '14 at 08:58
@DaanTimmer I mentioned that array is at the end of struct, but I added a note to emphasis this point – Dabo Apr 29 '14 at 09:04

score 1 · Answer 2 · answered Apr 29 '14 at 07:17

1

You may try this:

struct st { ....

int index[6];

....
}

Also you may check function malloc() and realloc() in C

On a side note:

You may check STL container like std::vector which encapsulate the associated memory management.

answered Apr 29 '14 at 07:17

Rahul Tripathi

168,305
31
280
331

4

STL containers don't apply to applications written in `C`. – moooeeeep Apr 29 '14 at 07:33
@moooeeeep:- Yes I understand that. Thats why ihave written it on a side note! But thanks for pointing that! – Rahul Tripathi Apr 29 '14 at 08:09
I have an existing structure which has array of size 1. Now I need to increase its size. – Phalgun Sirga Apr 29 '14 at 08:44
@PhalgunSirga:- Yes thats what realloc and malloc are used for! – Rahul Tripathi Apr 29 '14 at 08:45

score 1 · Answer 3 · answered Apr 29 '14 at 07:17

1

An array defined that way exists on the stack. In order to dynamically change the size you will need to use allocate on the heap using malloc realloc and free.

answered Apr 29 '14 at 07:17

ioseph

1,887
20
25

3

What do you mean by "exists on the stack" ? Structures and fixed length array can live on the heap too ! – Nicolas Repiquet Apr 29 '14 at 07:27
Some further reading I found out globals such as OP's can live on either, or somewhere else, implementation specific. Thanks for the clarification! – ioseph Apr 30 '14 at 02:51
You can also allocate a fixed length array on the heap : int (*my_array)[10] = malloc(sizeof *my_array); – Nicolas Repiquet Apr 30 '14 at 14:59

How to increase the size of an array in C

3 Answers3