Declare array insde a structure in c

Question

Hi i would like declare a array of size x, this size will be know at runtime (not a static declatrion). how to do that. An efficient way to do that.

I had structure like this

    #define SIZE 50
    struct abc {
                  int students[SIZE];
                  int age;
   }

i would like read SIZE from some point at run time, instead of predefining it.

Edit : can we allocate memory for entire structure including array dynamically.

        strcut abc {
                     int *students; // should point to array of SIZE.
                     int age;
       }s;

can we get sizeof struct = size of entire struct(including array); ?

Rishikesh Raje · Accepted Answer · 2016-03-18 10:42:46Z

5

If the size is known at runtime, you need to use dynamic allocation.

struct abc 
{
  int *students;
  int age;
}

Then in code,

struct abc var;
var.students = malloc(size*sizeof(int));

and at the end of your code

free(var.students);

answered Mar 18, 2016 at 10:42

Rishikesh Raje

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4 Comments

Vasu Over a year ago

how to handle 2d array, say arry[SIZE] [FIX] ?

Rishikesh Raje Over a year ago

see this link

Vasu Over a year ago

second index is predefined, in this can plz provide example. for this use case.

Rishikesh Raje Over a year ago

In this case also you need to use a 2D array and malloc it as shown in the above link. Only thing is that FIX will be a constant.

kwarnke · Accepted Answer · 2016-03-18 16:52:07Z

1

There are two maybe simpler solution possible. You can use a empty sized array at the end of the struct:

struct abc {
    int age;
    int students[];
};

unsigned count = 10;
struct abc* foo = malloc(sizeof(*foo) + (count * sizeof(foo->students[0])));

Here you need only one malloc() call if you want both, the struct and the array being allocated dynamically. And also only one free. For a 2d array you have to alloc it as a 1d array and calculate the indices for your own.

An other solution could be:

unsigned count = 10;
struct abcd {
    int age;
    int students[count];
};

Simple use a variable length array in the struct direct. Here you can use 2d as usual.

answered Mar 18, 2016 at 16:52

kwarnke

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1 Comment

Vasu Over a year ago

Hey thannks, how can we declare a pointer for array inside struct, then allocate memory for enitre struct including array ?

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