Declaration and definition of union

Question

Why does this code work:

#include<stdio.h>
int main(){
 union var{
 int a, b, c;
 };
 printf("%ld", sizeof(union var));
 return 0;
}

My doubt is that isn't union var a declaration only, and during declaration no memory is allocated. So why does this code print 4?

You are asking the code to print the size of the *type.* You can do the same with `int`, without actually declaring an `int` variable. — Beta, Feb 15 '21 at 15:33
and you should be using the `%z` (probably `%zu`) format specifier: https://stackoverflow.com/questions/2524611/how-can-one-print-a-size-t-variable-portably-using-the-printf-family — yano, Feb 15 '21 at 15:35
@Beta Does a union or structure declaration have memory associated with it? — programmer, Feb 15 '21 at 15:57
See this https://stackoverflow.com/questions/21584541/does-structure-declaration-occupies-memory — programmer, Feb 15 '21 at 15:59

score 3 · Answer 1 · answered Feb 15 '21 at 16:03

3

It prints 4 because that's how big a union var variable is. The fact that there are no union var variables in your program, is completely irrelevant. If you created one, for example by writing union var myUnionVar;, it would use 4 bytes of memory

You can also do this with structs:

struct list_node {
    struct list_node *next;
    int key;
    int value;
};

// note that sizeof returns a size_t which should be printed with %zu
printf("%zu", sizeof(struct list_node)); // prints 12 or 16, probably

answered Feb 15 '21 at 16:03

user253751

57,427
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Can you please elaborate " It prints 4 because that's how big a union var variable is." – programmer Feb 15 '21 at 16:05
@programmer Do you know what variables are and what structs are? And how unions are kinda like structs but not really? – user253751 Feb 15 '21 at 16:05
Though I know the differences between structures and union, I don't see how it is relevant to this question. Anyways, please tell what particular difference you want me to know – programmer Feb 15 '21 at 16:09
@programmer Do you know what an int variable is? – user253751 Feb 15 '21 at 16:10
@programmer _how big a variable variable is_ = _the size a variable uses in memory_. An `int` typically uses 4 bytes. – Jabberwocky Feb 15 '21 at 16:12
Ah I got it now, union is a user defined data type just like int.. So it would print how many bytes would be occupied by a variable of that type if a variable of that type would be declared, right? – programmer Feb 15 '21 at 16:18
@programmer Yes. – user253751 Feb 15 '21 at 16:22

score 2 · Answer 2 · answered Feb 15 '21 at 16:19

sizeof is an operator that yields the size in bytes of its argument (right-hand-side operand). The operand can either be a type or a variable. Most of the time, the result is a constant that can be evaluated at compile time.

union var is a type, therefore with sizeof(union var) you are asking "what size would occupy a variable if it had the type union var?" The answer is 4 bytes.

Declaration and definition of union

2 Answers2