sizeof union in C

Question

The sizeof a union is equal to the size of the largest member in it. But in the following union, the size is being shown to be 8 Bytes. Size of int and float is 4 Bytes. Why does it show 8 Bytes?

union data {
    int i;
    float f;
    char *str;
};

Maybe try printing the `sizeof` each of the elements separately as one of them is obviously 8 bytes — Chris Turner, Jul 10 '17 at 15:57
I have X, Y and Z. This is causing W. I've checked X, Y and they are not the cause of W. What could cause W? — Eugene Sh., Jul 10 '17 at 15:59
*Size of int and float is 4 Bytes.* How did you determine this? And what about the `char *`? — lurker, Jul 10 '17 at 16:00
@JonnySchubert Its `union`, not a `struct`. How would packing affect a `union`? — chux - Reinstate Monica, Jul 10 '17 at 16:03
@SergioFormiggini It has a pretty loose relation to the CPU bitness. — Eugene Sh., Jul 10 '17 at 16:05
What kind of machine architecture and compiler are you using — Jonny Schubert, Jul 10 '17 at 16:06
@EugeneSh. On my I3 with Linux 64 the pointers are 8 byte length. On the other hand is very easy to verify what the compiler generates. I'm sure that the result is due to the pointer length! — Sir Jo Black, Jul 10 '17 at 16:09
minus for unbelievable laziness. You wrote this silly post, wasted our time and you did not check what your pointer size is. — 0___________, Jul 10 '17 at 16:10
@SergioFormiggini What is happening is not related to what *might* happen. Moreover, sizes to different types *on the same machine* are not guaranteed to be the same size. https://stackoverflow.com/questions/1241205/are-all-data-pointers-the-same-size-in-one-platform-for-all-data-types — Eugene Sh., Jul 10 '17 at 16:13
@Eugene Sh. Another useless discussion. Ridiculous topic .Graet companion — 0___________, Jul 10 '17 at 16:16
If `int` and `float` are 4 bytes wide on your system, but the size of your union is 8 bytes wide, then it's pretty clear that on your system `char *` is 8 bytes wide. — John Bode, Jul 10 '17 at 16:25

score 6 · Answer 1 · edited Jul 10 '17 at 17:29

6

sizeof(char *) is the size of the pointer. It's usually 4 for 32-bit machine, and 8 for 64-bit machine. Given your result, I take it you're on a 64-bit machine.

edited Jul 10 '17 at 17:29

Carcigenicate

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answered Jul 10 '17 at 16:05

Daniel Almeida

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klutt · Answer 2 · 2017-07-10T16:39:42.740

6

#include <stdio.h>

union data_t {
  int i;
  float f;
  char *str;
};

int main()
{
  union data_t data;
  printf("%zu\n%zu\n%zu\n", sizeof (data.i), sizeof (data.f), sizeof (data.str));
  printf("\nsizeof(char*): %zu\nsizeof(char): %zu\n", sizeof(char *), sizeof(char));
}

Output:

$ ./a.out 
4
4
8

sizeof(char*): 8
sizeof(char): 1

edited Jul 10 '17 at 16:39

answered Jul 10 '17 at 16:06

klutt

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@chux Fixed it. – klutt Jul 10 '17 at 16:40

yash darak · Answer 3 · 2017-07-19T20:21:50.513

Size of union = size of the largest data type used

The memory occupied by a union will be large enough to hold the largest member of the union. It doesn't matter what is currently in use. For example,

union Data 
 {
   int i;
   float f;
   char str[20];
 } data;

Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. It means a single variable, i.e., same memory location, can be used to store multiple types of data. Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by a character string.

score 2 · Answer 4 · answered Jul 10 '17 at 16:07

2

Try the code below on a Windows 64/Linux 64 and the result should be 8, on a 32 bit system should be 4.

int main(void)
{
    char * str;

    printf("%d\n",sizeof(str));

    return 0;

}

answered Jul 10 '17 at 16:07

Sir Jo Black

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sizeof union in C

4 Answers4