Union Assigned Values in C

Question

Given the following union definition:

typedef union{
int i;
char ch;
float f;}record;



record a;
//a.i = 10;
a.ch = 'A';
//a.f = 10.56;
printf("printing a.i: %p \n", a.i);
printf("printing a.ch: %c \n", a.ch);
printf("printing a.f: %f \n", a.f);
return 0;

I get the following output:

printing a.i: 65
printing a.ch: A
printing a.f: 0.000000

Why does a.i not print 0 (the default value for undefined integers) but instead the ASCII value for 'A'. Does this somehow have access to a.ch??

All members of a union *share* memory. Because of this, a union can be used for [*type- punning*](https://en.wikipedia.org/wiki/Type_punning) where you pretend one type is another. — Some programmer dude, Feb 10 '18 at 20:30
It should also be noted that the result of this depends on the system [*endianness*](https://en.wikipedia.org/wiki/Endianness), and how the compiler initializes (or not) the bytes that are not explicitly initialized. — Some programmer dude, Feb 10 '18 at 20:33
`%p` is not the correct format for an `int`, your compiler should have told you. (And please indent code correctly before posting.) — Jens Gustedt, Feb 10 '18 at 20:50

score 2 · Answer 1 · edited Feb 10 '18 at 20:39

2

You get 65, the code of 'A', because field i shares a portion of its space with the char field.

Overall, though, your program’s behavior with a local union is undefined, because you read the entire int after writing only a portion of it that overlaps with the char.

Initializing the union would fix the problem. However, there is no guarantee that the read would give you 65 in the lower byte. This behavior is implementation-specific.

edited Feb 10 '18 at 20:39

Jean-François Fabre

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answered Feb 10 '18 at 20:35

Sergey Kalinichenko

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65 is the code of `A` not `a`. – Jean-François Fabre Feb 10 '18 at 20:38
@Jean-FrançoisFabre Right. I hate typing answers on an iPhone... – Sergey Kalinichenko Feb 10 '18 at 20:39
1

not bad for a phone answer! – Jean-François Fabre Feb 10 '18 at 20:39

Union Assigned Values in C

1 Answers1