Unexpected output in C code

Question

The following code prints AA:

#include <stdio.h>
int main()
{
    for(int i;i;i--)
        printf("A");
    return 0;
}

Why the initial value of variable i is 2, and not some garbage value?

Is the lifetime of variable i static or automatic?

This program exhibits undefined behavior for using the value of an object with **automatic storage duration** while it is uninitialized. — EOF, Jul 12 '16 at 14:18
@Coder With UB, any value is candidate garbage for an `int`. (including a trap) — chux - Reinstate Monica, Jul 12 '16 at 14:19
@chux Garbage values appear randomly. But if you run this code multiple times, initial value of a variable will always be 2. How? — user300046, Jul 12 '16 at 14:21
"Garbage values appear randomly" is not true. Undefined behavior is Undefined behavior. "run this code multiple times" is hardly _always_. — chux - Reinstate Monica, Jul 12 '16 at 14:23
@Coder Going for [top billing](https://m.reddit.com/r/todayilearned/comments/22rl06/til_in_vaudeville_the_straight_mans_name_usually/)? — chux - Reinstate Monica, Jul 12 '16 at 14:25
If you have gdb, try this: `start` the program. Type `watch -l i` to set a hardware watchpoint at `i`'s location. Type `run` to restart the program. Look at all the points where the functions called before main touch that location. — Mark Plotnick, Jul 12 '16 at 14:54

score 2 · Answer 1 · edited Jul 12 '16 at 14:23

2

Apparently the variable i is not initialized. This means that the behavior of the implementation is undefined. And value of i is garbage value only. Here it is 2.

edited Jul 12 '16 at 14:23

VatsalSura

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answered Jul 12 '16 at 14:17

Codor

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@EugeneSh. Thanks for the edit. – Codor Jul 12 '16 at 14:24

score 1 · Accepted Answer · answered Jul 12 '16 at 14:26

1

The scope of the variable i is the entire for-statement (including its body). It has the automatic storage duration and will be destroyed after exiting the loop. You may not declare it as having the static storage duration as for example

for(static int i;i;i--)
    printf("A");

As the variable i was not initialized it has an indetermined value that can be a trap value.

answered Jul 12 '16 at 14:26

Vlad from Moscow

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If it is a trap value , would it still cause UB ? – ameyCU Jul 12 '16 at 14:29
@ameyCU Yes it can result in exception. – Vlad from Moscow Jul 12 '16 at 14:31

Unexpected output in C code

2 Answers2