C : integer variables randomly change values

Question

I just started learning the C language. I have a good history with C# and Java though.

#include <stdio.h>     
#include <stdlib.h>
#include "info.h"
int main()
{
    int day = 24, month = 3, year = 2016;

    char name[] = "Ahmad\0";
    strcpy(name, "Ahmad(strcpy-ed string)\0"); // <-- LINE 8

    printf("%s made this program on %d-%d-%d\n", name, day, month, year);

    return 0;
}

As you can see the values have been assigned to day, month and year. But the problem is the output has competely different values. The output is this

Ahmad(strcpy-ed string) made this program on 1920234272-1684352377-1885565556

What's more interesting is if i delete line 8 it works properly. Why is this happening?

When you copy something larger than the container's capacity, shit happens. — user3528438, Mar 24 '16 at 18:11
This is an overflow... `Ahmad\0` (6 positions array), and `"Ahmad(strcpy-ed string)\0"` (more than 6, I am lazy to count it) being written over it — Bonatti, Mar 24 '16 at 18:15
Making sure by reading the documentation is better than "having thought". At least once you encounter a problem it is high time. — too honest for this site, Mar 24 '16 at 18:39

score 1 · Accepted Answer · answered Mar 24 '16 at 18:12

You are copying more bytes into name[] than are allocated for it-- C does not stop you from doing this. The extra bytes are overwriting other things, in this case your other variables. You are creating undefined behavior, which is a very bad thing in C programs.

score 0 · Answer 2 · answered Mar 24 '16 at 18:11

0

name only has place for 8 characters, then you write into other memory, probably your other variables.

answered Mar 24 '16 at 18:11

Jens Gustedt

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C : integer variables randomly change values

2 Answers2