string run time error working only with getchar why?

Question

i am using visual studio 2013 for desktop and i have run time fail with the string i don't know why this isn't working.

    int main(void)
    {
         char string[MAX1] = "THANK ";
         char you[MAX2] = "u";
         strncat(string, you, 1); //without fix this line or the next one
         printMessage(string); 
         return 0;
    }

it is running only when i am putting getchar at the end. it is working with other programs like notepad++.

MAX1=7. MAX2=2.

@SouravGhosh - Now, now. It could be a global object with a field that is a function pointer, can't it :P — StoryTeller - Unslander Monica, Mar 13 '17 at 09:34
Possible duplicate of [Why is this simple strcat crashing at runtime?](http://stackoverflow.com/questions/27872079/why-is-this-simple-strcat-crashing-at-runtime) — StoryTeller - Unslander Monica, Mar 13 '17 at 09:36

score 0 · Answer 1 · answered Mar 14 '17 at 11:26

the concatenated contents of string[] (if it worked) would be THANK u'\0' I.E. 8 characters.

however, only 7 characters were allocated.

The call to strcat() will not stop, so writes beyond the bounds of the array string[]. The result is undefined behavior and can lead to a seg fault event.

However, when using the strncat() the trailing NUL byte will not be copied. so string[] is now a unterminated char array. and the call to printMessage() will not know where to stop printing characters, Again undefined behavior.

string run time error working only with getchar why?

1 Answers1