-1

I tried to run this code in C and expected runtime error but actually it ran without errors. Can you tell me the reason of why this happens?

char str[10];
scanf("%s",str);
printf("%s",str);

As I initialized the size of array as 10, how can code prints string of more than 10 letters?

Andrew Li
  • 55,805
  • 14
  • 125
  • 143
Saniket
  • 47
  • 1
  • 3
    Undefined behavior. FYI: in your `str` is enough space for __9__ characters. – tkausl Aug 28 '16 at 06:10
  • Possible duplicate of [How to read string from keyboard using C](http://stackoverflow.com/questions/7709452/how-to-read-string-from-keyboard-using-c) – Yogesh Rathi Aug 28 '16 at 06:12
  • scanf fuction read an arbitrary length of string,so your code has an undefined behavior. – llinvokerl Aug 28 '16 at 06:23

3 Answers3

6

As soon as you read or write from an array outside of its bounds, you invoke undefined behavior.

Whenever this happens, the program may do anything it wants. It is even allowed to play you a birthday song although it's not your birthday, or to transfer money from your bank account. Or it may crash, or delete files, or just pretend nothing bad happened.

In your case, it did the latter, but it is in no way guaranteed.

To learn further details about this phenomenon, read something about exploiting buffer overflows, this is a large topic.

Roland Illig
  • 40,703
  • 10
  • 88
  • 121
1

C doesn't perform any bounds checking on the array. This can lead to buffer overflows attack on your executable.

The bound checking should be done at the user end to make it anti-buffer overflow.

Instead of typing in magic numbers when taking input from fgets in an array, always use the sizeof(array) - 1 operator on the array to take in that much, -1 for leaving a space for '\0' character.

Dark Innocence
  • 1,389
  • 9
  • 17
0

This is a good question. And the answer
is that it there is indeed a memory problem
The string is read and stored from the address of str
up until the length of the actual read string demands,
and it exceeds the place you allocated for it.

Now, it may be not crash immediately, or even ever for
short programs, but it's very likely that when you expand
the program, and define other variables, this string will
overrun them, creating weird bugs of all kinds, and it may
eventually also crash.

In short, this is a real error, but it's not uncommon to have
memory bugs like this one which do not affect at first, but
do create bugs or crash the program later.

Israel Unterman
  • 13,158
  • 4
  • 28
  • 35