How to use scanf for a char array input?

Question

I set a char array of size of 10 and want to check the real size of the input permitted.

I tested

123456789; 1234567890; 123456789123456789

Interestingly, all of them passed and got the right output which are

123456789; 1234567890; 123456789123456789

It confused me a lot because I thought the last two are wrong input.

Does that make sense or is it a compiler difference?

This is the code

#include <stdio.h>
main()    
{    
char input[10];    
scanf("%s", input);    
printf(input);    
}  '

Naltroc · Accepted Answer · 2016-07-15 20:59:43Z

6

The scanf() with format specifier %s scans the input until a space is encountered. In your case, what you are seeing is undefined behavior. Your array can hold 10 chars, but you are writing out of its boundaries.

While you are getting an expected answer now, this is not always guarnateed and may instead cause a crash.

It is advisable to use a function such as fgets() takes care of buffer overflow.

edited Jul 15, 2016 at 20:59

Naltroc

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answered Feb 6, 2015 at 12:50

Gopi

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1 Comment

GermanNerd Over a year ago

With input that is long enough, you are guaranteed to cause a crash. The reason is that with increasing input, the individual bytes get written to higher memory addresses. In most machines today, the stack grows downwards. Therefore your buffer overrun will destroy the stack above main(), and when main() terminates, all sort of hell will happen. If you have a stack protector, it will detect it; otherwise, you'll likely create a segfault. But even worse tthings could happen. This a preferred route for malware attacks.

Ahmad Yoosofan · Accepted Answer · 2019-01-02 06:36:03Z

0

You can use

scanf("%9s", input);

answered Jan 2, 2019 at 6:36

Ahmad Yoosofan

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