C语言scanf复制多余的字符串

Question

#include <stdio.h>
#include <string.h>
int main()
{
    char x[5];
    char y[10];
    scanf("%s",x);
    scanf("%s",y);
    printf("%s %s",x,y);
}

I typed "hello world" in the terminal, but the printf result is "helloworld world". Is there something wrong I'm doing?

Answer 1

When you declare

char x[5];
char y[10];

In memory, you have something like:

                     1 1 1 1 1 
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 
+---------+-------------------+
|. . . . .|. . . . . . . . . .| 
+---------+-------------------+
 ^         ^
 \-- x     \-- y

After the first scanf , you write hello\\0 in x

                     1 1 1 1 1 
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 
+---------+-------------------+
|h e l l o|0 . . . . . . . . .| 
+---------+-------------------+

After the second scanf , you write world\\0 in y

                     1 1 1 1 1 
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 
+---------+-------------------+
|h e l l o|w o r l d 0 . . . .| 
+---------+-------------------+

When you type printf("%s", x); you ask to type to write from x to the first \\0 , so "helloworld" is printed.

When you type printf("%s", y); you ask to type to write from y to the first \\0 , so "world" is printed.

Note:

The memory arrangement is not deterministic, you should have all kind of other results, even crashes...

You can find a good tutorial to use scanf here: http://sekrit.de/webdocs/c/beginners-guide-away-from-scanf.html

Answer 2

Never, never, really never use the "%s" format specifier without a WIDTH to tell scanf() how many characters it may put into the memory pointed to by the argument.

char str[5];
scanf("%4s", str);

Answer 3

Strings in C are terminated by a null byte. So the string "hello" actually needs 6 bytes to store: 5 for the letters in "hello" and one for the null byte.

The variable x is only 5 bytes wide, so it's not big enough to store the string "hello". As a result, attempting to write this string to x writes past the end of the array. This invokes undefined behavior , which in this case manifests as writing to an adjacent variable.

Increase the size of x to prevent overrunning the array. You should also use a field width in the format specifier to specify the maximum number of characters to read:

char x[6];
char y[10];
scanf("%5s",x);
scanf("%9s",y);