I was trying to get input from user but when i set a variable for it it also adds it self to another variable

Code

#include <stdio.h>

int main()
{

    printf("Enter String: ");
    char a[10];
    scanf("%s", a);

    char t[8] = "00:00:00";
    printf("%s\n", t);

}

Terminal

enter string: asd
00:00:00asd

CodePudding user response：

Because "00:00:00" needs 9 space (including the terminating '\0' ) and you only give it 8.

CodePudding user response：

Your computer stores the variables on the stack. All variables are added to stack but the stack grows to the lowest addresses.

Until the scanf, the stack lookes like:

Variable a:

The scanf function reads in a line and terminates it with a '0' character which equals 0. That's why we have a 0 in cell -6 The upper addresses are not used because your input line has only three characters and the terminating 0.

Now comes the variable t:

As you can see, we get a contigous block of memory starting from -17 to -6 with the the string "00:00:00asd" and a terminating 0.

This block is being printed 00:00:00asd. This issue is called buffer overflow or especially string overflow.

You can fix it with this modified code but you will have similiar issues if you enter a line during during scanf that is longer than 9 characters.

#include <stdio.h>

int main()
{

    printf("Enter String: ");
    char a[10];
    scanf("%s", a);

    char t[9] = "00:00:00";
    printf("%s\n", t);

}

florian@florian-desktop:~$ ./a.out 
Enter String: asdfasdfasd
00:00:00
*** stack smashing detected ***: terminated

Instead you could use fgets: How to read from stdin with fgets()? (Steve Emmersons answer)