string behaviour in C

Question

I am reading a C course (it is dutch so probably you won't know) and there is a small exercise to understand string behaviour. Therefor i created a small C program to start the exercise but already the first output of my program is (for me) astonishing.

Source of my C program :

#include <string.h>
#include <stdio.h>

void printString(char *string)
{
    printf("0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19\n");
    printf("%c ",string[0]);
    printf("%c ",string[1]);
    printf("%c ",string[2]);
    printf("%c ",string[3]);
    printf("%c ",string[4]);
    printf("%c ",string[5]);
    printf("%c ",string[6]);
    printf("%c ",string[7]);
    printf("%c ",string[8]);
    printf("%c ",string[9]);
    printf("%c  ",string[10]);
    printf("%c  ",string[11]);
    printf("%c  ",string[12]);
    printf("%c  ",string[13]);
    printf("%c  ",string[14]);
    printf("%c  ",string[15]);
    printf("%c  ",string[16]);
    printf("%d  ",string[17]);
    printf("%d  ",string[18]);
    printf("%d\n",string[19]);
}

void main(){

    char str[20];

    strcpy(str,"Dag grootmoeder!");
    printString(str);
}

I compiled with gcc (no special switches) and ran the program several times : (For the English speaking people Dag grootmoeder! == Hi grandma!)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

D a g   g r o o t m o  e  d  e  r  !    94  -90  111

$./oefString 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

D a g   g r o o t m o  e  d  e  r  !    51  -12  96

$./oefString 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

D a g   g r o o t m o  e  d  e  r  !    -17  -117  28

$./oefString 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

D a g   g r o o t m o  e  d  e  r  !    96  15  -28

$./oefString 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

D a g   g r o o t m o  e  d  e  r  !    -20  -46  -18

$./oefString 

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

D a g   g r o o t m o  e  d  e  r  !    68  -75  58

Here is the question :

1) Why do I get rubbish values for the last 3 indexes of str ? At first I was also printf()'ing them with %c and noticed the chars changed, that is why I used %d thereafter to display the integer values.

2) Why do these values change? I do nothing more then copying the same string using strcpy() into str.

Thx for taking time to read and even more thanx for those who respond !

Jorn

Answer 1

You're simply acessing the memory past the end of the string. You didn't fill it so you can find anything there - so that's why on each run you find something else.

In C "strings" are actually 0-terminated arrays (or pointers to memory). So if you print the characters using "%d" you will notice the very last element, right after !, is 0.

If you want it to be predictable, you can initialize your string before using it:

memset(str, 0, sizeof(str));

or

char str[20] = {0,};

As a side note, it's int main, not void main.