struct with multiple char arrays problem

Why is output of this code


(i can't use string class)

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

struct S
 char a[10];
 char b[20];

int main()
 struct S* test = (S*)malloc(sizeof(S));

 strcpy(test->a, "1234567890");
 strcpy(test->b, "asdfg");

 printf("%s\n%s", test->a, test->b);

 return 0;


The string you've put in test->a is eleven characters long including the terminating null character: 1234567890\0. When you copy it into a, that null character ends up in the first character of b. You then overwrite it with the string you copy into b, so that in memory you have:

a - - - - - - - - - b - - - - - - - - - - - - - - - - - - -
1 2 3 4 5 6 7 8 9 0 a s d f g \0
        a's terminating null was here.

You then print a (starting from the '1'), and b (starting from the 'a'), producing that output.

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