Why is the last output "eU2"?

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

void main()
{
    char str1[] = "See the stone set in your eyes";
    char str2[] = "U2";
    char* ptr;
    ptr = &str1[3];//the stone...
    printf("%d\n", str1 - ptr); // -3
    ptr[-1] = 0;// del s
    ptr = (  ptr) 1;  
    printf("%s\n", ptr); // he stone set in your eyes
    strcpy(ptr, str1 1); // ee the stone set in your eyes
    strcat(ptr-2, str2); 
    printf("%s\n", ptr);
}

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CodePudding user response：

Based on Steve Summit's comments, I replaced ptr = ( ptr) 1, which leads to undefined behavior, with two repetitions of ptr .

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

int main(void)
{
    char str1[] = "See the stone set in your eyes";
    char str2[] = "U2";
    char* ptr;
    ptr = &str1[3];// points at the space before "the stone..." 
    printf("%ld\n", str1 - ptr); // -3
    ptr[-1] = 0; // replaces the second e of see with 0 in str1
    ptr  ;
    ptr  ; // points to the h of "the stone..." 
    printf("%s\n", ptr); // he stone set in your eyes
    strcpy(ptr, str1 1); // ptr points to "e\0 the stone set in your eyes"
    strcat(ptr-2, str2); // concatenates the string "e\0" with "U2\0"
    printf("%s\n", ptr); // prints eU2
}