query on variable setting in unions

[lalits]

Write your question here.
Setting value for id.d[0],id.d[1]....id.d[7] via sscanf,
since its union other member which is a struct (len and s[]) should have garbage value but in below program it is set correctly.

Need to understand how id.p.len and id.p.s[] are correctly set

#include<stdio.h> 
int main(void) 
{ 
   int i;
   char *p;
   union
   {
     struct
     {
        char len;  char s[31];
     } p;
     int d[8];
   } id;
   memset(&id, 0, sizeof(id));
   p = "186519317 253100044 236521544";
 sscanf(p, "%d%d%d%d%d%d%d%d", &id.d[0], &id.d[1],&id.d[2], &id.d[3], &id.d[4], &id.d[5],&id.d[6], &id.d[7]);

for (i = id.p.len-1; i >= 0; i--)
   {id.p.s[i] = id.p.s[i] ^ ( id.p.s[i+1] ^ (i * 3)); 
   } 
   printf("Value of id.p.s: %s\n",id.p.s);
} 

[dhayden]

but in below program it is set correctly.

What is the "correct" value? When I run this (after fixing compilation errors and warnings) I get

Value of id.p.s: XWJGBNWJ470

on a little-endian host and

Value of id.p.s: SMARTNA_DEV

on a big-endian host.

I'm guessing that the big-endian version is what you're looking for. It works because the compiler just happened to lay out the union in the way that the code expects.

[lalits]

yes the second one is the one I m getting and is correct but how id.p.len in for loop gets value , I don't get why it works

[dhayden]

The important thing to realize is that it works by accident. This isn't portable code and it relies on undefined behavior.

Converting the numbers to hex you get 0b1e0f15 0f16000c 0e190848.
If the union is laid out with p.len occupying the same location as the first byte of d[0], then that value is 0xb (11 decimal), which is what it ends up being on a big-endian machine .

[lalits]

Sorry for asking dump question aagain
To what I understand is
-union id in above example will have size 32 bytes (256 bits).
-when we assign id.d[0] = 186519317
then underlying storage will be binary as shown below
001100010011100000110110001101010011000100111001001100110011000100110111(rest 0 ahead to fill upto 256)
-Since its union if I try to access variable len it shd take first 1 byte (8 bit) which is 00110001
-00110001 is 49 but in above eg getting 11 (which is correct :) )

Any reason to consider hexadecimal format ? also is my above understanding incorrect.

Thanks a lot for your help and sorry if my question is to dumb

[dhayden]

We're getting different values for the binary value. As I said, I get get 0b(hex) for the first byte, which is what you're seeing. How do you get

00110001001110000011011000

... for the binary of 186519317?

[lalits]

Actully i converted online website will check again ,
I tried to make reverse of the program meaning from text SMARTNA_DEV want to get the string P i hardcoded.
But xor part could not fig out how we can get.

[lalits]

Need help how we can get the original string back from the final string
Below is the decryption part code I wrote but since don't know how to put hexadecimal value to a variable , I am unot getting original string back

#include<stdio.h> 
#include<string.h>
int main(void) 
{ 
   int i;
   char *q;     /* pointer variable declaration */
   int a;
  
   union
   {
     struct
     {
        char len;  char s[31];
     } p;      //1 byte;31 byte = 32 byte
     int d[8]; //4 bytes * 8 = 32
    
   } id;
  
   memset(&id, 0, sizeof(id));
   
   //Encrypting Part
   q = "186519317 253100044 236521544"; /* store address of var in pointer variable*/
   sscanf(q, "%d%d%d%d%d%d%d%d", &id.d[0], &id.d[1],&id.d[2], &id.d[3], &id.d[4], &id.d[5],&id.d[6], &id.d[7]);      
   for (i = id.p.len-1; i >= 0; i--)
      id.p.s[i] = id.p.s[i] ^ ( id.p.s[i+1] ^ (i * 3)); 
   
    
   //DECRYPTING part
   char *source = "SMARTNA_DEV";
 
   int m;
   char rs[31];
   for (m = strlen(source)-1; m >=0 ; m--)
      rs[m] = ((m*3) ^ source[m+1]) ^ source[m];      
  
} 

[dhayden]

You have the right idea but
- clear id
- put the result back into id
- don't forget to set the length:

    int m;
    memset(&id, 0, sizeof(id));
    id.p.len = strlen(source);
    for (m = strlen(source) - 1; m >= 0; m--) {
        id.p.s[m] = ((m * 3) ^ source[m + 1]) ^ source[m];
    }
    cout << "encrypted numbers:";
    for (int i=0; i<8; ++i) {
        cout << ' ' << id.d[i];
    }
    cout << '\n';

[lalits]

Wow , you made it so simple .
:( , Don't know why I couold not get it thanks a ton.