sth about excutable string?

[Prissy]

#include <stdio.h>
int main(int a)
{
char* x = "bB_Z#^B_Z#h1AI.BMB5#VB2>2:B>>=6#RB@1.>>L12#NB6N:>BN#JFHL1@D6#";
while(a = *x / 4) a -= 8, printf("\n%*s" + !!a, a, "_/_/_/" + *x++ % 4 * 2);
getchar();
return 0;
}

OK,It's really a funny program,but why? Why no errs? How does it work?
Who can explain?
Thanks.

[jsmith]

The string is not executable. The program uses the ASCII value of each character in the following way.

First note that x is incremented by one every printf(), and we only ever look at the character pointed to by x. Which means that we look at each character in the string.

a = *x / 4. So a is given the value of the ASCII value of the character divided by 4. The loop ends when *x/4 == 0, which is only the case when *x < 4. Since the string is NULL terminated, the NULL character at the end stops the loop.

Note the comma operator a -= 8, ...

Comma is just a way to do two things on one line of code. Replace the comma with a semicolon and add braces around the two lines of the while loop and the program is unchanged.

First subtract 8 from a.

printf( "\n%*s" + !!a )

Neat. !!a is a programming trick used to convert any nonzero value to 1 and zero to zero. "\n%*s" is a character pointer. One can add to pointers; if a is now 1, adding 1 to the character pointer yields "%*s" -- a nice way to not print out the \n every time.

The same trick is used on the last part of the printf:

printf( "_/_/_/" + *x++ % 4 * 2 );

Again, we add either 0, 2, 4, or 6 to the char* string to either print out one set of _/, two, three, or no sets. (Note the * 2 is there because _/ is 2 characters).

EDIT:

The one other detail I omitted was use of the %*s in the format string.
%10s, for example, says to use a field width of 10 to print the string.
%*s says to use the next argument as the field width. The code passes
in a. This is how the program manages to print blanks.

Very nice obfuscated program. The char* string encodes three pieces of information in each byte: 1) whether or not to print a newline; 2) the field width; 3) the number of _/ to print.

[jsmith]

This does the same thing (not very cleanly):

#include <iostream>
#include <string>

using namespace std;

int main()
{
    char* x = "bB_Z#^B_Z#h1AI.BMB5#VB2>2:B>>=6#RB@1.>>L12#NB6N:>BN#JFHL1@D6#";
    for( int a; a = *x / 4; ++x ) {
        a -= 8;
        if( a == 0 )
            cout << endl;
        else if( a > 0 )
        {
            int numChars = 6 - 2 * ( *x % 4 );
            int numSpaces = a - numChars;

            if( numSpaces > 0 )
                cout << string( numSpaces, ' ' );
            for( ; numChars > 0; numChars -= 2 ) 
                 cout << "_/";
        }
    }
}

[Prissy]

Got it .
Thank u so much~