Valhalla Legends Archive

I wrote some code that acts similarely to recursing over main, except without the messy call stack.  It runs on my x86 slackware 9.1 install, and I won't guarentee it'll work anywhere else:

#include <stdio.h>
int i;
int main(int argc, char **argv)
{
   int buffer[argc];

   printf("%d\t", i++);

   if(i < 100)
       buffer[7] -= 5;

   return 0;
}


*grin*

<edit> sample output:
iago@laptop:~/Projects/C$ gcc test.cpp ; a.out
0       1       2       3       4       5       6       7       8       9       10   
11      12      13      14      15      16      17      18      19      20      
21      22      23      24      25      26      27      28      29      30      
31      32      33      34      35      36      37      38      39      40      
41      42      43      44      45      46      47      48      49      50      
51      52      53      54      55      56      57      58      59      60      
61      62      63      64      65      66      67      68      69      70      
71      72      73      74      75      76      77      78      79      80      
81      82      83      84      85      86      87      88      89      90      
91      92      93      94      95      96      97      98   99      iago@laptop:~/Projects/C$


How does this work iago?  Does it reposition the instruction register so that the code points back to the printf instruction?  Because there is no loop...

Quote from: iago on April 26, 2004, 03:15 PM

Code Select Expand

#include <stio.h>
int i;
int main(int argc, char **argv)
{
   int buffer[argc];

   printf("%d\t", i++);

   if(i < 100)
       buffer[7] -= 5;

   return 0;
}


*grin*

#include <stdio.h>

It's interesting code, but I don't quite understand the point of having argc in "buffer[argc]". To be honest, I don't understand how you can pass a variable as the size argument to an auto array. I was expecting a compilation error there, but it compiled just fine. The program runs ok too, but prints rather much on stderr while running.

Now that you mention it, I don't know either.  I used argc simply to confuse the guy at work I was showing it to

Myndfyre - it repositions its return address to the "call" instruction.

stdio?  :p

Quote from: Mephisto on April 26, 2004, 05:54 PM
stdio?  :p

bah, typo.. I deleted some global variables I stopped using and had to retype the include

Quote from: Adron on April 26, 2004, 04:16 PM
It's interesting code, but I don't quite understand the point of having argc in "buffer[argc]". To be honest, I don't understand how you can pass a variable as the size argument to an auto array. I was expecting a compilation error there, but it compiled just fine. The program runs ok too, but prints rather much on stderr while running.

It does when I compile it (under windows). I don't understand this either!

I thought you could only use a constant value/expression when declaring arrays...At least according to what I was taught you can't use a variable to declare an array in the subscript, because a variables *varies* and the compiler needs to know exactly how much to allocate for the array.

Edit: Apparantly you can declare an array with a variable in the subscript...It makes sense to me too... :p  I never tested it, but should've, so I wrote this very simple program.  


int main() {
   int x = 5;
   int array[x];
   return 0;
}

Quote from: Mephisto on April 27, 2004, 09:13 AM
I thought you could only use a constant value/expression when declaring arrays...

Perhaps argc/argv are special?  I have no idea why it works


Here's something else interesting that I noticed:
int main()
{
       main();
       return 0;
}

This will not compile on Linux.  I get the error:
iago@laptop:~/Projects/C$ gcc -Wall test2.cpp
/tmp/cc8BvbGZ.o(.eh_frame+0x11): undefined reference to `__gxx_personality_v0'
collect2: ld returned 1 exit status
iago@laptop:~/Projects/C$

Quote from: Mephisto on April 27, 2004, 09:13 AM
I thought you could only use a constant value/expression when declaring arrays...At least according to what I was taught you can't use a variable to declare an array in the subscript, because a variables *varies* and the compiler needs to know exactly how much to allocate for the array.

Edit: Apparantly you can declare an array with a variable in the subscript...It makes sense to me too... :p  I never tested it, but should've, so I wrote this very simple program.  

Code Select Expand


int main() {
   int x = 5;
   int array[x];
   return 0;
}

Did you try running that with warnings?

I get this:
iago@laptop:~/Projects/C$ gcc -Wall test2.cpp
test2.cpp: In function `int main()':
test2.cpp:4: warning: unused variable `int array[((x - 1) + 1)]'
iago@laptop:~/Projects/C$

it declares that variable "array" very weirdly :/

<edit>
I tried it on c, not c++, and it still works
iago@laptop:~/Projects/C$ gcc -Wall test.c
test.c: In function `main':
test.c:4: warning: unused variable `array'
iago@laptop:~/Projects/C$

Ouch, my brain

In case somebody's curious, this program:

#include <stdio.h>

int main()
{
       int x;

       printf("What is the value of x?\n");
       scanf("%d", &x);

       int array[x];

       for(int i = 0; i < x; i++)
       {
               array[i] = 0;
       }

       return 0;
}


compiles fine, on Linux, and produces the main function:

main:
.LFB1:
       pushl   %ebp
.LCFI0:
       movl    %esp, %ebp
.LCFI1:
       subl    $24, %esp
.LCFI2:
       andl    $-16, %esp
       movl    $0, %eax
       subl    %eax, %esp
       movl    %esp, -12(%ebp)
       subl    $12, %esp
       pushl   $.LC0
.LCFI3:
       call    printf
       addl    $16, %esp
       subl    $8, %esp
       leal    -4(%ebp), %eax
       pushl   %eax
       pushl   $.LC1
       call    scanf
       addl    $16, %esp
       movl    -4(%ebp), %eax
       decl    %eax
       sall    $2, %eax
       addl    $4, %eax
       addl    $15, %eax
       shrl    $4, %eax
       sall    $4, %eax
       subl    %eax, %esp
       movl    %esp, -16(%ebp)
       movl    $0, -8(%ebp)
.L2:
       movl    -8(%ebp), %eax
       cmpl    -4(%ebp), %eax
       jl      .L5
       jmp     .L3
.L5:
       movl    -8(%ebp), %eax
       movl    -16(%ebp), %edx
       movl    $0, (%edx,%eax,4)
       leal    -8(%ebp), %eax
       incl    (%eax)
       jmp     .L2
.L3:
       movl    -12(%ebp), %esp
       movl    $0, %eax
       leave
       ret


I don't have time right now to figure out how this is working, but feel free

Btw, has anyone tried compiling with ANSI-compliance enabled?
I'm pretty sure this is a GCC extension.
Edit: I checked (Comeau) and it's standard C99. Interesting...

As for how it works...

Quote from: iago on April 27, 2004, 09:28 AM

Code Select Expand

       subl    %eax, %esp


Quote from: Yoni on April 27, 2004, 10:10 AM
Btw, has anyone tried compiling with ANSI-compliance enabled?
I'm pretty sure this is a GCC extension.
Edit: I checked (Comeau) and it's standard C99. Interesting...

As for how it works...

Quote from: iago on April 27, 2004, 09:28 AM

Code Select Expand

       subl    %eax, %esp


Wow,that's smart, it subtracts the variable from esp  *impressed*

iago@laptop:~/Projects/C$ gcc -ansi -fno-nonansi-builtins -Wall test2.cpp
iago@laptop:~/Projects/C$

works fine

<edit> I think -ansi means standard C89.

I had no idea this was allowed. hmm.

Well, I copied your code straight over:


error C2466: cannot allocate an array of constant size 0
error C2057: expected constant expression
error C2133: 'buffer' : unknown size


Although I think I see how it works now.. Kind've intentionally overflowing the stack - clever

Quote from: Arta[vL] on April 27, 2004, 02:21 PM
Well, I copied your code straight over:

Code Select Expand


error C2466: cannot allocate an array of constant size 0
error C2057: expected constant expression
error C2133: 'buffer' : unknown size


Although I think I see how it works now.. Kind've intentionally overflowing the stack - clever

Try making my array a constant size of "1"