Let's global!

Today, I and Zektonic were working on a program to be exploited by us only. We incorporated a few techniques to make the experience tough, even for us. The details will come out after the assignment is due.

However, during studying the variable allocation. We discovered something very eerie. Look at the two tables below, generated by a very similar program, taking note of the location of the variables of the right table ( in1 , in3 , in4 ) :

gcc version 4.1.2 20061020 (prerelease) (Debian 4.1.1-17) Kernel 2.6.8-2-386	gcc version 3.2.2 20030222 (Red Hat Linux 3.2.2-5) Kernel 2.4.20-8smp
int main(){ int a; int b[10]; int c[10]; char d; char e[10]; char f; int g; printf("%p\n",&a); printf("%p\n",&b); printf("%p\n",&c); printf("%p\n",&d); printf("%p\n",&e); printf("%p\n",&f); printf("%p\n",&g); }	int main(){ int in1; int in2; char b1[16]; int in3; char b2[16]; int in4; char b3[16]; printf("in1 %p\n",in1); printf("in2 %p\n",in2); printf("in3 %p\n",in3); printf("in4 %p\n",in4); printf("b1 %p\n",b1); printf("b2 %p\n",b2); printf("b3 %p\n",b3); }
0xbffff890 0xbffff868 0xbffff840 0xbffff83f 0xbffff835 0xbffff834 0xbffff830	in1 0x80483ce in2 0xbfffed28 in3 0x80482a6 in4 0x40015a38 b1 0xbfffed00 b2 0xbfffece0 b3 0xbfffecc0

Follow ups ( 2007/1/30 00:51 ) :
After re-entry into the system, I reviewed the source code. As seen above, the difference turns out to be the reference operator ( & , the ampersand ).

If you do not understand, look at this modified program and it's output.

Source :
int main(){
int in1;
int in2;
char b1[16];
int in3;
char b2[16];
int in4;
char b3[16];
in1 = 5;
printf("in1 %d %p\n",in1,in1);
printf("in2 %d %p\n",in2,in2);
printf("in3 %d %p\n",in3,in3);
printf("in4 %d %p\n",in4,in4);
printf("b1 %d %p\n",b1,b1);
printf("b2 %d %p\n",b2,b2);
printf("b3 %d %p\n",b3,b3);
}

Output :
in1 5 0x5
in2 -1073745912 0xbffff008
in3 134513318 0x80482a6
in4 1073830456 0x40015a38
b1 -1073745952 0xbfffefe0
b2 -1073745984 0xbfffefc0
b3 -1073746016 0xbfffefa0


Do the values in the output look obvious? They are exactly uninitialized variables you often see in C programs.