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.
