hi,
I'm actually writing a program in C and i'm fighting with an address
dereferencing since some days now.
The program run on a non *n*x system and I can't produce the error on
my linux system so i can only describe it... sorry.
In my system depending if i (un)comment the commented lines in the
next code, the 1st or the 2nd string is printed.
uncommented -> pointers are the same
commented -> pointers aren't the same
I'm compiling with (to run on my VM):
-std=gnu99 -Wall -Werror -nostdinc -Wstrict-aliasing=2 -fno-builtin
-I../include -I../
and (on linux):
-std=gnu99 -Wall -Werror -Wstrict-aliasing=2
ggc version : gcc (GCC) 3.4.6
GNU assembler 2.16.1
the linkage is a bit different too as my binary is neither ELF nor a.out ...
The asm inline part was a way to escape the problem as the compiled
asm wasn't working...
But after that "patch" an incrementation of "ap" : ap++; after the
assignation wasn't working and in the asm code the increment wasn't on
the good (register+offset)/address so ...
the code :
int the_bug(const char *fmt, ...)
{
char **ap;
char **s = (char **)(&fmt);
/* __asm__ volatile ( */
/* "\tleal %1, %%eax\n" */
/* "\tmovl %%eax,%0\n" */
/* "\tmovl %1, %%eax\n" */
/* "\tmovl %2, %%ebx \n" */
/* "\tmovl %%eax, (%%ebx)\n" */
/* : "=m"(ap) : "m" (fmt), "m"(ap)); */
if (*s == (char *)fmt)
write(1, "they are the same\n", 18);
else
write(1, "they aren't the same\n", 21);
return (0);
}
the ASM generated (uncommented and commented)
(as shown in biew (->objdump doesn't work because of the "not ELF format")) :
push bp | push
bp
mov bp, sp | mov
bp, sp
sub (w) sp, +08 | sub (w)
sp, +08
lea ax, [di+08] | lea
ax, [di+08]
mov [di-08], ax | mov
[di-08], ax
lea ax, [di+08] |
mov [di-04], ax |
mov ax, [di+08] |
mov bx, [di-04] |
mov [bp+di], ax |
mov ax, [di-08] | mov
ax, [di-08]
mov ax, [bx+si] | mov
ax, [bx+si]
cmp ax, [di+08] | cmp
ax, [di+08]
jne file:000029B4 DIFF | jne
file:000029A6
sub (w) sp, +04 | sub (w)
sp, +04
push (w) +0D | push (w) +0D
push (w) 3AA8 | push (w) 3AA8
add [bx+si], al | add
[bx+si], al
push (w) 01 | push (w)
01
calln file:0000295D | calln
file:0000295D
??? (w) di | ???
(w) di
add (w) sp, +10 | add (w)
sp, +10
jmps file:000029C8 DIFF | jmps
file:000029BA
sub (w) sp, +04 | sub (w)
sp, +04
push (w) +10 | push (w)
+10
push (w) 3AB6 | push (w) 3AB6
add [bx+si], al | add
[bx+si], al
push (w) 01 | push
(w) 01
calln file:0000295D | calln
file:0000295D
??? (w) di | ???
(w) di
add (w) sp, +10 | add (w)
sp, +10
mov ax, 0000 | mov
ax, 0000
add [bx+si], al | add
[bx+si], al
leave | leave
retn | retn
If anyone has an idea... to reproduce the error, ..... or (better :))
correct it...
--
Thibaud
