Hello,
I think there is a little problem under gcc ! It seems that gcc thinks
that a code block will never be reach and so do not generate its code,
but it's wrong :
-- test.c START --
void foo_printf(char *str)
{
}
int foo()
{
//assembly for sys_clone()
asm volatile (
"xor %ebx, %ebx \n"
"xor %ecx, %ecx \n"
"xor %edx, %edx \n"
"mov $120, %eax \n"
"int $0x80 \n"
"test %eax, %eax \n"
"jnz father \n"
);
asm volatile ("child:");
foo_printf("in child\n");
goto __end;
asm volatile ("father:");
foo_printf("in father\n");
__end:
return 0;
}
int main()
{
foo();
return 0;
}
-- test.c END --
Then generated assembly is this one :
00000000 <foo_printf>:
0: 55 push %ebp
1: 89 e5 mov %esp,%ebp
3: 5d pop %ebp
4: c3 ret
00000005 <foo>:
5: 55 push %ebp
6: 89 e5 mov %esp,%ebp
8: 83 ec 04 sub $0x4,%esp
b: 31 db xor %ebx,%ebx
d: 31 c9 xor %ecx,%ecx
f: 31 d2 xor %edx,%edx
11: b8 78 00 00 00 mov $0x78,%eax
16: cd 80 int $0x80
18: 85 c0 test %eax,%eax
1a: 0f 85 fc ff ff ff jne 1c <foo+0x17>
00000020 <child>:
20: c7 04 24 00 00 00 00 movl $0x0,(%esp)
27: e8 fc ff ff ff call 28 <child+0x8>
2c: b8 00 00 00 00 mov $0x0,%eax
31: c9 leave
32: c3 ret
00000033 <main>:
Father was removed because of "goto __end", simply because gcc thought
that this code part will never be reached. But process is cloned() and
it should have taken into account the "jnz father"
The problem is the same for other conditions that make gcc think code
will never be reached, like :
while (1)
foo_printf("in child\n");
OR
foo_printf("in child\n");
return 0;
Here are command line information and code generated :
$ gcc -Wall -save-temps -c test.c -o test.o
no warnings, but
$ gcc -Wall -save-temps test.c -o test
test.o: In function `foo':
test.c:(.text+0x1c): undefined reference to `father'
collect2: ld returned 1 exit status
Ok, because label were removed from assembly !
HERE ARE GENERATED FILES :
-- test.s START --
.file "test.c"
.text
.globl foo_printf
.type foo_printf, @function
foo_printf:
pushl %ebp
movl %esp, %ebp
popl %ebp
ret
.size foo_printf, .-foo_printf
.section .rodata
.LC0:
.string "in child\n"
.text
.globl foo
.type foo, @function
foo:
pushl %ebp
movl %esp, %ebp
subl $4, %esp
#APP
xor %ebx, %ebx
xor %ecx, %ecx
xor %edx, %edx
mov $120, %eax
int $0x80
test %eax, %eax
jnz father
child:
#NO_APP
movl $.LC0, (%esp)
call foo_printf
.L3:
movl $0, %eax
leave
ret
.size foo, .-foo
.globl main
.type main, @function
main:
pushl %ebp
movl %esp, %ebp
subl $8, %esp
andl $-16, %esp
movl $0, %eax
addl $15, %eax
addl $15, %eax
shrl $4, %eax
sall $4, %eax
subl %eax, %esp
call foo
movl $0, %eax
leave
ret
.size main, .-main
.section .note.GNU-stack,"",@progbits
.ident "GCC: (GNU) 3.4.6 (Debian 3.4.6-4)"
-- test.s END --
-- test.i START --
# 1 "test.c"
# 1 "<built-in>"
# 1 "<command line>"
# 1 "test.c"
void foo_printf(char *str)
{
}
int foo()
{
asm volatile (
"xor %ebx, %ebx \n"
"xor %ecx, %ecx \n"
"xor %edx, %edx \n"
"mov $120, %eax \n"
"int $0x80 \n"
"test %eax, %eax \n"
"jnz father \n"
);
asm volatile ("child:");
foo_printf("in child\n");
goto __end;
asm volatile ("father:");
foo_printf("in father\n");
__end:
return 0;
}
int main()
{
foo();
return 0;
}
-- test.i END --
-- GCC INFO --
$ gcc -v
Reading specs from /usr/lib/gcc/i486-linux-gnu/3.4.6/specs
Configured with: ../src/configure -v --enable-languages=c,c++,f77,pascal
--prefix=/usr --libexecdir=/usr/lib
--with-gxx-include-dir=/usr/include/c++/3.4 --enable-shared
--with-system-zlib --enable-nls --without-included-gettext
--program-suffix=-3.4 --enable-__cxa_atexit --enable-clocale=gnu
--enable-libstdcxx-debug --with-tune=i686 i486-linux-gnu
Thread model: posix
gcc version 3.4.6 (Debian 3.4.6-4)
$ uname -a
Linux sushi 2.6.20 #7 Mon Feb 26 10:31:01 CET 2007 i686 GNU/Linux
thanks
stephane
