Dummy

bong.mau · 08-31-2005, 11:39 AM

Hi

I need a program that do nothing !
it must contains only a return to the caller.
like iefbr14 on ibm mainframes.....: an assembler pgm containing a branch to register 14,a return to the caller...
i 'm a linux newbie,but not a computer newbie,i immagine kde is written in
C language,so if someone know where to download an alreday compiled C
pgm that "do nothing" or have patience to instruct me to write correct statements and issue the make and/or make install command ,i will reach my target.
thanks in advance
Maurizio

jtshaw · 08-31-2005, 11:48 AM

I might be confused by what your asking here....

Code:

/* dummy.c */

int main () {
     return 0;
}

Build instructions:
gcc -Wall -o dummy dummy.c

Execute instructions
./dummy

nodger · 08-31-2005, 02:48 PM

that program does something. returns a value.

jtshaw · 08-31-2005, 02:56 PM

Quote:

it must contains only a return to the caller.

If you want nothing returned then

void main() {return;} or simply void main() {} would do the trick... however, in Linux a program is suppose to return 0 or an error code apon completion of it's execution to truly follow all the rules.

Also... so you know... KDE is written in C++ utilizing the QT API.

slackie1000 · 08-31-2005, 02:59 PM

hi there,
could you have a "void main" ??
what about...

Code:

/* dummy.c */
void main () {
}

the gcc compiler gives a warning but you actually gets a program that do nothing..
regards,
slackie1000

jtshaw · 08-31-2005, 03:04 PM

Quote:

Originally posted by slackie1000
hi there,
could you have a "void main" ??
what about...

Code:

/* dummy.c */ void main () { }

the gcc compiler gives a warning but you actually gets a program that do nothing..
regards,
slackie1000

Sure you can do that... if your interested it actually causes a one line decrease in the assembly code created:

int main:

Code:

    .file   "dummy.c"
    .text
.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
    movl    $0, %eax <--- THIS IS THE DIFFERENCE
    leave
    ret
    .size   main, .-main
    .section    .note.GNU-stack,"",@progbits
    .ident  "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

void main:

Code:

    .file   "dummy.c"
    .text
.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
    leave
    ret
    .size   main, .-main
    .section    .note.GNU-stack,"",@progbits
    .ident  "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

I compiled this without any optimizations.. using -O3 or -Os would produce less code.

slackie1000 · 08-31-2005, 03:10 PM

Quote:

Originally posted by jtshaw
Sure you can do that... if your interested it actually causes a one line decrease in the assembly code created:
int main:

Code:

.file "dummy.c" .text .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 movl $0, %eax <--- THIS IS THE DIFFERENCE leave ret .size main, .-main .section .note.GNU-stack,"",@progbits .ident "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

void main:

Code:

.file "dummy.c" .text .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 leave ret .size main, .-main .section .note.GNU-stack,"",@progbits .ident "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

I compiled this without any optimizations.. using -O3 or -Os would produce less code.

woo!! that was nice... thanks for posting that jtshaw!!!
for me what was also interesting is that both executables had the same size with the default compilation: 4355

Code:

gcc -o something dummy.c

cheers,
slackie1000

Hko · 08-31-2005, 03:12 PM

Quote:

Code:

    .file   "dummy.c"
    .text
.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
    movl    $0, %eax <--- THIS IS THE DIFFERENCE
    leave
    ret
    .size   main, .-main
    .section    .note.GNU-stack,"",@progbits
    .ident  "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

Looks like quite a lot, for a program that's supposed to do nothing...
:-)

jtshaw · 08-31-2005, 03:15 PM

And people say functions have no overhead....

The smallest I could get the assembly output was:

Code:

    .file   "dummy.c"
    .section    .note.GNU-stack,"",@progbits
    .ident  "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

That was compiling a file with nothing but a comment in it

Unfortunetly you can't link that, because to link and build an exe you must have a main.

The smallest I could get it that would link was

Code:

    .file   "dummy.c"
    .text
.globl main
    .type   main, @function
main:
    pushl   %ebp
    movl    %esp, %ebp
    popl    %ebp
    ret
    .size   main, .-main
    .section    .note.GNU-stack,"",@progbits
    .ident  "GCC: (GNU) 3.4.4 (Gentoo 3.4.4-r1, ssp-3.4.4-1.0, pie-8.7.8)"

GCC apparently doesn't bother to make sure you use the stack pointer before it saves it.

bong.mau · 09-01-2005, 12:06 PM

Hi
First :thanks to all people that answered to my question.
i was using the dummy pgm in the mainframe environment to
reach this target...
immagine a product that starts many other pgm that may be uneseful.
the program starting is not ruled by parameters...
so i renamed the dummy pgm with the name of the original to be substituted...
the iefbr14 contains a return to register 14,then the return code is 0
because the operation is always successfull.
so the main product think to have started a sub program but instead it starts a dummy.
if the C environment does not provide automatically a return code 0
meaning successfull operation,then the dummy pgm must contain a
return code 0 setting.

Maurizio

aluser · 09-01-2005, 12:29 PM

Quote:

The smallest I could get it that would link was ...

If you change languages, you can do better

Code:

13:20 aluser@alf:~/test/asm$ cat small.s
.globl _start
.text
_start:
        movl $1, %eax
        int $0x80
13:20 aluser@alf:~/test/asm$ as -o small.o small.s
13:20 aluser@alf:~/test/asm$ ld -o small small.o
13:20 aluser@alf:~/test/asm$ ./small
13:20 aluser@alf:~/test/asm$ echo $?
0
13:20 aluser@alf:~/test/asm$ ls -l small
-rwxr-xr-x  1 aluser aluser 656 Sep  1 13:20 small

Obviously, this is specific to linux x86...

It calls the exit() syscall with whatever is in %ebx as the exit value. On my system that happens to be 0.

If you don't call exit from _start, it'll segfault; _start doesn't have a calling function. When you compile something with gcc and it links with libc, libc sets up a _start that calls your main() and exits with main's return code; I've bypassed that step.

I've read it's actually possible to get a working executabe down to the size of the ELF header by putting code in unused parts of the header. You would have to create the entire executable in a hex editor at that point.