using write() instead of puts() in a trivial program?

[Charles Ulrich]

Greetings,

I hope my greenness isn't showing too bad by asking this, but I ran across
this trivial program today that left me flabbergasted:

#define MESSAGE "This account is currently not available.\n"

int
main(int argc, char *argv[])
{
write(STDOUT_FI LENO, MESSAGE, sizeof(MESSAGE) );
_exit(1);
}

This is an excerpt from the newly-rewritten nologin(8) in FreeBSD. I am
not willing to accept that the person who committed this code is a fool,
thus I have to conclude that I just don't understand why exactly the
following isn't good enough for such a trivial program:

puts("This account is currently not available.");

Perhaps write() would be appropriate in a program which did intensive
amounts of output, but the meat of this program is literally 1 line long.

I tried doing a Google search, but as you can imagine, the commonality of
the "write" keyword throws off any results that might be useful, so I was
wondering some kind, generous person here could clue me in on what's going
on here. I'd greatly appreciate it.

Charles Ulrich

[José de Paula]

Em Mon, 12 Jan 2004 21:10:59 +0000, Alexander Bartolich escreveu:
<snip>

write(2) is a system call.
_exit(2) is a system call.
By using nothing but system calls the total size of the resulting
binary can be minimized. See option -nostdlib of gcc.

That's it. /sbin/nologin isn't linked to any library, not even libc. The
result?

$ ls -lh /sbin/nologin
-r-xr-xr-x 1 root wheel 1,9K 4 Jun 2003 /sbin/nologin

FreeBSD 5.1 i386. A hand-tweaked assembly program could hardly be smaller.
Well, it could, but that's another topic.

--
Emacs is a great operating system---it lacks a good editor, though.

[Grumble]

José de Paula wrote:

That's it. /sbin/nologin isn't linked to any library, not even libc.
The result?

$ ls -lh /sbin/nologin
-r-xr-xr-x 1 root wheel 1,9K 4 Jun 2003 /sbin/nologin

FreeBSD 5.1 i386. A hand-tweaked assembly program could hardly be smaller.
Well, it could, but that's another topic.

1900 bytes just to make two system calls?

It could be made much smaller :-)

e.g. http://www.muppetlabs.com/~breadbox/...ny/teensy.html

[Dan Pop]

In <pa************ *************** *@ig.com.br> =?iso-8859-1?q?Jos=E9_de_P aula?= <jo***********@ ig.com.br> writes:

Em Mon, 12 Jan 2004 21:10:59 +0000, Alexander Bartolich escreveu:

<snip>

write(2) is a system call.
_exit(2) is a system call.

By using nothing but system calls the total size of the resulting
binary can be minimized. See option -nostdlib of gcc.

That's it. /sbin/nologin isn't linked to any library, not even libc.

Then, the linker would complain about the unresolved write and _exit
symbols. Even this program requires minimal library support, write() and
_exit() usually being wrappers to syscall().

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Dan Pop]

In <bu**********@n ews-rocq.inria.fr> Grumble <in*****@kma.eu .org> writes:

José de Paula wrote:

That's it. /sbin/nologin isn't linked to any library, not even libc.
The result?

$ ls -lh /sbin/nologin
-r-xr-xr-x 1 root wheel 1,9K 4 Jun 2003 /sbin/nologin

FreeBSD 5.1 i386. A hand-tweaked assembly program could hardly be smaller.
Well, it could, but that's another topic.

1900 bytes just to make two system calls?

It could be made much smaller :-)

The size of the smallest C program is determined by the size of the
startup module:

fangorn:~/tmp 110> cat test.c
int main(void)
{
return 0;
}
fangorn:~/tmp 111> gcc -s test.c
fangorn:~/tmp 112> ls -l a.out
-rwxr-xr-x 1 danpop sysprog 2912 Jan 19 15:50 a.out*

If I want to get a smaller binary, that still works, I have to start
cheating:

fangorn:~/tmp 123> cat cheat.c
int main()
{
_exit(0);
}
fangorn:~/tmp 124> gcc -s -static -nostdlib cheat.c -lc
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 08048080
fangorn:~/tmp 125> ls -l a.out
-rwxr-xr-x 1 danpop sysprog 740 Jan 19 15:56 a.out*
fangorn:~/tmp 126> ./a.out
fangorn:~/tmp 127> echo $status
0

The ld warning was caused by the missing startup module, but this
minimal program doesn't really need it. The first program above, however,
does:

fangorn:~/tmp 128> gcc -s -static -nostdlib test.c -lc
/usr/bin/ld: warning: cannot find entry symbol _start; defaulting to 08048080
fangorn:~/tmp 129> ./a.out
Segmentation fault
fangorn:~/tmp 130> ls -l a.out
-rwxr-xr-x 1 danpop sysprog 588 Jan 19 16:00 a.out*

The missing syscall support from the statically linked executable
made it both shorter and non-functional.

Dan
--
Dan Pop
DESY Zeuthen, RZ group
Email: Da*****@ifh.de

[Grumble]

Dan Pop wrote:

The size of the smallest C program is determined by the size of the
startup module:

And the size of the smallest ELF binary is determined by the size of
the ELF header ;-)

(Jose mentioned hand-tweaked assembly under FreeBSD.)