Date: Tue, 01 Sep 1998 18:40:42 +0000 From: Mike Smith <mike@smith.net.au> To: John Polstra <jdp@polstra.com> Cc: Mike Smith <mike@smith.net.au>, "Andrew Reilly" <reilly@zeta.org.au>, current@FreeBSD.ORG Subject: Re: ELF binaries size Message-ID: <199809011840.SAA02659@dingo.cdrom.com> In-Reply-To: Your message of "Tue, 01 Sep 1998 18:34:11 MST." <199809020134.SAA16805@austin.polstra.com>
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> > > > No, we map both text and data read-only.
> > >
> > > No, the text is read-only and the data is copy-on-write.
> >
> > Is this an ELF requirement, or just an arbitrary change?
>
> I don't understand what you're getting at. A program has to be able
> to write its data. That's where the variables are.
dingo:/tmp>cat a.c
char *foo = "test";
int bar;
void main(void)
{
foo[0] = 0;
}
dingo:/tmp>./a.out
Bus error (core dumped)
dingo:/tmp>nm a.out | sort
00001020 F /usr/lib/aout/crt0.o
00001020 T start
00001560 F /var/tmp/ccMd26351.o
00001560 t gcc2_compiled.
00001568 T _main
00001580 F __main.o
00001580 T ___do_global_dtors
000015a8 T ___do_global_ctors
000015f4 T ___main
00001620 F _exit.o
00001940 T _etext
00002000 d __DYNAMIC
0000207c D ___progname
0000208c D _foo
0000209c D __exit_dummy_ref
000020ac D __exit_dummy_decl
000020bc D _edata
000020c4 B ___DTOR_LIST__
000020cc B ___CTOR_LIST__
000020d4 B ___ldso_version
000020d8 B ___ldso_entry
000020dc B _environ
000020e0 B _errno
000020e4 B _bar
000020e8 B _end
As you can see, 'foo' is in the data segment, and it's pretty clearly
read-only. If you were to compile with -fwritable_strings, gcc would
emit code to copy it to the bss.
The data segment only contains constant data, including constant
initialisers for non-global initialised variables.
As you can see from 'bar', variables live in the bss.
--
\\ Sometimes you're ahead, \\ Mike Smith
\\ sometimes you're behind. \\ mike@smith.net.au
\\ The race is long, and in the \\ msmith@freebsd.org
\\ end it's only with yourself. \\ msmith@cdrom.com
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