From: crossd@gmail.com (Dan Cross)
Subject: [TUHS] /bin/true (was basic tools / Universal Unix)
Date: Sun, 22 Oct 2017 21:11:25 -0400 [thread overview]
Message-ID: <CAEoi9W4e_jp3LQ-haRp037v8u=AgzH4KDfj60+bFq1CxHhHwtQ@mail.gmail.com> (raw)
In-Reply-To: <201710222300.v9MN0dN4135736@tahoe.cs.Dartmouth.EDU>
On Sun, Oct 22, 2017 at 7:00 PM, Doug McIlroy <doug at cs.dartmouth.edu> wrote:
>> macOS requires you to have a data section aligned to 4K, even if you
>> don't use it. The resulting binary is a little over 8K; again, mostly
>> zeros.
>
> Not quite. The classic empty executable file for /bin/true works
> on OS X. That is not just a clever trick;it's a natural consequence
> of Kernighan's ancient prrecept: do nothing gracefully. Conceivably
> the 4K data section is, too--if the page has no physical presence
> until it is accessed.
Oh sure! Sorry; the mention of the 4K data section was meant to be
orthogonal to the empty-shell file thing. However, one still cannot
'exec' an empty shell script, even on OS X:
: hurricane; mkdir t
: hurricane; cd t
/Users/dcross/t
: hurricane; touch true
: hurricane; cat>dotrue.c
int main() { execl("./true", "true", 0); write(2, "Boo\n", 4); return 1; }
: hurricane; chmod +x ./true
: hurricane; ./true
: hurricane; echo $?
0
: hurricane; make dotrue
cc dotrue.c -o dotrue
dotrue.c:1:14: warning: implicit declaration of function 'execl' is
invalid in C99 [-Wimplicit-function-declaration]
int main() { execl("./true", "true", 0); write(2, "Boo\n", 4); return 1; }
^
dotrue.c:1:42: warning: implicit declaration of function 'write' is
invalid in C99 [-Wimplicit-function-declaration]
int main() { execl("./true", "true", 0); write(2, "Boo\n", 4); return 1; }
^
2 warnings generated.
: hurricane; ./dotrue
Boo
: hurricane; echo $?
1
: hurricane; cp /usr/bin/true .
: hurricane; ./dotrue
: hurricane; echo $?
0
: hurricane; file true
true: Mach-O 64-bit executable x86_64
: hurricane; ls -l true
-rwxr-xr-x 1 dcross eng 17760 Oct 22 21:08 true
: hurricane; cd ..
/Users/dcross
: hurricane; rm -fr t
: hurricane;
(The system-provided /usr/bin/true is >16KB!!)
- Dan C.
next prev parent reply other threads:[~2017-10-23 1:11 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-10-22 23:00 Doug McIlroy
2017-10-23 1:11 ` Dan Cross [this message]
-- strict thread matches above, loose matches on Subject: below --
2017-10-19 14:52 /bin/true (was [TUHS] " Ron Natalie
2017-10-19 21:23 ` Steffen Nurpmeso
2017-10-19 21:43 ` Chet Ramey
2017-10-19 23:00 ` [TUHS] /bin/true (was " Dave Horsfall
2017-10-19 23:14 ` Grant Taylor
2017-10-19 23:23 ` Lyndon Nerenberg
2017-10-19 23:27 ` Kurt H Maier
2017-10-22 4:18 ` Dave Horsfall
2017-10-20 12:10 ` Chet Ramey
2017-10-19 21:43 ` /bin/true (was [TUHS] " Dave Horsfall
2017-10-19 22:04 ` Ronald Natalie
2017-10-19 23:25 ` [TUHS] /bin/true (was " Dave Horsfall
[not found] ` <CAEoi9W7YZ7YXUip0JTMGip3Nd0czgdjqRCMRcK2GYmDJsckuDg@mail.gmail.com>
[not found] ` <CAEoi9W4zdJ3+RXjK5-5rAJ6rwx_0kx6N-bn1U61=txJGyLT_mw@mail.gmail.com>
2017-10-20 1:27 ` Dan Cross
2017-10-20 1:31 ` Lyndon Nerenberg
2017-10-20 2:05 ` Ronald Natalie
2017-11-28 16:21 ` Nemo
2017-11-28 17:56 ` Warner Losh
2017-11-28 18:26 ` Dan Cross
2017-11-28 18:41 ` Warner Losh
2017-11-28 19:09 ` Dan Cross
2017-11-28 20:34 ` Clem Cole
2017-11-28 22:42 ` Ralph Corderoy
2017-11-28 18:34 ` Bakul Shah
2017-11-28 23:25 ` Ralph Corderoy
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