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tac: new applet. ~240 bytes.

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Copyright (C) 2003  Yang Xiaopeng  <yxp at hanwang.com.cn>
Copyright (C) 2007  Natanael Copa  <natanael.copa@gmail.com>
Copyright (C) 2007  Tito Ragusa    <farmatito@tiscali.it>
This commit is contained in:
Denis Vlasenko
2008-01-06 06:27:17 +00:00
parent 56ea65ca5f
commit bcd5fc12ec
5 changed files with 30 additions and 8 deletions
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25
docs/nofork_noexec.txt
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@@ -2,7 +2,7 @@
Unix shells traditionally execute some commands internally in the attempt
to dramatically speed up execution. It will be slow as hell if for every
"echo blah" shell will fork and exec /bin/echo. For this end, shells
"echo blah" shell will fork and exec /bin/echo. To this end, shells
have to _reimplement_ these commands internally.
Busybox is unique in this regard because it already is a collection
@@ -11,14 +11,20 @@ for speeding up busybox shells, and more. NOEXEC and NOFORK applets
are exactly those applets which are eligible for these tricks.
Applet will be subject to NOFORK/NOEXEC tricks if it is marked as such
in applets.h. CONFIG_FEATURE_PREFER_APPLETS is a config option which
in applets.h. FEATURE_PREFER_APPLETS is a config option which
globally enables usage of NOFORK/NOEXEC tricks.
If it is enabled, FEATURE_SH_STANDALONE can be enabled too,
and then shells will use NOFORK/NOEXEC tricks for ordinary commands.
NB: shell builtins use these tricks regardless of FEATURE_SH_STANDALONE
or FEATURE_PREFER_APPLETS.
If you want to call a program and wait for it, use spawn_and_wait(argv).
It will check whether argv[0] is an applet name and will optionally
do NOFORK/NOEXEC thing.
In C, if you want to call a program and wait for it, use
spawn_and_wait(argv), BB_EXECVP(prog,argv) or BB_EXECLP(prog,argv0,...).
They check whether program name is an applet name and optionally
do NOFORK/NOEXEC thing depending on configuration.
NOEXEC
NOEXEC
NOEXEC applet should work correctly if another applet forks and then
executes exit(<applet>_main(argc,argv)) in the child. The rules
@@ -32,9 +38,10 @@ roughly are:
* ...
NOEXEC applets save only one half of fork+exec overhead.
NOEXEC trick is disabled for NOMMU compile.
NOEXEC trick is disabled for NOMMU build.
NOFORK
NOFORK
NOFORK applet should work correctly if another applet simply runs
<applet>_main(argc,argv) and then continues with its business (xargs,
@@ -55,6 +62,8 @@ on what applet can/cannot do:
* if you allocate memory, you can use xmalloc() only on the very first
allocation. All other allocations should use malloc[_or_warn]().
After first allocation, you cannot use any xfuncs.
Otherwise, failing xfunc will return to caller applet
without freeing malloced data!
* All allocated data, opened files, signal handlers, termios settings,
O_NONBLOCK flags etc should be freed/closed/restored prior to return.
* ...