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
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
* ...
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,
-find, shells can do it). This poses much more serious limitations
-on what applet can/cannot do:
+<applet>_main(argc,argv) and then continues with its business.
+xargs, find, shells do it (grep for "spawn_and_wait" and
+"run_nofork_applet" to find more users).
+
+This poses much more serious limitations on what applet can do:
* all NOEXEC limitations apply.
* do not ever exit() or exec().
is taken from xfunc_error_retval.
- fflush_stdout_and_exit(n) is ok to use.
* do not use shared global data, or save/restore shared global data
- prior to returning. (e.g. bb_common_bufsiz1 is off-limits).
+ (e.g. bb_common_bufsiz1) prior to returning.
- getopt32() is ok to use. You do not need to save/restore option_mask32,
it is already done by core code.
* 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.
* ...
are probably not worth the effort.
Any NOFORK applet is also a NOEXEC applet.
+
+
+ Relevant CONFIG options
+
+FEATURE_PREFER_APPLETS
+ BB_EXECVP(cmd, argv) will try to exec /proc/self/exe
+ if command's name matches some applet name
+ applet tables will contain NOFORK/NOEXEC bits
+ spawn_and_wait(argv) will do NOFORK/NOEXEC tricks
+
+FEATURE_SH_STANDALONE (needs FEATURE_PREFER_APPLETS=y)
+ shells will try to exec /proc/self/exe if command's name matches
+ some applet name
+ shells will do NOEXEC trick on NOEXEC applets
+
+FEATURE_SH_NOFORK (needs FEATURE_PREFER_APPLETS=y)
+ shells will do NOFORK trick on NOFORK applets