Update TODO with mention of the CONFIG->ENABLE migration and ruminations

about FEATURE_CLEAN_UP.
This commit is contained in:
Rob Landley 2005-09-16 14:58:55 +00:00
parent af0dd596a8
commit a882126fee

65
TODO
View File

@ -115,3 +115,68 @@ Memory Allocation
free for an alloca()), and this means that various cleanup loops that just
call free might also be optimized out by the compiler if written right, so
we can yank those #ifdefs too, and generally clean up the code.
---
Switch CONFIG_SYMBOLS to ENABLE_SYMBOLS
In busybox 1.0 and earlier, configuration was done by CONFIG_SYMBOLS
that were either defined or undefined to indicate whether the symbol was
selected in the .config file. They were used with #ifdefs, ala:
#ifdef CONFIG_SYMBOL
if (other_test) {
do_code();
}
#endif
In 1.1, we have new ENABLE_SYMBOLS which are always defined (as 0 or 1),
meaning you can still use them for preprocessor tests by replacing
"#ifdef CONFIG_SYMBOL" with "#if ENABLE_SYMBOL". But more importantly, we
can use them as a true or false test in normal C code:
if (ENABLE_SYMBOL && other_test) {
do_code();
}
(Optimizing away if() statements that resolve to a constant value
is known as "dead code elimination", an optimization so old and simple that
Turbo Pascal for DOS did it twenty years ago. Even modern mini-compilers
like the Tiny C Compiler (tcc) and the Small Device C Compiler (SDCC)
perform dead code elimination.)
Right now, busybox.h is #including both "config.h" (defining the
CONFIG_SYMBOLS) and "bb_config.h" (defining the ENABLE_SYMBOLS). At some
point in the future, it would be nice to wean ourselves off of the
CONFIG versions. (Among other things, some defective build environments
leak the Linux kernel's CONFIG_SYMBOLS into the system's standard #include
files. We've experienced collisions before.)
---
FEATURE_CLEAN_UP
This is more an unresolved issue than a to-do item. More thought is needed.
Normally we rely on exit() to free memory, close files, and unmap segments
for us. This makes most calls to free(), close(), and unmap() optional in
busybox applets that don't intend to run for very long, and optional stuff
can be omitted to save size.
The idea was raised that we could simulate fork/exit with setjmp/longjmp
for _really_ brainless embedded systems, or speed up the standalone shell
by not forking. Doing so would require a reliable FEATURE_CLEAN_UP.
Unfortunately, this isn't as easy as it sounds.
The problem is, lots of things exit(), sometimes unexpectedly (xmalloc())
and sometimes reliably (bb_perror_msg_and_die() or show_usage()). This
jumps out of the normal flow control and bypasses any cleanup code we
put at the end of our applets.
It's possible to add hooks to libbb functions like xmalloc() and bb_xopen()
to add their entries to a linked list, which could be traversed and
freed/closed automatically. (This would need to be able to free just the
entries after a checkpoint to be usable for a forkless standalone shell.
You don't want to free the shell's own resources.)
Right now, FEATURE_CLEAN_UP is more or less a debugging aid, to make things
like valgrind happy. It's also documentation of _what_ we're trusting
exit() to clean up for us. But new infrastructure to auto-free stuff would
render the existing FEATURE_CLEAN_UP code redundant.
For right now, exit() handles it just fine.