Keeping data small

When many applets are compiled into busybox, all rw data and
bss for each applet are concatenated. Including those from libc,
if static bbox is built. When bbox is started, _all_ this data
is allocated, not just that one part for selected applet.

What "allocated" exactly means, depends on arch.
On nommu it's probably bites the most, actually using real
RAM for rwdata and bss. On i386, bss is lazily allocated
by COWed zero pages. Not sure about rwdata - also COW?

Small experiment measures "parasitic" bbox memory consumption.
Here we start 1000 "busybox sleep 10" in parallel.
bbox binary is practically allyesconfig static one,
built against uclibc:

bash-3.2# nmeter '%t %c %b %m %p %[pn]'
23:17:28 ..........  0  0 168M  0 147
23:17:29 ..........  0  0 168M  0 147
23:17:30 U.........  0  0 168M  1 147
23:17:31 SU........  0 188k 181M 244 391
23:17:32 SSSSUUU...  0  0 223M 757 1147
23:17:33 UUU.......  0  0 223M  0 1147
23:17:34 U.........  0  0 223M  1 1147
23:17:35 ..........  0  0 223M  0 1147
23:17:36 ..........  0  0 223M  0 1147
23:17:37 S.........  0  0 223M  0 1147
23:17:38 ..........  0  0 223M  1 1147
23:17:39 ..........  0  0 223M  0 1147
23:17:40 ..........  0  0 223M  0 1147
23:17:41 ..........  0  0 210M  0 906
23:17:42 ..........  0  0 168M  1 147
23:17:43 ..........  0  0 168M  0 147

This requires 55M of memory. Thus 1 trivial busybox applet
takes 55k of userspace memory (nmeter doesn't account for kernel-side
allocations). Definitely can be improved.

Thus we should avoid large global data in our applets,
and should minimize usage of libc functions which implicitly use
such structures in libc.

	Example 1

One example how to reduce global data usage is in
archival/libunarchive/decompress_unzip.c:

/* This is somewhat complex-looking arrangement, but it allows
 * to place decompressor state either in bss or in
 * malloc'ed space simply by changing #defines below.
 * Sizes on i386:
 * text    data     bss     dec     hex
 * 5256       0     108    5364    14f4 - bss
 * 4915       0       0    4915    1333 - malloc
 */
#define STATE_IN_BSS 0
#define STATE_IN_MALLOC 1

This example completely eliminates globals in that module.
Required memory is allocated in inflate_gunzip() [its main module]
and then passed down to all subroutines which need to access globals
as a parameter.

	Example 2

In case you don't want to pass this additional parameter everywhere,
take a look at archival/gzip.c. Here all global data is replaced by
singe global pointer (ptr_to_globals) to allocated storage.

In order to not duplicate ptr_to_globals in every applet, you can
reuse single common one. It is defined in libbb/messages.c
as void *ptr_to_globals, but is NOT declared in libbb.h.
You first define a struct:

struct my_globals { int a; char buf[1000]; };

and then declare that ptr_to_globals is a pointer to it:

extern struct my_globals *ptr_to_globals;
#define G (*ptr_to_globals)

Linker magic enures that these two merge into single pointer object.
Now initialize it in <applet>_main():

	ptr_to_globals = xzalloc(sizeof(G));

and you can reference "globals" by G.a, G.buf and so on, in any function.