3 When many applets are compiled into busybox, all rw data and
4 bss for each applet are concatenated. Including those from libc,
5 if static bbox is built. When bbox is started, _all_ this data
6 is allocated, not just that one part for selected applet.
8 What "allocated" exactly means, depends on arch.
9 On nommu it's probably bites the most, actually using real
10 RAM for rwdata and bss. On i386, bss is lazily allocated
11 by COWed zero pages. Not sure about rwdata - also COW?
13 In order to keep bbox NOMMU and small-mem systems friendly
14 we should avoid large global data in our applets, and should
15 minimize usage of libc functions which implicitly use
16 such structures in libc.
18 Small experiment measures "parasitic" bbox memory consumption.
19 Here we start 1000 "busybox sleep 10" in parallel.
20 bbox binary is practically allyesconfig static one,
23 bash-3.2# nmeter '%t %c %b %m %p %[pn]'
24 23:17:28 .......... 0 0 168M 0 147
25 23:17:29 .......... 0 0 168M 0 147
26 23:17:30 U......... 0 0 168M 1 147
27 23:17:31 SU........ 0 188k 181M 244 391
28 23:17:32 SSSSUUU... 0 0 223M 757 1147
29 23:17:33 UUU....... 0 0 223M 0 1147
30 23:17:34 U......... 0 0 223M 1 1147
31 23:17:35 .......... 0 0 223M 0 1147
32 23:17:36 .......... 0 0 223M 0 1147
33 23:17:37 S......... 0 0 223M 0 1147
34 23:17:38 .......... 0 0 223M 1 1147
35 23:17:39 .......... 0 0 223M 0 1147
36 23:17:40 .......... 0 0 223M 0 1147
37 23:17:41 .......... 0 0 210M 0 906
38 23:17:42 .......... 0 0 168M 1 147
39 23:17:43 .......... 0 0 168M 0 147
41 This requires 55M of memory. Thus 1 trivial busybox applet
47 One example how to reduce global data usage is in
48 archival/libunarchive/decompress_unzip.c:
50 /* This is somewhat complex-looking arrangement, but it allows
51 * to place decompressor state either in bss or in
52 * malloc'ed space simply by changing #defines below.
54 * text data bss dec hex
55 * 5256 0 108 5364 14f4 - bss
56 * 4915 0 0 4915 1333 - malloc
58 #define STATE_IN_BSS 0
59 #define STATE_IN_MALLOC 1
61 (see the rest of the file to get the idea)
63 This example completely eliminates globals in that module.
64 Required memory is allocated in inflate_gunzip() [its main module]
65 and then passed down to all subroutines which need to access 'globals'
71 In case you don't want to pass this additional parameter everywhere,
72 take a look at archival/gzip.c. Here all global data is replaced by
73 single global pointer (ptr_to_globals) to allocated storage.
75 In order to not duplicate ptr_to_globals in every applet, you can
76 reuse single common one. It is defined in libbb/messages.c
77 as struct globals *const ptr_to_globals, but the struct globals is
78 NOT defined in libbb.h. You first define your own struct:
80 struct globals { int a; char buf[1000]; };
82 and then declare that ptr_to_globals is a pointer to it:
84 #define G (*ptr_to_globals)
86 ptr_to_globals is declared as constant pointer.
87 This helps gcc understand that it won't change, resulting in noticeably
88 smaller code. In order to assign it, use PTR_TO_GLOBALS macro:
90 PTR_TO_GLOBALS = xzalloc(sizeof(G));
92 Typically it is done in <applet>_main().
94 Now you can reference "globals" by G.a, G.buf and so on, in any function.
99 There is one big common buffer in bss - bb_common_bufsiz1. It is a much
100 earlier mechanism to reduce bss usage. Each applet can use it for
101 its needs. Library functions are prohibited from using it.
103 'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer:
105 #define G (*(struct globals*)&bb_common_bufsiz1)
107 Be careful, though, and use it only if
108 sizeof(struct globals) <= sizeof(bb_common_bufsiz1).
113 You have to initialize it by hand. xzalloc() can be helpful in clearing
114 allocated storage to 0, but anything more must be done by hand.
116 All global variables are prefixed by 'G.' now. If this makes code
117 less readable, use #defines:
119 #define dev_fd (G.dev_fd)
120 #define sector (G.sector)
125 If applet doesn't use much of global data, converting it to use
126 one of above methods is not worth the resulting code obfuscation.
127 If you have less than ~300 bytes of global data - don't bother.
130 gcc's data alignment problem
132 The following attribute added in vi.c:
135 static struct termios term_orig __attribute__ ((aligned (4)));
136 static struct termios term_vi __attribute__ ((aligned (4)));
138 reduced bss size by 32 bytes, because gcc sometimes aligns structures to
139 ridiculously large values. asm output diff for above example:
143 .section .bss.term_orig,"aw",@nobits
146 .type term_orig, @object
150 .section .bss.term_vi,"aw",@nobits
153 .type term_vi, @object
156 gcc doesn't seem to have options for altering this behaviour.