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 busybox is built. When busybox 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? In order to keep busybox NOMMU and small-mem systems friendly we should avoid large global data in our applets, and should minimize usage of libc functions which implicitly use such structures. Small experiment to measure "parasitic" bbox memory consumption: here we start 1000 "busybox sleep 10" in parallel. busybox binary is practically allyesconfig static one, built against uclibc. Run on x86-64 machine with 64-bit kernel: bash-3.2# nmeter '%t %c %m %p %[pn]' 23:17:28 .......... 168M 0 147 23:17:29 .......... 168M 0 147 23:17:30 U......... 168M 1 147 23:17:31 SU........ 181M 244 391 23:17:32 SSSSUUU... 223M 757 1147 23:17:33 UUU....... 223M 0 1147 23:17:34 U......... 223M 1 1147 23:17:35 .......... 223M 0 1147 23:17:36 .......... 223M 0 1147 23:17:37 S......... 223M 0 1147 23:17:38 .......... 223M 1 1147 23:17:39 .......... 223M 0 1147 23:17:40 .......... 223M 0 1147 23:17:41 .......... 210M 0 906 23:17:42 .......... 168M 1 147 23:17:43 .......... 168M 0 147 This requires 55M of memory. Thus 1 trivial busybox applet takes 55k of memory on 64-bit x86 kernel. On 32-bit kernel we need ~26k per applet. Script: i=1000; while test $i != 0; do echo -n . busybox sleep 30 & i=$((i - 1)) done echo wait (Data from NOMMU arches are sought. Provide 'size busybox' output too) Example 1 One example how to reduce global data usage is in archival/libarchive/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 (see the rest of the file to get the idea) This example completely eliminates globals in that module. Required memory is allocated in unpack_gz_stream() [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 single 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 struct globals *const ptr_to_globals, but the struct globals is NOT defined in libbb.h. You first define your own struct: struct globals { int a; char buf[1000]; }; and then declare that ptr_to_globals is a pointer to it: #define G (*ptr_to_globals) ptr_to_globals is declared as constant pointer. This helps gcc understand that it won't change, resulting in noticeably smaller code. In order to assign it, use SET_PTR_TO_GLOBALS macro: SET_PTR_TO_GLOBALS(xzalloc(sizeof(G))); Typically it is done in _main(). Now you can reference "globals" by G.a, G.buf and so on, in any function. bb_common_bufsiz1 There is one big common buffer in bss - bb_common_bufsiz1. It is a much earlier mechanism to reduce bss usage. Each applet can use it for its needs. Library functions are prohibited from using it. 'G.' trick can be done using bb_common_bufsiz1 instead of malloced buffer: #define G (*(struct globals*)&bb_common_bufsiz1) Be careful, though, and use it only if globals fit into bb_common_bufsiz1. Since bb_common_bufsiz1 is BUFSIZ + 1 bytes long and BUFSIZ can change from one libc to another, you have to add compile-time check for it: if (sizeof(struct globals) > sizeof(bb_common_bufsiz1)) BUG__globals_too_big(); Drawbacks You have to initialize it by hand. xzalloc() can be helpful in clearing allocated storage to 0, but anything more must be done by hand. All global variables are prefixed by 'G.' now. If this makes code less readable, use #defines: #define dev_fd (G.dev_fd) #define sector (G.sector) Word of caution If applet doesn't use much of global data, converting it to use one of above methods is not worth the resulting code obfuscation. If you have less than ~300 bytes of global data - don't bother. Finding non-shared duplicated strings strings busybox | sort | uniq -c | sort -nr gcc's data alignment problem The following attribute added in vi.c: static int tabstop; static struct termios term_orig __attribute__ ((aligned (4))); static struct termios term_vi __attribute__ ((aligned (4))); reduces bss size by 32 bytes, because gcc sometimes aligns structures to ridiculously large values. asm output diff for above example: tabstop: .zero 4 .section .bss.term_orig,"aw",@nobits - .align 32 + .align 4 .type term_orig, @object .size term_orig, 60 term_orig: .zero 60 .section .bss.term_vi,"aw",@nobits - .align 32 + .align 4 .type term_vi, @object .size term_vi, 60 gcc doesn't seem to have options for altering this behaviour. gcc 3.4.3 and 4.1.1 tested: char c = 1; // gcc aligns to 32 bytes if sizeof(struct) >= 32 struct { int a,b,c,d; int i1,i2,i3; } s28 = { 1 }; // struct will be aligned to 4 bytes struct { int a,b,c,d; int i1,i2,i3,i4; } s32 = { 1 }; // struct will be aligned to 32 bytes // same for arrays char vc31[31] = { 1 }; // unaligned char vc32[32] = { 1 }; // aligned to 32 bytes -fpack-struct=1 reduces alignment of s28 to 1 (but probably will break layout of many libc structs) but s32 and vc32 are still aligned to 32 bytes. I will try to cook up a patch to add a gcc option for disabling it. Meanwhile, this is where it can be disabled in gcc source: gcc/config/i386/i386.c int ix86_data_alignment (tree type, int align) { #if 0 if (AGGREGATE_TYPE_P (type) && TYPE_SIZE (type) && TREE_CODE (TYPE_SIZE (type)) == INTEGER_CST && (TREE_INT_CST_LOW (TYPE_SIZE (type)) >= 256 || TREE_INT_CST_HIGH (TYPE_SIZE (type))) && align < 256) return 256; #endif Result (non-static busybox built against glibc): # size /usr/srcdevel/bbox/fix/busybox.t0/busybox busybox text data bss dec hex filename 634416 2736 23856 661008 a1610 busybox 632580 2672 22944 658196 a0b14 busybox_noalign Keeping code small Set CONFIG_EXTRA_CFLAGS="-fno-inline-functions-called-once", produce "make bloatcheck", see the biggest auto-inlined functions. Now, set CONFIG_EXTRA_CFLAGS back to "", but add NOINLINE to some of these functions. In 1.16.x timeframe, the results were (annotated "make bloatcheck" output): function old new delta expand_vars_to_list - 1712 +1712 win lzo1x_optimize - 1429 +1429 win arith_apply - 1326 +1326 win read_interfaces - 1163 +1163 loss, leave w/o NOINLINE logdir_open - 1148 +1148 win check_deps - 1148 +1148 loss rewrite - 1039 +1039 win run_pipe 358 1396 +1038 win write_status_file - 1029 +1029 almost the same, leave w/o NOINLINE dump_identity - 987 +987 win mainQSort3 - 921 +921 win parse_one_line - 916 +916 loss summarize - 897 +897 almost the same do_shm - 884 +884 win cpio_o - 863 +863 win subCommand - 841 +841 loss receive - 834 +834 loss 855 bytes saved in total. scripts/mkdiff_obj_bloat may be useful to automate this process: run "scripts/mkdiff_obj_bloat NORMALLY_BUILT_TREE FORCED_NOINLINE_TREE" and select modules which shrank.