LXR busybox/libbb/common

   1/* vi: set sw=4 ts=4: */
   2/*
   3 * Utility routines.
   4 *
   5 * Copyright (C) 2016 Denys Vlasenko
   6 *
   7 * Licensed under GPLv2, see file LICENSE in this source tree.
   8 */
   9//config:config FEATURE_USE_BSS_TAIL
  10//config:       bool "Use the end of BSS page"
  11//config:       default n
  12//config:       help
  13//config:       Attempt to reclaim a small unused part of BSS.
  14//config:
  15//config:       Executables have the following parts:
  16//config:       = read-only executable code and constants, also known as "text"
  17//config:       = read-write data
  18//config:       = non-initialized (zeroed on demand) data, also known as "bss"
  19//config:
  20//config:       At link time, "text" is padded to a full page. At runtime, all "text"
  21//config:       pages are mapped RO and executable.
  22//config:
  23//config:       "Data" starts on the next page boundary, but is not padded
  24//config:       to a full page at the end. "Bss" starts wherever "data" ends.
  25//config:       At runtime, "data" pages are mapped RW and they are file-backed
  26//config:       (this includes a small portion of "bss" which may live in the last
  27//config:       partial page of "data").
  28//config:       Pages which are fully in "bss" are mapped to anonymous memory.
  29//config:
  30//config:       "Bss" end is usually not page-aligned. There is an unused space
  31//config:       in the last page. Linker marks its start with the "_end" symbol.
  32//config:
  33//config:       This option will attempt to use that space for bb_common_bufsiz1[]
  34//config:       array. If it fits after _end, it will be used, and COMMON_BUFSIZE
  35//config:       will be enlarged from its guaranteed minimum size of 1 kbyte.
  36//config:       This may require recompilation a second time, since value of _end
  37//config:       is known only after final link.
  38//config:
  39//config:       If you are getting a build error like this:
  40//config:               appletlib.c:(.text.main+0xd): undefined reference to '_end'
  41//config:       disable this option.
  42
  43//kbuild:lib-y += common_bufsiz.o
  44
  45#include "libbb.h"
  46#include "common_bufsiz.h"
  47
  48#if !ENABLE_FEATURE_USE_BSS_TAIL
  49
  50/* We use it for "global" data via *(struct global*)bb_common_bufsiz1.
  51 * Since gcc insists on aligning struct global's members, it would be a pity
  52 * (and an alignment fault on some CPUs) to mess it up. */
  53char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
  54
  55#else
  56
  57# ifndef setup_common_bufsiz
  58/* For now, this is never used:
  59 * scripts/generate_BUFSIZ.sh never generates "malloced" bufsiz1:
  60 *      enum { COMMON_BUFSIZE = 1024 };
  61 *      extern char *const bb_common_bufsiz1;
  62 *      void setup_common_bufsiz(void);
  63 * This has proved to be worse than the approach of defining
  64 * larger bb_common_bufsiz1[] array.
  65 */
  66
  67/*
  68 * It is not defined as a dummy macro.
  69 * It means we have to provide this function.
  70 */
  71char *const bb_common_bufsiz1 __attribute__ ((section (".data")));
  72void setup_common_bufsiz(void)
  73{
  74        if (!bb_common_bufsiz1)
  75                *(char**)&bb_common_bufsiz1 = xzalloc(COMMON_BUFSIZE);
  76}
  77# else
  78#  ifndef bb_common_bufsiz1
  79   /* bb_common_bufsiz1[] is not aliased to _end[] */
  80char bb_common_bufsiz1[COMMON_BUFSIZE] ALIGNED(sizeof(long long));
  81#  endif
  82# endif
  83
  84#endif
  85