sysdeps/generic/getenv.c

   1 /* Copyright (C) 1991, 1992, 1994, 1996, 1998 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Library General Public License as
   6    published by the Free Software Foundation; either version 2 of the
   7    License, or (at your option) any later version.
   8
   9    The GNU C Library is distributed in the hope that it will be useful,
  10    but WITHOUT ANY WARRANTY; without even the implied warranty of
  11    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  12    Library General Public License for more details.
  13
  14    You should have received a copy of the GNU Library General Public
  15    License along with the GNU C Library; see the file COPYING.LIB.  If not,
  16    write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  17    Boston, MA 02111-1307, USA.  */
  18
  19 #include <endian.h>
  20 #include <errno.h>
  21 #include <stdint.h>
  22 #include <stdlib.h>
  23 #include <string.h>
  24 #include <unistd.h>
  25
  26 #ifndef HAVE_GNU_LD
  27 #define __environ       environ
  28 #endif
  29
  30 /* Return the value of the environment variable NAME.  This implementation
  31    is tuned a bit in that it assumes no environment variable has an empty
  32    name which of course should always be true.  We have a special case for
  33    one character names so that for the general case we can assume at least
  34    two characters which we can access.  By doing this we can avoid using the
  35    `strncmp' most of the time.  */
  36 char *
  37 getenv (name)
  38      const char *name;
  39 {
  40   const size_t len = strlen (name);
  41   char **ep;
  42   uint16_t name_start;
  43
  44   if (__environ == NULL || name[0] == '\0')
  45     return NULL;
  46
  47   if (name[1] == '\0')
  48     {
  49       /* The name of the variable consists of only one character.  Therefore
  50          the first two characters of the environment entry are this character
  51          and a '=' character.  */
  52 #if __BYTE_ORDER == __LITTLE_ENDIAN
  53       name_start = ('=' << 8) | *(const unsigned char *) name;
  54 #else
  55 # if __BYTE_ORDER == __BIG_ENDIAN
  56       name_start = '=' | ((*(const unsigned char *) name) << 8);
  57 # else
  58  #error "Funny byte order."
  59 # endif
  60 #endif
  61       for (ep = __environ; *ep != NULL; ++ep)
  62         {
  63 #if _STRING_ARCH_unaligned
  64           uint16_t ep_start = *(uint16_t *) *ep;
  65 #else
  66           uint16_t ep_start = (((unsigned char *) *ep)[0]
  67                                | (((unsigned char *) *ep)[1] << 8));
  68 #endif
  69           if (name_start == ep_start)
  70             return &(*ep)[2];
  71         }
  72     }
  73   else
  74     {
  75 #if _STRING_ARCH_unaligned
  76       name_start = *(const uint16_t *) name;
  77 #else
  78       name_start = (((const unsigned char *) name)[0]
  79                     | (((const unsigned char *) name)[1] << 8));
  80 #endif
  81       len -= 2;
  82       name += 2;
  83
  84       for (ep = __environ; *ep != NULL; ++ep)
  85         {
  86 #if _STRING_ARCH_unaligned
  87           uint16_t ep_start = *(uint16_t *) *ep;
  88 #else
  89           uint16_t ep_start = (((unsigned char *) *ep)[0]
  90                                | (((unsigned char *) *ep)[1] << 8));
  91 #endif
  92
  93           if (name_start == ep_start && !strncmp (*ep + 2, name, len)
  94               && (*ep)[len + 2] == '=')
  95             return &(*ep)[len + 3];
  96         }
  97     }
  98
  99   return NULL;
 100 }