stdlib/getenv.c

   1 /* Copyright (C) 1991,92,94,96,98,99,2002,2005 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 Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the 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    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, write to the Free
  16    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  17    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
  27 /* Return the value of the environment variable NAME.  This implementation
  28    is tuned a bit in that it assumes no environment variable has an empty
  29    name which of course should always be true.  We have a special case for
  30    one character names so that for the general case we can assume at least
  31    two characters which we can access.  By doing this we can avoid using the
  32    `strncmp' most of the time.  */
  33 char *
  34 getenv (name)
  35      const char *name;
  36 {
  37   size_t len = strlen (name);
  38   char **ep;
  39   uint16_t name_start;
  40
  41   if (__environ == NULL || name[0] == '\0')
  42     return NULL;
  43
  44   if (name[1] == '\0')
  45     {
  46       /* The name of the variable consists of only one character.  Therefore
  47          the first two characters of the environment entry are this character
  48          and a '=' character.  */
  49 #if __BYTE_ORDER == __LITTLE_ENDIAN || !_STRING_ARCH_unaligned
  50       name_start = ('=' << 8) | *(const unsigned char *) name;
  51 #else
  52 # if __BYTE_ORDER == __BIG_ENDIAN
  53       name_start = '=' | ((*(const unsigned char *) name) << 8);
  54 # else
  55  #error "Funny byte order."
  56 # endif
  57 #endif
  58       for (ep = __environ; *ep != NULL; ++ep)
  59         {
  60 #if _STRING_ARCH_unaligned
  61           uint16_t ep_start = *(uint16_t *) *ep;
  62 #else
  63           uint16_t ep_start = (((unsigned char *) *ep)[0]
  64                                | (((unsigned char *) *ep)[1] << 8));
  65 #endif
  66           if (name_start == ep_start)
  67             return &(*ep)[2];
  68         }
  69     }
  70   else
  71     {
  72 #if _STRING_ARCH_unaligned
  73       name_start = *(const uint16_t *) name;
  74 #else
  75       name_start = (((const unsigned char *) name)[0]
  76                     | (((const unsigned char *) name)[1] << 8));
  77 #endif
  78       len -= 2;
  79       name += 2;
  80
  81       for (ep = __environ; *ep != NULL; ++ep)
  82         {
  83 #if _STRING_ARCH_unaligned
  84           uint16_t ep_start = *(uint16_t *) *ep;
  85 #else
  86           uint16_t ep_start = (((unsigned char *) *ep)[0]
  87                                | (((unsigned char *) *ep)[1] << 8));
  88 #endif
  89
  90           if (name_start == ep_start && !strncmp (*ep + 2, name, len)
  91               && (*ep)[len + 2] == '=')
  92             return &(*ep)[len + 3];
  93         }
  94     }
  95
  96   return NULL;
  97 }
  98 libc_hidden_def (getenv)