stdlib/getenv.c

   1 /* Copyright (C) 1991-2017 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, see
  16    <http://www.gnu.org/licenses/>.  */
  17
  18 #include <endian.h>
  19 #include <errno.h>
  20 #include <stdint.h>
  21 #include <stdlib.h>
  22 #include <string.h>
  23 #include <unistd.h>
  24
  25
  26 /* Return the value of the environment variable NAME.  This implementation
  27    is tuned a bit in that it assumes no environment variable has an empty
  28    name which of course should always be true.  We have a special case for
  29    one character names so that for the general case we can assume at least
  30    two characters which we can access.  By doing this we can avoid using the
  31    `strncmp' most of the time.  */
  32 char *
  33 getenv (const char *name)
  34 {
  35   size_t len = strlen (name);
  36   char **ep;
  37   uint16_t name_start;
  38
  39   if (__environ == NULL || name[0] == '\0')
  40     return NULL;
  41
  42   if (name[1] == '\0')
  43     {
  44       /* The name of the variable consists of only one character.  Therefore
  45          the first two characters of the environment entry are this character
  46          and a '=' character.  */
  47 #if __BYTE_ORDER == __LITTLE_ENDIAN || !_STRING_ARCH_unaligned
  48       name_start = ('=' << 8) | *(const unsigned char *) name;
  49 #else
  50       name_start = '=' | ((*(const unsigned char *) name) << 8);
  51 #endif
  52       for (ep = __environ; *ep != NULL; ++ep)
  53         {
  54 #if _STRING_ARCH_unaligned
  55           uint16_t ep_start = *(uint16_t *) *ep;
  56 #else
  57           uint16_t ep_start = (((unsigned char *) *ep)[0]
  58                                | (((unsigned char *) *ep)[1] << 8));
  59 #endif
  60           if (name_start == ep_start)
  61             return &(*ep)[2];
  62         }
  63     }
  64   else
  65     {
  66 #if _STRING_ARCH_unaligned
  67       name_start = *(const uint16_t *) name;
  68 #else
  69       name_start = (((const unsigned char *) name)[0]
  70                     | (((const unsigned char *) name)[1] << 8));
  71 #endif
  72       len -= 2;
  73       name += 2;
  74
  75       for (ep = __environ; *ep != NULL; ++ep)
  76         {
  77 #if _STRING_ARCH_unaligned
  78           uint16_t ep_start = *(uint16_t *) *ep;
  79 #else
  80           uint16_t ep_start = (((unsigned char *) *ep)[0]
  81                                | (((unsigned char *) *ep)[1] << 8));
  82 #endif
  83
  84           if (name_start == ep_start && !strncmp (*ep + 2, name, len)
  85               && (*ep)[len + 2] == '=')
  86             return &(*ep)[len + 3];
  87         }
  88     }
  89
  90   return NULL;
  91 }
  92 libc_hidden_def (getenv)