glibc/sysdeps/generic/getenv.c

/* Copyright (C) 1991, 92, 94, 96, 98, 99 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Library General Public License as
   published by the Free Software Foundation; either version 2 of the
   License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Library General Public License for more details.

   You should have received a copy of the GNU Library General Public
   License along with the GNU C Library; see the file COPYING.LIB.  If not,
   write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
   Boston, MA 02111-1307, USA.  */

#include <endian.h>
#include <errno.h>
#include <stdint.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#ifndef	HAVE_GNU_LD
#define	__environ	environ
#endif

/* Return the value of the environment variable NAME.  This implementation
   is tuned a bit in that it assumes no environment variable has an empty
   name which of course should always be true.  We have a special case for
   one character names so that for the general case we can assume at least
   two characters which we can access.  By doing this we can avoid using the
   `strncmp' most of the time.  */
char *
getenv (name)
     const char *name;
{
  size_t len = strlen (name);
  char **ep;
  uint16_t name_start;

  if (__environ == NULL || name[0] == '\0')
    return NULL;

  if (name[1] == '\0')
    {
      /* The name of the variable consists of only one character.  Therefore
	 the first two characters of the environment entry are this character
	 and a '=' character.  */
#if __BYTE_ORDER == __LITTLE_ENDIAN || !_STRING_ARCH_unaligned
      name_start = ('=' << 8) | *(const unsigned char *) name;
#else
# if __BYTE_ORDER == __BIG_ENDIAN
      name_start = '=' | ((*(const unsigned char *) name) << 8);
# else
 #error "Funny byte order."
# endif
#endif
      for (ep = __environ; *ep != NULL; ++ep)
	{
#if _STRING_ARCH_unaligned
	  uint16_t ep_start = *(uint16_t *) *ep;
#else
	  uint16_t ep_start = (((unsigned char *) *ep)[0]
			       | (((unsigned char *) *ep)[1] << 8));
#endif
	  if (name_start == ep_start)
	    return &(*ep)[2];
	}
    }
  else
    {
#if _STRING_ARCH_unaligned
      name_start = *(const uint16_t *) name;
#else
      name_start = (((const unsigned char *) name)[0]
		    | (((const unsigned char *) name)[1] << 8));
#endif
      len -= 2;
      name += 2;

      for (ep = __environ; *ep != NULL; ++ep)
	{
#if _STRING_ARCH_unaligned
	  uint16_t ep_start = *(uint16_t *) *ep;
#else
	  uint16_t ep_start = (((unsigned char *) *ep)[0]
			       | (((unsigned char *) *ep)[1] << 8));
#endif

	  if (name_start == ep_start && !strncmp (*ep + 2, name, len)
	      && (*ep)[len + 2] == '=')
	    return &(*ep)[len + 3];
	}
    }

  return NULL;
}