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glibc/stdlib/getenv.c
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/* Copyright (C) 1991-2016 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 Lesser General Public | |
License as published by the Free Software Foundation; either | |
version 2.1 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 | |
Lesser General Public License for more details. | |
You should have received a copy of the GNU Lesser General Public | |
License along with the GNU C Library; if not, see | |
<http://www.gnu.org/licenses/>. */ | |
#include <endian.h> | |
#include <errno.h> | |
#include <stdint.h> | |
#include <stdlib.h> | |
#include <string.h> | |
#include <unistd.h> | |
/* 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 (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 | |
name_start = '=' | ((*(const unsigned char *) name) << 8); | |
#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; | |
} | |
libc_hidden_def (getenv) |