+#if !_LIBC
+# include "malloca.h"
+#endif
+
+#if _LIBC || !HAVE_SETENV
+
+#if !_LIBC
+# define __environ environ
+#endif
+
+#if _LIBC
+/* This lock protects against simultaneous modifications of `environ'. */
+# include <bits/libc-lock.h>
+__libc_lock_define_initialized (static, envlock)
+# define LOCK __libc_lock_lock (envlock)
+# define UNLOCK __libc_lock_unlock (envlock)
+#else
+# define LOCK
+# define UNLOCK
+#endif
+
+/* In the GNU C library we must keep the namespace clean. */
+#ifdef _LIBC
+# define setenv __setenv
+# define clearenv __clearenv
+# define tfind __tfind
+# define tsearch __tsearch
+#endif
+
+/* In the GNU C library implementation we try to be more clever and
+ allow arbitrarily many changes of the environment given that the used
+ values are from a small set. Outside glibc this will eat up all
+ memory after a while. */
+#if defined _LIBC || (defined HAVE_SEARCH_H && defined HAVE_TSEARCH \
+ && defined __GNUC__)
+# define USE_TSEARCH 1
+# include <search.h>
+typedef int (*compar_fn_t) (const void *, const void *);
+
+/* This is a pointer to the root of the search tree with the known
+ values. */
+static void *known_values;
+
+# define KNOWN_VALUE(Str) \
+ ({ \
+ void *value = tfind (Str, &known_values, (compar_fn_t) strcmp); \
+ value != NULL ? *(char **) value : NULL; \
+ })
+# define STORE_VALUE(Str) \
+ tsearch (Str, &known_values, (compar_fn_t) strcmp)
+
+#else
+# undef USE_TSEARCH
+
+# define KNOWN_VALUE(Str) NULL
+# define STORE_VALUE(Str) do { } while (0)
+