setenv.c

/* Copyright (C) 1992-2025 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
   <https://www.gnu.org/licenses/>.  */

#if HAVE_CONFIG_H
# include <config.h>
#endif

#include <assert.h>
#include <setenv.h>

/* Pacify GCC; see the commentary about VALLEN below.  This is needed
   at least through GCC 4.9.2.  Pacify GCC for the entire file, as
   there seems to be no way to pacify GCC selectively, only for the
   place where it's needed.  Do not use DIAG_IGNORE_NEEDS_COMMENT
   here, as it's not defined yet.  */
#pragma GCC diagnostic ignored "-Wmaybe-uninitialized"

#include <errno.h>
#if !_LIBC
# if !defined errno && !defined HAVE_ERRNO_DECL
extern int errno;
# endif
# define __set_errno(ev) ((errno) = (ev))
#endif

#if _LIBC || HAVE_STDLIB_H
# include <stdlib.h>
#endif
#if _LIBC || HAVE_STRING_H
# include <string.h>
#endif
#if _LIBC || HAVE_UNISTD_H
# include <unistd.h>
#endif

#if !_LIBC
# define __environ	environ
# ifndef HAVE_ENVIRON_DECL
extern char **environ;
# endif
#endif

#if _LIBC
/* This lock protects against simultaneous modifications of `environ'.  */
# include <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 unsetenv __unsetenv
# 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>

/* 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)

#endif

/* Allocate a new environment array and put it o the
   __environ_array_list.  Returns NULL on memory allocation
   failure.  */
static struct environ_array *
__environ_new_array (size_t required_size)
{
  /* No backing array yet, or insufficient room.  */
  size_t new_size;
  if (__environ_array_list == NULL
      || __environ_array_list->allocated * 2 < required_size)
    /* Add some unused space for future growth.  */
    new_size = required_size + 16;
  else
    new_size = __environ_array_list->allocated * 2;

  /* Zero-initialize everything, so that getenv can only
     observe valid or null pointers.  */
  char **new_array = calloc (new_size, sizeof (*new_array));
  if (new_array == NULL)
    return NULL;

  struct environ_array *target_array = malloc (sizeof (*target_array));
  if (target_array == NULL)
    {
      free (new_array);
      return NULL;
    }

  target_array->allocated = new_size;
  target_array->array = new_array;
  assert (new_size >= target_array->allocated);

  /* Put it onto the list.  */
  target_array->next = __environ_array_list;
  __environ_array_list = target_array;
  return target_array;
}

int
__add_to_environ (const char *name, const char *value, const char *combined,
		  int replace)
{
  /* Compute lengths before locking, so that the critical section is
     less of a performance bottleneck.  VALLEN is needed only if
     COMBINED is null (unfortunately GCC is not smart enough to deduce
     this; see the #pragma at the start of this file).  Testing
     COMBINED instead of VALUE causes setenv (..., NULL, ...)  to dump
     core now instead of corrupting memory later.  */
  const size_t namelen = strlen (name);
  size_t vallen;
  if (combined == NULL)
    vallen = strlen (value) + 1;

  LOCK;

  /* We have to get the pointer now that we have the lock and not earlier
     since another thread might have created a new environment.   */
  char **start_environ = atomic_load_relaxed (&__environ);
  char **ep = start_environ;

  /* This gets written to __environ in the end.  */
  char **result_environ = start_environ;

  /* Size of the environment if *ep == NULL.  */
  if (ep != NULL)
    for (; *ep != NULL; ++ep)
      if (strncmp (*ep, name, namelen) == 0 && (*ep)[namelen] == '=')
	break;

  if (ep == NULL || __glibc_likely (*ep == NULL))
    {
      /* The scanning loop above reached the end of the environment.
	 Add a new string to it.  */
      replace = true;

      /* + 2 for the new entry and the terminating NULL.  */
      size_t required_size = (ep - start_environ) + 2;
      if (__environ_is_from_array_list (start_environ)
	  && required_size <= __environ_array_list->allocated)
	/* The __environ array is ours, and we have room in it.  We
	   can use ep as-is.  Add a null terminator in case current
	   usage is less than previous usage.  */
	ep[1] = NULL;
      else
	{
	  /* We cannot use __environ as is and need to copy over the
	     __environ contents into an array managed via
	     __environ_array_list.  */

	  struct environ_array *target_array;
	  if (__environ_array_list != NULL
	      && required_size <= __environ_array_list->allocated)
	    /* Existing array has enough room.  Contents is copied below.  */
	    target_array = __environ_array_list;
	  else
	    {
	      /* Allocate a new array.  */
	      target_array = __environ_new_array (required_size);
	      if (target_array == NULL)
		{
		  UNLOCK;
		  return -1;
		}
	    }

	  /* Copy over the __environ array contents.  This forward
	     copy slides backwards part of the array if __environ
	     points into target_array->array.  This happens if an
	     application makes an assignment like:

	       environ = &environ[1];

	     The forward copy avoids clobbering values that still
	     needing copying.  This code handles the case
	     start_environ == ep == NULL, too.  */
	  size_t i;
	  for (i = 0; start_environ + i < ep; ++i)
	    /* Regular store because unless there has been direct
	       manipulation of the environment, target_array is still
	       a private copy.  */
	    target_array->array[i] = atomic_load_relaxed (start_environ + i);

	  /* This is the new place where we should add the element.  */
	  ep = target_array->array + i;

	  /* Add the null terminator in case there was a pointer there
	     previously.  */
	  ep[1] = NULL;

	  /* And __environ should be repointed to our array.  */
	  result_environ = target_array->array;
	}
    }

  if (replace || *ep == NULL)
    {
      char *np;

      /* Use the user string if given.  */
      if (combined != NULL)
	np = (char *) combined;
      else
	{
	  const size_t varlen = namelen + 1 + vallen;
#ifdef USE_TSEARCH
	  char *new_value = malloc (varlen);
	  if (new_value == NULL)
	    {
	      UNLOCK;
	      return -1;
	    }
# ifdef _LIBC
	  __mempcpy (__mempcpy (__mempcpy (new_value, name, namelen), "=", 1),
		     value, vallen);
# else
	  memcpy (new_value, name, namelen);
	  new_value[namelen] = '=';
	  memcpy (&new_value[namelen + 1], value, vallen);
# endif

	  np = KNOWN_VALUE (new_value);
	  if (__glibc_likely (np == NULL))
#endif
	    {
#ifdef USE_TSEARCH
	      np = new_value;
#endif
	      /* And remember the value.  */
	      STORE_VALUE (np);
	    }
#ifdef USE_TSEARCH
	  else
	    free (new_value);
#endif
	}

      /* Use release MO so that loads are sufficient to observe the
	 pointer contents because the CPU carries the dependency for
	 us.  This also acts as a thread fence, making getenv
	 async-signal-safe.  */
      atomic_store_release (ep, np);
      atomic_store_release (&__environ, result_environ);
    }

  UNLOCK;

  return 0;
}

int
setenv (const char *name, const char *value, int replace)
{
  if (name == NULL || *name == '\0' || strchr (name, '=') != NULL)
    {
      __set_errno (EINVAL);
      return -1;
    }

  return __add_to_environ (name, value, NULL, replace);
}

int
unsetenv (const char *name)
{
  size_t len;
  char **ep;

  if (name == NULL || *name == '\0' || strchr (name, '=') != NULL)
    {
      __set_errno (EINVAL);
      return -1;
    }

  len = strlen (name);

  LOCK;

  ep = atomic_load_relaxed (&__environ);
  if (ep != NULL)
    while (true)
      {
	char *entry = atomic_load_relaxed (ep);
	if (entry == NULL)
	  break;
	if (strncmp (entry, name, len) == 0 && entry[len] == '=')
	  {
	    /* Found it.  Remove this pointer by moving later ones back.  */
	    char **dp = ep;

	    while (true)
	      {
		char *next_value = atomic_load_relaxed (dp + 1);
		/* This store overwrites a value that has been
		   removed, or that has already been written to a
		   previous value.  Release MO so that this store does
		   not get reordered before the counter update in the
		   previous loop iteration.  */
		atomic_store_release (dp, next_value);
		/* Release store synchronizes with acquire loads in
		   getenv.  Non-atomic update because there is just
		   one writer due to the lock.

		   See discussion of the counter check in getenv for
		   an explanation why this is sufficient synchronization.  */
		atomic_store_release (&__environ_counter,
				      atomic_load_relaxed (&__environ_counter)
				      + 1);
		if (next_value == NULL)
		  break;
		++dp;
	      }
	    /* Continue the loop in case NAME appears again.  */
	  }
	else
	  ++ep;
      }

  UNLOCK;

  return 0;
}

/* The `clearenv' was planned to be added to POSIX.1 but probably
   never made it.  Nevertheless the POSIX.9 standard (POSIX bindings
   for Fortran 77) requires this function.  */
int
clearenv (void)
{
  LOCK;
  char **start_environ = atomic_load_relaxed (&__environ);
  if (__environ_is_from_array_list (start_environ))
    {
      /* Store null pointers to avoid strange effects when the array
	 is reused in setenv.  */
      for (char **ep = start_environ; *ep != NULL; ++ep)
	atomic_store_relaxed (ep, NULL);
      /* Update the counter similar to unsetenv, so that the writes in
	 setenv do not need to update the counter.  */
      atomic_store_release (&__environ_counter,
			    atomic_load_relaxed (&__environ_counter) + 1);
    }

  atomic_store_relaxed (&__environ, NULL);
  UNLOCK;

  return 0;
}
#ifdef _LIBC
void
__libc_setenv_freemem (void)
{
  /* Remove all traces.  */
  clearenv ();

  /* Clear all backing arrays.  */
  while (__environ_array_list != NULL)
    {
      free (__environ_array_list->array);
      void *ptr = __environ_array_list;
      __environ_array_list = __environ_array_list->next;
      free (ptr);
    }

  /* Now remove the search tree.  */
  __tdestroy (known_values, free);
  known_values = NULL;
}

# undef setenv
# undef unsetenv
# undef clearenv
weak_alias (__setenv, setenv)
weak_alias (__unsetenv, unsetenv)
weak_alias (__clearenv, clearenv)
#endif