text: #include "jemalloc/internal/jemalloc_internal.h"
  #ifndef JEMALLOC_ZONE
  #  error "This source file is for zones on Darwin (OS X)."
  #endif
  
  /*
   * The malloc_default_purgeable_zone() function is only available on >= 10.6.
   * We need to check whether it is present at runtime, thus the weak_import.
   */
  extern malloc_zone_t *malloc_default_purgeable_zone(void)
  JEMALLOC_ATTR(weak_import);
  
  /******************************************************************************/
  /* Data. */
  
  static malloc_zone_t *default_zone, *purgeable_zone;
  static malloc_zone_t jemalloc_zone;
  static struct malloc_introspection_t jemalloc_zone_introspect;
  
  /******************************************************************************/
  /* Function prototypes for non-inline static functions. */
  
  static size_t	zone_size(malloc_zone_t *zone, void *ptr);
  static void	*zone_malloc(malloc_zone_t *zone, size_t size);
  static void	*zone_calloc(malloc_zone_t *zone, size_t num, size_t size);
  static void	*zone_valloc(malloc_zone_t *zone, size_t size);
  static void	zone_free(malloc_zone_t *zone, void *ptr);
  static void	*zone_realloc(malloc_zone_t *zone, void *ptr, size_t size);
  #if (JEMALLOC_ZONE_VERSION >= 5)
  static void	*zone_memalign(malloc_zone_t *zone, size_t alignment,
  #endif
  #if (JEMALLOC_ZONE_VERSION >= 6)
      size_t size);
  static void	zone_free_definite_size(malloc_zone_t *zone, void *ptr,
      size_t size);
  #endif
  static void	*zone_destroy(malloc_zone_t *zone);
  static size_t	zone_good_size(malloc_zone_t *zone, size_t size);
  static void	zone_force_lock(malloc_zone_t *zone);
  static void	zone_force_unlock(malloc_zone_t *zone);
  
  /******************************************************************************/
  /*
   * Functions.
   */
  
  static size_t
  zone_size(malloc_zone_t *zone, void *ptr)
  {
  
  	/*
  	 * There appear to be places within Darwin (such as setenv(3)) that
  	 * cause calls to this function with pointers that *no* zone owns.  If
  	 * we knew that all pointers were owned by *some* zone, we could split
  	 * our zone into two parts, and use one as the default allocator and
  	 * the other as the default deallocator/reallocator.  Since that will
  	 * not work in practice, we must check all pointers to assure that they
  	 * reside within a mapped extent before determining size.
  	 */
  	return (ivsalloc(tsdn_fetch(), ptr));
  }
  
  static void *
  zone_malloc(malloc_zone_t *zone, size_t size)
  {
  
  	return (je_malloc(size));
  }
  
  static void *
  zone_calloc(malloc_zone_t *zone, size_t num, size_t size)
  {
  
  	return (je_calloc(num, size));
  }
  
  static void *
  zone_valloc(malloc_zone_t *zone, size_t size)
  {
  	void *ret = NULL; /* Assignment avoids useless compiler warning. */
  
  	je_posix_memalign(&ret, PAGE, size);
  
  	return (ret);
  }
  
  static void
  zone_free(malloc_zone_t *zone, void *ptr)
  {
  
  	if (ivsalloc(tsdn_fetch(), ptr) != 0) {
  		je_free(ptr);
  		return;
  	}
  
  	free(ptr);
  }
  
  static void *
  zone_realloc(malloc_zone_t *zone, void *ptr, size_t size)
  {
  
  	if (ivsalloc(tsdn_fetch(), ptr) != 0)
  		return (je_realloc(ptr, size));
  
  	return (realloc(ptr, size));
  }
  
  #if (JEMALLOC_ZONE_VERSION >= 5)
  static void *
  zone_memalign(malloc_zone_t *zone, size_t alignment, size_t size)
  {
  	void *ret = NULL; /* Assignment avoids useless compiler warning. */
  
  	je_posix_memalign(&ret, alignment, size);
  
  	return (ret);
  }
  #endif
  
  #if (JEMALLOC_ZONE_VERSION >= 6)
  static void
  zone_free_definite_size(malloc_zone_t *zone, void *ptr, size_t size)
  {
  	size_t alloc_size;
  
  	alloc_size = ivsalloc(tsdn_fetch(), ptr);
  	if (alloc_size != 0) {
  		assert(alloc_size == size);
  		je_free(ptr);
  		return;
  	}
  
  	free(ptr);
  }
  #endif
  
  static void *
  zone_destroy(malloc_zone_t *zone)
  {
  
  	/* This function should never be called. */
  	not_reached();
  	return (NULL);
  }
  
  static size_t
  zone_good_size(malloc_zone_t *zone, size_t size)
  {
  
  	if (size == 0)
  		size = 1;
  	return (s2u(size));
  }
  
  static void
  zone_force_lock(malloc_zone_t *zone)
  {
  
  	if (isthreaded)
  		jemalloc_prefork();
  }
  
  static void
  zone_force_unlock(malloc_zone_t *zone)
  {
  
  	/*
  	 * Call jemalloc_postfork_child() rather than
  	 * jemalloc_postfork_parent(), because this function is executed by both
  	 * parent and child.  The parent can tolerate having state
  	 * reinitialized, but the child cannot unlock mutexes that were locked
  	 * by the parent.
  	 */
  	if (isthreaded)
  		jemalloc_postfork_child();
  }
  
  static void
  zone_init(void)
  {
  
  	jemalloc_zone.size = (void *)zone_size;
  	jemalloc_zone.malloc = (void *)zone_malloc;
  	jemalloc_zone.calloc = (void *)zone_calloc;
  	jemalloc_zone.valloc = (void *)zone_valloc;
  	jemalloc_zone.free = (void *)zone_free;
  	jemalloc_zone.realloc = (void *)zone_realloc;
  	jemalloc_zone.destroy = (void *)zone_destroy;
  	jemalloc_zone.zone_name = "jemalloc_zone";
  	jemalloc_zone.batch_malloc = NULL;
  	jemalloc_zone.batch_free = NULL;
  	jemalloc_zone.introspect = &jemalloc_zone_introspect;
  	jemalloc_zone.version = JEMALLOC_ZONE_VERSION;
  #if (JEMALLOC_ZONE_VERSION >= 5)
  	jemalloc_zone.memalign = zone_memalign;
  #endif
  #if (JEMALLOC_ZONE_VERSION >= 6)
  	jemalloc_zone.free_definite_size = zone_free_definite_size;
  #endif
  #if (JEMALLOC_ZONE_VERSION >= 8)
  	jemalloc_zone.pressure_relief = NULL;
  #endif
  
  	jemalloc_zone_introspect.enumerator = NULL;
  	jemalloc_zone_introspect.good_size = (void *)zone_good_size;
  	jemalloc_zone_introspect.check = NULL;
  	jemalloc_zone_introspect.print = NULL;
  	jemalloc_zone_introspect.log = NULL;
  	jemalloc_zone_introspect.force_lock = (void *)zone_force_lock;
  	jemalloc_zone_introspect.force_unlock = (void *)zone_force_unlock;
  	jemalloc_zone_introspect.statistics = NULL;
  #if (JEMALLOC_ZONE_VERSION >= 6)
  	jemalloc_zone_introspect.zone_locked = NULL;
  #endif
  #if (JEMALLOC_ZONE_VERSION >= 7)
  	jemalloc_zone_introspect.enable_discharge_checking = NULL;
  	jemalloc_zone_introspect.disable_discharge_checking = NULL;
  	jemalloc_zone_introspect.discharge = NULL;
  #  ifdef __BLOCKS__
  	jemalloc_zone_introspect.enumerate_discharged_pointers = NULL;
  #  else
  	jemalloc_zone_introspect.enumerate_unavailable_without_blocks = NULL;
  #  endif
  #endif
  }
  
  static malloc_zone_t *
  zone_default_get(void)
  {
  	malloc_zone_t **zones = NULL;
  	unsigned int num_zones = 0;
  
  	/*
  	 * On OSX 10.12, malloc_default_zone returns a special zone that is not
  	 * present in the list of registered zones. That zone uses a "lite zone"
  	 * if one is present (apparently enabled when malloc stack logging is
  	 * enabled), or the first registered zone otherwise. In practice this
  	 * means unless malloc stack logging is enabled, the first registered
  	 * zone is the default.  So get the list of zones to get the first one,
  	 * instead of relying on malloc_default_zone.
  	 */
  	if (KERN_SUCCESS != malloc_get_all_zones(0, NULL,
  	    (vm_address_t**)&zones, &num_zones)) {
  		/*
  		 * Reset the value in case the failure happened after it was
  		 * set.
  		 */
  		num_zones = 0;
  	}
  
  	if (num_zones)
  		return (zones[0]);
  
  	return (malloc_default_zone());
  }
  
  /* As written, this function can only promote jemalloc_zone. */
  static void
  zone_promote(void)
  {
  	malloc_zone_t *zone;
  
  	do {
  		/*
  		 * Unregister and reregister the default zone.  On OSX >= 10.6,
  		 * unregistering takes the last registered zone and places it
  		 * at the location of the specified zone.  Unregistering the
  		 * default zone thus makes the last registered one the default.
  		 * On OSX < 10.6, unregistering shifts all registered zones.
  		 * The first registered zone then becomes the default.
  		 */
  		malloc_zone_unregister(default_zone);
  		malloc_zone_register(default_zone);
  
  		/*
  		 * On OSX 10.6, having the default purgeable zone appear before
  		 * the default zone makes some things crash because it thinks it
  		 * owns the default zone allocated pointers.  We thus
  		 * unregister/re-register it in order to ensure it's always
  		 * after the default zone.  On OSX < 10.6, there is no purgeable
  		 * zone, so this does nothing.  On OSX >= 10.6, unregistering
  		 * replaces the purgeable zone with the last registered zone
  		 * above, i.e. the default zone.  Registering it again then puts
  		 * it at the end, obviously after the default zone.
  		 */
  		if (purgeable_zone != NULL) {
  			malloc_zone_unregister(purgeable_zone);
  			malloc_zone_register(purgeable_zone);
  		}
  
  		zone = zone_default_get();
  	} while (zone != &jemalloc_zone);
  }
  
  JEMALLOC_ATTR(constructor)
  void
  zone_register(void)
  {
  
  	/*
  	 * If something else replaced the system default zone allocator, don't
  	 * register jemalloc's.
  	 */
  	default_zone = zone_default_get();
  	if (!default_zone->zone_name || strcmp(default_zone->zone_name,
  	    "DefaultMallocZone") != 0)
  		return;
  
  	/*
  	 * The default purgeable zone is created lazily by OSX's libc.  It uses
  	 * the default zone when it is created for "small" allocations
  	 * (< 15 KiB), but assumes the default zone is a scalable_zone.  This
  	 * obviously fails when the default zone is the jemalloc zone, so
  	 * malloc_default_purgeable_zone() is called beforehand so that the
  	 * default purgeable zone is created when the default zone is still
  	 * a scalable_zone.  As purgeable zones only exist on >= 10.6, we need
  	 * to check for the existence of malloc_default_purgeable_zone() at
  	 * run time.
  	 */
  	purgeable_zone = (malloc_default_purgeable_zone == NULL) ? NULL :
  	    malloc_default_purgeable_zone();
  
  	/* Register the custom zone.  At this point it won't be the default. */
  	zone_init();
  	malloc_zone_register(&jemalloc_zone);
  
  	/* Promote the custom zone to be default. */
  	zone_promote();
  }
  