1 files changed, 42 insertions, 6 deletions
diff --git a/manual/memory.texi b/manual/memory.texi
index c957c2f9cf..28c619f899 100644
--- a/manual/memory.texi
+++ b/manual/memory.texi
@@ -731,6 +731,34 @@ should make sure to restore all the hooks to their previous value.  When
 coming back from the recursive call, all the hooks should be resaved
 since a hook might modify itself.
 
+@comment malloc.h
+@comment GNU
+@defvar __malloc_initialize_hook
+The value of this variable is a pointer to a function that is called
+once when the malloc implementation is initialized.  This is a weak
+variable, so it can be overridden in the application with a definition
+like the following:
+
+@smallexample
+void (*@var{__malloc_initialize_hook}) (void) = my_init_hook;
+@end smallexample
+@end defvar
+
+An issue to look out for is the time at which the malloc hook functions
+can be safely installed.  If the hook functions call the malloc-related
+functions recursively, it is necessary that malloc has already properly
+initialized itself at the time when @code{__malloc_hook} etc. is
+assigned to.  On the other hand, if the hook functions provide a
+complete malloc implementation of their own, it is vital that the hooks
+are assigned to @emph{before} the very first @code{malloc} call has
+completed, because otherwise a chunk obtained from the ordinary,
+un-hooked malloc may later be handed to @code{__free_hook}, for example.
+
+In both cases, the problem can be solved by setting up the hooks from
+within a user-defined function pointed to by
+@code{__malloc_initialize_hook}---then the hooks will be set up safely
+at the right time.
+
 Here is an example showing how to use @code{__malloc_hook} and
 @code{__free_hook} properly.  It installs a function that prints out
 information every time @code{malloc} or @code{free} is called.  We just
@@ -743,8 +771,21 @@ static void *(*old_malloc_hook) (size_t);
 static void (*old_free_hook) (void*);
 
 /* Prototypes for our hooks.  */
+static void *my_init_hook (void);
 static void *my_malloc_hook (size_t);
-static void my_free_hook(void*);
+static void my_free_hook (void*);
+
+/* Override initializing hook from the C library. */
+void (*__malloc_initialize_hook) (void) = my_init_hook;
+
+static void
+my_init_hook (void)
+@{
+  old_malloc_hook = __malloc_hook;
+  old_free_hook = __free_hook;
+  __malloc_hook = my_malloc_hook;
+  __free_hook = my_free_hook;
+@}
 
 static void *
 my_malloc_hook (size_t size)
@@ -787,11 +828,6 @@ my_free_hook (void *ptr)
 main ()
 @{
   ...
-  old_malloc_hook = __malloc_hook;
-  old_free_hook = __free_hook;
-  __malloc_hook = my_malloc_hook;
-  __free_hook = my_free_hook;
-  ...
 @}
 @end smallexample