From f7af5e52d7ada47a38d309aea76e166be27f6288 Mon Sep 17 00:00:00 2001
From: Steve Vinoski <vinoski@ieee.org>
Date: Thu, 3 Mar 2011 10:07:15 -0500
Subject: clarify NIF resource object deallocation documentation
In the erl_nif documentation, clarify how and when NIF resource
objects can be deallocated. Specifically, add focus for the case of
calling enif_release_resource immediately after obtaining a resource
term from enif_make_resource, since this is likely to be a common
approach NIFs use to manage resources.
Also fix a couple misspellings in the erl_nif documentation.
---
erts/doc/src/erl_nif.xml | 49 +++++++++++++++++++++++++++++-------------------
1 file changed, 30 insertions(+), 19 deletions(-)
(limited to 'erts')
diff --git a/erts/doc/src/erl_nif.xml b/erts/doc/src/erl_nif.xml
index 5987ddbd5e..4bbd4e2a54 100644
--- a/erts/doc/src/erl_nif.xml
+++ b/erts/doc/src/erl_nif.xml
@@ -193,9 +193,9 @@ ok
A handle ("safe pointer") to this memory block can then be returned to Erlang by the use of
<seealso marker="#enif_make_resource">enif_make_resource</seealso>.
The term returned by <c>enif_make_resource</c>
- is totally opaque in nature. It can be stored and passed between processses
- on the same node, but the only real end usage is to pass it back as argument to a NIF.
- The NIF can then do <seealso marker="#enif_get_resource">enif_get_resource</seealso>
+ is totally opaque in nature. It can be stored and passed between processes
+ on the same node, but the only real end usage is to pass it back as an argument to a NIF.
+ The NIF can then call <seealso marker="#enif_get_resource">enif_get_resource</seealso>
and get back a pointer to the memory block that is guaranteed to still be
valid. A resource object will not be deallocated until the last handle term
has been garbage collected by the VM and the resource has been
@@ -212,17 +212,7 @@ ok
the garbage collector or <c>enif_release_resource</c>). Resource types
are uniquely identified by a supplied name string and the name of the
implementing module.</p>
- <p>Resource types support upgrade in runtime by allowing a loaded NIF
- library to takeover an already existing resource type and thereby
- "inherit" all existing objects of that type. The destructor of the new
- library will thereafter be called for the inherited objects and the
- library with the old destructor function can be safely unloaded. Existing
- resource objects, of a module that is upgraded, must either be deleted
- or taken over by the new NIF library. The unloading of a library will be
- postponed as long as there exist resource objects with a destructor
- function in the library.
- </p>
- <p>Here is a template example of how to create and return a resource object.</p>
+ <marker id="enif_resource_example"/><p>Here is a template example of how to create and return a resource object.</p>
<p/>
<code type="none">
ERL_NIF_TERM term;
@@ -240,8 +230,13 @@ ok
/* resource now only owned by "Erlang" */
}
return term;
-}
-</code>
+ </code>
+ <p>Note that once <c>enif_make_resource</c> creates the term to
+ return to Erlang, the code can choose to either keep its own
+ native pointer to the allocated struct and release it later, or
+ release it immediately and rely solely on the garbage collector
+ to eventually deallocate the resource object when it collects
+ the term.</p>
<p>Another usage of resource objects is to create binary terms with
user defined memory management.
<seealso marker="#enif_make_resource_binary">enif_make_resource_binary</seealso>
@@ -251,6 +246,16 @@ ok
this can be a binary term consisting of data from a <c>mmap</c>'ed file.
The destructor can then do <c>munmap</c> to release the memory
region.</p>
+ <p>Resource types support upgrade in runtime by allowing a loaded NIF
+ library to takeover an already existing resource type and thereby
+ "inherit" all existing objects of that type. The destructor of the new
+ library will thereafter be called for the inherited objects and the
+ library with the old destructor function can be safely unloaded. Existing
+ resource objects, of a module that is upgraded, must either be deleted
+ or taken over by the new NIF library. The unloading of a library will be
+ postponed as long as there exist resource objects with a destructor
+ function in the library.
+ </p>
</item>
<tag>Threads and concurrency</tag>
<item><p>A NIF is thread-safe without any explicit synchronization as
@@ -368,7 +373,7 @@ ok
environments between NIF calls. </p>
<p>A <em>process independent environment</em> is created by calling
<seealso marker="#enif_alloc_env">enif_alloc_env</seealso>. It can be
- used to store terms beteen NIF calls and to send terms with
+ used to store terms between NIF calls and to send terms with
<seealso marker="#enif_send">enif_send</seealso>. A process
independent environment with all its terms is valid until you explicitly
invalidates it with <seealso marker="#enif_free_env">enif_free_env</seealso>
@@ -832,8 +837,14 @@ typedef enum {
<fsummary>Create an opaque handle to a resource object</fsummary>
<desc><p>Create an opaque handle to a memory managed resource object
obtained by <seealso marker="#enif_alloc_resource">enif_alloc_resource</seealso>.
- No ownership transfer is done, the resource object still needs to be released by
- <seealso marker="#enif_release_resource">enif_release_resource</seealso>.</p>
+ No ownership transfer is done, as the resource object still needs to be released by
+ <seealso marker="#enif_release_resource">enif_release_resource</seealso>,
+ but note that the call to <c>enif_release_resource</c> can occur
+ immediately after obtaining the term from <c>enif_make_resource</c>,
+ in which case the resource object will be deallocated when the
+ term is garbage collected. See the
+ <seealso marker="#enif_resource_example">example of creating and
+ returning a resource object</seealso> for more details.</p>
<p>Note that the only defined behaviour of using a resource term in
an Erlang program is to store it and send it between processes on the
same node. Other operations such as matching or <c>term_to_binary</c>
--
cgit v1.2.3