Document the use of g_autofree and g_autoptr in glib for automatic
freeing of memory.

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
---
 CODING_STYLE.rst | 85 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 85 insertions(+)

diff --git a/CODING_STYLE.rst b/CODING_STYLE.rst
index 4501d87352..39397f0f6f 100644
--- a/CODING_STYLE.rst
+++ b/CODING_STYLE.rst
@@ -441,6 +441,91 @@ In addition, QEMU assumes that the compiler does not use the latitude
 given in C99 and C11 to treat aspects of signed '<<' as undefined, as
 documented in the GNU Compiler Collection manual starting at version 4.0.
 
+Automatic memory deallocation
+=============================
+
+QEMU has a mandatory dependency either the GCC or CLang compiler. As
+such it has the freedom to make use of a C language extension for
+automatically running a cleanup function when a stack variable goes
+out of scope. This can be used to simplify function cleanup paths,
+often allowing many goto jumps to be eliminated, through automatic
+free'ing of memory.
+
+The GLib2 library provides a number of functions/macros for enabling
+automatic cleanup:
+
+  `<https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html>`_
+
+Most notably:
+
+* g_autofree - will invoke g_free() on the variable going out of scope
+
+* g_autoptr - for structs / objects, will invoke the cleanup func created
+  by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This is
+  supported for most GLib data types and GObjects
+
+For example, instead of
+
+.. code-block:: c
+
+    int somefunc(void) {
+        int ret = -1;
+        char *foo = g_strdup_printf("foo%", "wibble");
+        GList *bar = .....
+
+        if (eek) {
+           goto cleanup;
+        }
+
+        ret = 0;
+
+      cleanup:
+        g_free(foo);
+        g_list_free(bar);
+        return ret;
+    }
+
+Using g_autofree/g_autoptr enables the code to be written as:
+
+.. code-block:: c
+
+    int somefunc(void) {
+        g_autofree char *foo = g_strdup_printf("foo%", "wibble");
+        g_autoptr (GList) bar = .....
+
+        if (eek) {
+           return -1;
+        }
+
+        return 0;
+    }
+
+While this generally results in simpler, less leak-prone code, there
+are still some caveats to beware of
+
+* Variables declared with g_auto* MUST always be initialized,
+  otherwise the cleanup function will use uninitialized stack memory
+
+* If a variable declared with g_auto* holds a value which must
+  live beyond the life of the function, that value must be saved
+  and the original variable NULL'd out. This can be simpler using
+  g_steal_pointer
+
+
+.. code-block:: c
+
+    char *somefunc(void) {
+        g_autofree char *foo = g_strdup_printf("foo%", "wibble");
+        g_autoptr (GList) bar = .....
+
+        if (eek) {
+           return NULL;
+        }
+
+        return g_steal_pointer(&foo);
+    }
+
+
 Error handling and reporting
 ============================
 
-- 
2.21.0

Daniel P. Berrangé <berrange@redhat.com> writes:

> Document the use of g_autofree and g_autoptr in glib for automatic
> freeing of memory.
>
> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>

> ---
>  CODING_STYLE.rst | 85 ++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 85 insertions(+)
>
> diff --git a/CODING_STYLE.rst b/CODING_STYLE.rst
> index 4501d87352..39397f0f6f 100644
> --- a/CODING_STYLE.rst
> +++ b/CODING_STYLE.rst
> @@ -441,6 +441,91 @@ In addition, QEMU assumes that the compiler does not use the latitude
>  given in C99 and C11 to treat aspects of signed '<<' as undefined, as
>  documented in the GNU Compiler Collection manual starting at version 4.0.
>
> +Automatic memory deallocation
> +=============================
> +
> +QEMU has a mandatory dependency either the GCC or CLang compiler. As
> +such it has the freedom to make use of a C language extension for
> +automatically running a cleanup function when a stack variable goes
> +out of scope. This can be used to simplify function cleanup paths,
> +often allowing many goto jumps to be eliminated, through automatic
> +free'ing of memory.
> +
> +The GLib2 library provides a number of functions/macros for enabling
> +automatic cleanup:
> +
> +  `<https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html>`_
> +
> +Most notably:
> +
> +* g_autofree - will invoke g_free() on the variable going out of scope
> +
> +* g_autoptr - for structs / objects, will invoke the cleanup func created
> +  by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This is
> +  supported for most GLib data types and GObjects
> +
> +For example, instead of
> +
> +.. code-block:: c
> +
> +    int somefunc(void) {
> +        int ret = -1;
> +        char *foo = g_strdup_printf("foo%", "wibble");
> +        GList *bar = .....
> +
> +        if (eek) {
> +           goto cleanup;
> +        }
> +
> +        ret = 0;
> +
> +      cleanup:
> +        g_free(foo);
> +        g_list_free(bar);
> +        return ret;
> +    }
> +
> +Using g_autofree/g_autoptr enables the code to be written as:
> +
> +.. code-block:: c
> +
> +    int somefunc(void) {
> +        g_autofree char *foo = g_strdup_printf("foo%", "wibble");
> +        g_autoptr (GList) bar = .....
> +
> +        if (eek) {
> +           return -1;
> +        }
> +
> +        return 0;
> +    }
> +
> +While this generally results in simpler, less leak-prone code, there
> +are still some caveats to beware of
> +
> +* Variables declared with g_auto* MUST always be initialized,
> +  otherwise the cleanup function will use uninitialized stack memory
> +
> +* If a variable declared with g_auto* holds a value which must
> +  live beyond the life of the function, that value must be saved
> +  and the original variable NULL'd out. This can be simpler using
> +  g_steal_pointer
> +
> +
> +.. code-block:: c
> +
> +    char *somefunc(void) {
> +        g_autofree char *foo = g_strdup_printf("foo%", "wibble");
> +        g_autoptr (GList) bar = .....
> +
> +        if (eek) {
> +           return NULL;
> +        }
> +
> +        return g_steal_pointer(&foo);
> +    }
> +
> +
>  Error handling and reporting
>  ============================


--
Alex Bennée