Re: [Qemu-devel] [PATCH v3 3/4] docs: document use of automatic cleanup functions in glib

[Eric Blake]

On 8/29/19 11:50 AM, Daniel P. Berrangé wrote:
> Document the use of g_autofree and g_autoptr in glib for automatic
> freeing of memory.
> 
> Signed-off-by: Daniel P. Berrangé 
> ---
>  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

s/either/on either/

> +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.
> +

> +
> +For example, instead of
> +
> +.. code-block:: c
> +
> +int somefunc(void) {

Should that { be on its own line to match our prevailing style?

> +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) {

here too

> +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) {

and again

> +g_autofree char *foo = g_strdup_printf("foo%", "wibble");
> +g_autoptr (GList) bar = .
> +
> +if (eek) {
> +   return NULL;
> +}
> +
> +return g_steal_pointer();
> +}
> +
> +
>  Error handling and reporting
>  
>  
> 

With those fixes,
Reviewed-by: Eric Blake 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.   +1-919-301-3226
Virtualization:  qemu.org | libvirt.org



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[Qemu-devel] [PATCH v3 3/4] docs: document use of automatic cleanup functions in glib

[Daniel P . Berrangé]

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

Signed-off-by: Daniel P. Berrangé 
---
 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:
+
+  ``_
+
+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();
+}
+
+
 Error handling and reporting
 
 
-- 
2.21.0