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Re: [Qemu-devel] [PATCH v2 3/4] docs: document use of automatic cleanup
From: |
Alex Bennée |
Subject: |
Re: [Qemu-devel] [PATCH v2 3/4] docs: document use of automatic cleanup functions in glib |
Date: |
Thu, 29 Aug 2019 17:55:47 +0100 |
User-agent: |
mu4e 1.3.4; emacs 27.0.50 |
Daniel P. Berrangé <address@hidden> writes:
> Document the use of g_autofree and g_autoptr in glib for automatic
> freeing of memory.
>
> Signed-off-by: Daniel P. Berrangé <address@hidden>
Reviewed-by: Alex Bennée <address@hidden>
> ---
> 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
- [Qemu-devel] [PATCH v2 0/4] docs: add docs about use of automatic cleanup functions, Daniel P . Berrangé, 2019/08/29
- [Qemu-devel] [PATCH v2 1/4] docs: convert README, CODING_STYLE and HACKING to RST syntax, Daniel P . Berrangé, 2019/08/29
- [Qemu-devel] [PATCH v2 2/4] docs: merge HACKING.rst contents into CODING_STYLE.rst, Daniel P . Berrangé, 2019/08/29
- [Qemu-devel] [PATCH v2 3/4] docs: document use of automatic cleanup functions in glib, Daniel P . Berrangé, 2019/08/29
- Re: [Qemu-devel] [PATCH v2 3/4] docs: document use of automatic cleanup functions in glib,
Alex Bennée <=
- [Qemu-devel] [PATCH v2 4/4] docs: split the CODING_STYLE doc into distinct groups, Daniel P . Berrangé, 2019/08/29
- Re: [Qemu-devel] [PATCH v2 0/4] docs: add docs about use of automatic cleanup functions, no-reply, 2019/08/29
- Re: [Qemu-devel] [PATCH v2 0/4] docs: add docs about use of automatic cleanup functions, no-reply, 2019/08/29
- Re: [Qemu-devel] [PATCH v2 0/4] docs: add docs about use of automatic cleanup functions, no-reply, 2019/08/29