Daniel P. Berrangé <berra...@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 <berra...@redhat.com> Reviewed-by: Alex Bennée <alex.ben...@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