Daniel P. Berrangé <berra...@redhat.com> writes:
> Document the use of g_autofree and g_autoptr in glib for automatic > freeing of memory, or other resource cleanup (eg mutex unlocking). > > Signed-off-by: Daniel P. Berrangé <berra...@redhat.com> > --- > CODING_STYLE.md | 101 ++++++++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 101 insertions(+) > > diff --git a/CODING_STYLE.md b/CODING_STYLE.md > index 9f4fc9dc77..f37b6c2d01 100644 > --- a/CODING_STYLE.md > +++ b/CODING_STYLE.md > @@ -479,3 +479,104 @@ terminate QEMU. > > Note that &error_fatal is just another way to exit(1), and &error_abort > is just another way to abort(). > + > + > +## 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 > + > + 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: > + > + 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 > + > + > + 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); > + } All good so far. > + > +The cleanup functions are not restricted to simply free'ing memory. The > +GMutexLocker class is a variant of GMutex that has automatic locking and > +unlocking at start and end of the enclosing scope > + > +In the following example, the `lock` in `MyObj` will be held for the > +precise duration of the `somefunc` function > + > + typedef struct { > + GMutex lock; > + } MyObj; > + > + char *somefunc(MyObj *obj) { > + g_autofree GMutexLocker *locker = g_mutex_locker_new(&obj->lock) > + g_autofree char *foo = g_strdup_printf("foo%", "wibble"); > + g_autoptr (GList) bar = ..... > + > + if (eek) { > + return NULL; > + } > + > + return g_steal_pointer(&foo); > + } I would personally prefer we get some RFC patches for auto-unlocking under our belt before we codify it's usage in our developer docs. Locking is a fickle beast at the best of times and I'd like to see where it benefits us before there is a rush to covert to the new style. For one thing the only uses I see of g_mutex_lock is in our tests, the main code base uses qemu_mutex_lock. How would we go about registering the clean-up functions for those in the code base? But apart from the lock stuff: Reviewed-by: Alex Bennée <alex.ben...@linaro.org> -- Alex Bennée