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); + } + +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); + } -- 2.21.0
