| Message ID | 20190829160710.8792-4-berrange@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | docs: add docs about use of automatic cleanup functions | expand |
Daniel P. Berrangé <berrange@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 <berrange@redhat.com> Reviewed-by: Alex Bennée <alex.bennee@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
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 ============================
Document the use of g_autofree and g_autoptr in glib for automatic freeing of memory. Signed-off-by: Daniel P. Berrangé <berrange@redhat.com> --- CODING_STYLE.rst | 85 ++++++++++++++++++++++++++++++++++++++++++++++++ 1 file changed, 85 insertions(+)