docs: document use of automatic cleanup functions in glib
Document the use of g_autofree and g_autoptr in glib for automatic freeing of memory. Reviewed-by: Alex Bennée <alex.bennee@linaro.org> Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
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@ -441,6 +441,91 @@ In addition, QEMU assumes that the compiler does not use the latitude
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given in C99 and C11 to treat aspects of signed '<<' as undefined, as
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documented in the GNU Compiler Collection manual starting at version 4.0.
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Automatic memory deallocation
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=============================
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QEMU has a mandatory dependency either the GCC or CLang compiler. As
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such it has the freedom to make use of a C language extension for
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automatically running a cleanup function when a stack variable goes
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out of scope. This can be used to simplify function cleanup paths,
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often allowing many goto jumps to be eliminated, through automatic
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free'ing of memory.
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The GLib2 library provides a number of functions/macros for enabling
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automatic cleanup:
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`<https://developer.gnome.org/glib/stable/glib-Miscellaneous-Macros.html>`_
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Most notably:
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* g_autofree - will invoke g_free() on the variable going out of scope
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* g_autoptr - for structs / objects, will invoke the cleanup func created
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by a previous use of G_DEFINE_AUTOPTR_CLEANUP_FUNC. This is
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supported for most GLib data types and GObjects
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For example, instead of
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.. code-block:: c
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int somefunc(void) {
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int ret = -1;
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char *foo = g_strdup_printf("foo%", "wibble");
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GList *bar = .....
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if (eek) {
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goto cleanup;
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}
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ret = 0;
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cleanup:
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g_free(foo);
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g_list_free(bar);
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return ret;
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}
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Using g_autofree/g_autoptr enables the code to be written as:
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.. code-block:: c
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int somefunc(void) {
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g_autofree char *foo = g_strdup_printf("foo%", "wibble");
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g_autoptr (GList) bar = .....
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if (eek) {
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return -1;
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}
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return 0;
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}
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While this generally results in simpler, less leak-prone code, there
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are still some caveats to beware of
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* Variables declared with g_auto* MUST always be initialized,
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otherwise the cleanup function will use uninitialized stack memory
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* If a variable declared with g_auto* holds a value which must
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live beyond the life of the function, that value must be saved
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and the original variable NULL'd out. This can be simpler using
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g_steal_pointer
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.. code-block:: c
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char *somefunc(void) {
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g_autofree char *foo = g_strdup_printf("foo%", "wibble");
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g_autoptr (GList) bar = .....
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if (eek) {
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return NULL;
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}
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return g_steal_pointer(&foo);
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}
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Error handling and reporting
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============================
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