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接前一篇文章：QEMU源码全解析28 —— QOM介绍（17）
本文内容参考：
《趣谈Linux操作系统》 —— 刘超，极客时间
《QEMU/KVM》源码解析与应用 —— 李强，机械工业出版社
特此致谢！
前文讲解了类属性的添加接口。本文讲解类属性的设置及获取接口。
每一种类属性都有自己的add函数，如布尔（bool）、字符串（str）、enum（枚举）等。它们都在qom/object.c中，逐个来看：
ObjectProperty *
object_class_property_add_bool(ObjectClass *klass, const char *name,bool (*get)(Object *, Error **),void (*set)(Object *, bool, Error **))
{BoolProperty *prop = g_malloc0(sizeof(*prop));prop->get = get;prop->set = set;return object_class_property_add(klass, name, "bool",get ? property_get_bool : NULL,set ? property_set_bool : NULL,NULL,prop);
}ObjectProperty *
object_class_property_add_enum(ObjectClass *klass, const char *name,const char *typename,const QEnumLookup *lookup,int (*get)(Object *, Error **),void (*set)(Object *, int, Error **))
{EnumProperty *prop = g_malloc(sizeof(*prop));prop->lookup = lookup;prop->get = get;prop->set = set;return object_class_property_add(klass, name, typename,get ? property_get_enum : NULL,set ? property_set_enum : NULL,NULL,prop);
}ObjectProperty *
object_class_property_add_str(ObjectClass *klass, const char *name,char *(*get)(Object *, Error **),void (*set)(Object *, const char *,Error **))
{StringProperty *prop = g_malloc0(sizeof(*prop));prop->get = get;prop->set = set;return object_class_property_add(klass, name, "string",get ? property_get_str : NULL,set ? property_set_str : NULL,NULL,prop);
}这次以str为例进行讲解。上一回已经讲过，类属性的添加是通过object_class_property_add接口完成的。而str类型属性的添加object_class_property_add_str函数则是调用了此接口。为了便于理解，再次贴出object_class_property_add函数代码，在qom/object.c中，如下：
ObjectProperty *
object_class_property_add(ObjectClass *klass,const char *name,const char *type,ObjectPropertyAccessor *get,ObjectPropertyAccessor *set,ObjectPropertyRelease *release,void *opaque)
{ObjectProperty *prop;assert(!object_class_property_find(klass, name));prop = g_malloc0(sizeof(*prop));prop->name = g_strdup(name);prop->type = g_strdup(type);prop->get = get;prop->set = set;prop->release = release;prop->opaque = opaque;g_hash_table_insert(klass->properties, prop->name, prop);return prop;
}property_set_str和property_get_str函数在前文都已给出过，这里再贴一下，在同文件（qom/object.c）中：
static void property_set_str(Object *obj, Visitor *v, const char *name,void *opaque, Error **errp)
{StringProperty *prop = opaque;char *value;if (!visit_type_str(v, name, &value, errp)) {return;}prop->set(obj, value, errp);g_free(value);
}static void property_get_str(Object *obj, Visitor *v, const char *name,void *opaque, Error **errp)
{StringProperty *prop = opaque;char *value;Error *err = NULL;value = prop->get(obj, &err);if (err) {error_propagate(errp, err);return;}visit_type_str(v, name, &value, errp);g_free(value);
}从这两个函数中就可以看到，StringProperty结构的set和get成员函数为调用object_class_property_add_str函数时传入的参数，形参分别为：void (*set)(Object *, const char *, Error **))和char *(*get)(Object *, Error **)。在object_property_add_str函数中使用了上边的property_set_str和property_get_str函数。在property_set_str函数中最终调用了形参void (*set)(Object *, const char *, Error **)对应的实参，即真正类型属性的设置函数；在property_get_str函数中最终调用了形参char *(*get)(Object *, Error **)对应的实参，即真正的类型属性的获取函数。
仍以machine为例，hw/core/machine.c的machine_class_init函数代码如下：
static void machine_class_init(ObjectClass *oc, void *data)
{MachineClass *mc = MACHINE_CLASS(oc);/* Default 128 MB as guest ram size */mc->default_ram_size = 128 * MiB;mc->rom_file_has_mr = true;/* numa node memory size aligned on 8MB by default.* On Linux, each node's border has to be 8MB aligned*/mc->numa_mem_align_shift = 23;object_class_property_add_str(oc, "kernel",machine_get_kernel, machine_set_kernel);object_class_property_set_description(oc, "kernel","Linux kernel image file");object_class_property_add_str(oc, "initrd",machine_get_initrd, machine_set_initrd);object_class_property_set_description(oc, "initrd","Linux initial ramdisk file");object_class_property_add_str(oc, "append",machine_get_append, machine_set_append);object_class_property_set_description(oc, "append","Linux kernel command line");object_class_property_add_str(oc, "dtb",machine_get_dtb, machine_set_dtb);object_class_property_set_description(oc, "dtb","Linux kernel device tree file");object_class_property_add_str(oc, "dumpdtb",machine_get_dumpdtb, machine_set_dumpdtb);object_class_property_set_description(oc, "dumpdtb","Dump current dtb to a file and quit");object_class_property_add(oc, "boot", "BootConfiguration",machine_get_boot, machine_set_boot,NULL, NULL);object_class_property_set_description(oc, "boot","Boot configuration");object_class_property_add(oc, "smp", "SMPConfiguration",machine_get_smp, machine_set_smp,NULL, NULL);object_class_property_set_description(oc, "smp","CPU topology");object_class_property_add(oc, "phandle-start", "int",machine_get_phandle_start, machine_set_phandle_start,NULL, NULL);object_class_property_set_description(oc, "phandle-start","The first phandle ID we may generate dynamically");object_class_property_add_str(oc, "dt-compatible",machine_get_dt_compatible, machine_set_dt_compatible);object_class_property_set_description(oc, "dt-compatible","Overrides the \"compatible\" property of the dt root node");object_class_property_add_bool(oc, "dump-guest-core",machine_get_dump_guest_core, machine_set_dump_guest_core);object_class_property_set_description(oc, "dump-guest-core","Include guest memory in a core dump");object_class_property_add_bool(oc, "mem-merge",machine_get_mem_merge, machine_set_mem_merge);object_class_property_set_description(oc, "mem-merge","Enable/disable memory merge support");object_class_property_add_bool(oc, "usb",machine_get_usb, machine_set_usb);object_class_property_set_description(oc, "usb","Set on/off to enable/disable usb");object_class_property_add_bool(oc, "graphics",machine_get_graphics, machine_set_graphics);object_class_property_set_description(oc, "graphics","Set on/off to enable/disable graphics emulation");object_class_property_add_str(oc, "firmware",machine_get_firmware, machine_set_firmware);object_class_property_set_description(oc, "firmware","Firmware image");object_class_property_add_bool(oc, "suppress-vmdesc",machine_get_suppress_vmdesc, machine_set_suppress_vmdesc);object_class_property_set_description(oc, "suppress-vmdesc","Set on to disable self-describing migration");object_class_property_add_link(oc, "confidential-guest-support",TYPE_CONFIDENTIAL_GUEST_SUPPORT,offsetof(MachineState, cgs),machine_check_confidential_guest_support,OBJ_PROP_LINK_STRONG);object_class_property_set_description(oc, "confidential-guest-support","Set confidential guest scheme to support");/* For compatibility */object_class_property_add_str(oc, "memory-encryption",machine_get_memory_encryption, machine_set_memory_encryption);object_class_property_set_description(oc, "memory-encryption","Set memory encryption object to use");object_class_property_add_link(oc, "memory-backend", TYPE_MEMORY_BACKEND,offsetof(MachineState, memdev), object_property_allow_set_link,OBJ_PROP_LINK_STRONG);object_class_property_set_description(oc, "memory-backend","Set RAM backend""Valid value is ID of hostmem based backend");object_class_property_add(oc, "memory", "MemorySizeConfiguration",machine_get_mem, machine_set_mem,NULL, NULL);object_class_property_set_description(oc, "memory","Memory size configuration");
}此函数中很多地方调用了object_class_property_add_str函数，以其中之一为例：
object_class_property_add_str(oc, "kernel",machine_get_kernel, machine_set_kernel);代码中的machine_set_kernel和machine_get_kernel函数就是传入object_class_property_add_str函数的与形参对应的实参，也即对于machine类的属性进行设置和获取时实际调用的函数。
至此，QEMU中的QOM相关的内容就简要介绍完了。欲知后事如何，且看下回分解。