2023-05-26：golang关于垃圾回收和析构函数的选择题，多数人会选错。_天天速递

2023-05-26：golang关于垃圾回收和析构的选择题，代码如下：

package mainimport ("fmt""runtime""time")type ListNode struct {Val  intNext *ListNode}func main0() {a := &ListNode{Val: 1}b := &ListNode{Val: 2}runtime.SetFinalizer(a, func(obj *ListNode) {fmt.Printf("a被回收--")})runtime.SetFinalizer(b, func(obj *ListNode) {fmt.Printf("b被回收--")})a.Next = bb.Next = a}func main() {main0()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()fmt.Print("结束")}

代码的运行结果是什么？并说明原因。注意析构是无序的。

(资料图片)

A. 结束

B. a被回收--b被回收--结束

C. b被回收--a被回收--结束

D. B和C都有可能

答案2023-05-26：

golang的垃圾回收算法跟java一样，都是根可达算法。代码中main0函数里a和b是互相引用，但是a和b没有外部引用。因此a和b会被当成垃圾被回收掉。而析构函数的调用不是有序的，所以B和C都有可能，答案选D。让我们看看答案是什么，如下：

看运行结果，答案不是选D，而是选A。这肯定会出乎很多人意料，golang的垃圾回收算法是根可达算法难不成是假的，大家公认的八股文难道是错的？有这个疑问是好事，但不能全盘否定。让我们看看析构函数的源码吧。代码在 src/runtime/mfinal.go中，如下：

// SetFinalizer sets the finalizer associated with obj to the provided// finalizer function. When the garbage collector finds an unreachable block// with an associated finalizer, it clears the association and runs// finalizer(obj) in a separate goroutine. This makes obj reachable again,// but now without an associated finalizer. Assuming that SetFinalizer// is not called again, the next time the garbage collector sees// that obj is unreachable, it will free obj.//// SetFinalizer(obj, nil) clears any finalizer associated with obj.//// The argument obj must be a pointer to an object allocated by calling// new, by taking the address of a composite literal, or by taking the// address of a local variable.// The argument finalizer must be a function that takes a single argument// to which obj"s type can be assigned, and can have arbitrary ignored return// values. If either of these is not true, SetFinalizer may abort the// program.//// Finalizers are run in dependency order: if A points at B, both have// finalizers, and they are otherwise unreachable, only the finalizer// for A runs; once A is freed, the finalizer for B can run.// If a cyclic structure includes a block with a finalizer, that// cycle is not guaranteed to be garbage collected and the finalizer// is not guaranteed to run, because there is no ordering that// respects the dependencies.//// The finalizer is scheduled to run at some arbitrary time after the// program can no longer reach the object to which obj points.// There is no guarantee that finalizers will run before a program exits,// so typically they are useful only for releasing non-memory resources// associated with an object during a long-running program.// For example, an os.File object could use a finalizer to close the// associated operating system file descriptor when a program discards// an os.File without calling Close, but it would be a mistake// to depend on a finalizer to flush an in-memory I/O buffer such as a// bufio.Writer, because the buffer would not be flushed at program exit.//// It is not guaranteed that a finalizer will run if the size of *obj is// zero bytes, because it may share same address with other zero-size// objects in memory. See https://go.dev/ref/spec#Size_and_alignment_guarantees.//// It is not guaranteed that a finalizer will run for objects allocated// in initializers for package-level variables. Such objects may be// linker-allocated, not heap-allocated.//// Note that because finalizers may execute arbitrarily far into the future// after an object is no longer referenced, the runtime is allowed to perform// a space-saving optimization that batches objects together in a single// allocation slot. The finalizer for an unreferenced object in such an// allocation may never run if it always exists in the same batch as a// referenced object. Typically, this batching only happens for tiny// (on the order of 16 bytes or less) and pointer-free objects.//// A finalizer may run as soon as an object becomes unreachable.// In order to use finalizers correctly, the program must ensure that// the object is reachable until it is no longer required.// Objects stored in global variables, or that can be found by tracing// pointers from a global variable, are reachable. For other objects,// pass the object to a call of the KeepAlive function to mark the// last point in the function where the object must be reachable.//// For example, if p points to a struct, such as os.File, that contains// a file descriptor d, and p has a finalizer that closes that file// descriptor, and if the last use of p in a function is a call to// syscall.Write(p.d, buf, size), then p may be unreachable as soon as// the program enters syscall.Write. The finalizer may run at that moment,// closing p.d, causing syscall.Write to fail because it is writing to// a closed file descriptor (or, worse, to an entirely different// file descriptor opened by a different goroutine). To avoid this problem,// call KeepAlive(p) after the call to syscall.Write.//// A single goroutine runs all finalizers for a program, sequentially.// If a finalizer must run for a long time, it should do so by starting// a new goroutine.//// In the terminology of the Go memory model, a call// SetFinalizer(x, f) “synchronizes before” the finalization call f(x).// However, there is no guarantee that KeepAlive(x) or any other use of x// “synchronizes before” f(x), so in general a finalizer should use a mutex// or other synchronization mechanism if it needs to access mutable state in x.// For example, consider a finalizer that inspects a mutable field in x// that is modified from time to time in the main program before x// becomes unreachable and the finalizer is invoked.// The modifications in the main program and the inspection in the finalizer// need to use appropriate synchronization, such as mutexes or atomic updates,// to avoid read-write races.func SetFinalizer(obj any, finalizer any) {if debug.sbrk != 0 {// debug.sbrk never frees memory, so no finalizers run// (and we don"t have the data structures to record them).return}e := efaceOf(&obj)etyp := e._typeif etyp == nil {throw("runtime.SetFinalizer: first argument is nil")}if etyp.kind&kindMask != kindPtr {throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")}ot := (*ptrtype)(unsafe.Pointer(etyp))if ot.elem == nil {throw("nil elem type!")}if inUserArenaChunk(uintptr(e.data)) {// Arena-allocated objects are not eligible for finalizers.throw("runtime.SetFinalizer: first argument was allocated into an arena")}// find the containing objectbase, _, _ := findObject(uintptr(e.data), 0, 0)if base == 0 {// 0-length objects are okay.if e.data == unsafe.Pointer(&zerobase) {return}// Global initializers might be linker-allocated.//var Foo = &Object{}//func main() {//runtime.SetFinalizer(Foo, nil)//}// The relevant segments are: noptrdata, data, bss, noptrbss.// We cannot assume they are in any order or even contiguous,// due to external linking.for datap := &firstmoduledata; datap != nil; datap = datap.next {if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {return}}throw("runtime.SetFinalizer: pointer not in allocated block")}if uintptr(e.data) != base {// As an implementation detail we allow to set finalizers for an inner byte// of an object if it could come from tiny alloc (see mallocgc for details).if ot.elem == nil || ot.elem.ptrdata != 0 || ot.elem.size >= maxTinySize {throw("runtime.SetFinalizer: pointer not at beginning of allocated block")}}f := efaceOf(&finalizer)ftyp := f._typeif ftyp == nil {// switch to system stack and remove finalizersystemstack(func() {removefinalizer(e.data)})return}if ftyp.kind&kindMask != kindFunc {throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")}ft := (*functype)(unsafe.Pointer(ftyp))if ft.dotdotdot() {throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")}if ft.inCount != 1 {throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())}fint := ft.in()[0]switch {case fint == etyp:// ok - same typegoto okargcase fint.kind&kindMask == kindPtr:if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {// ok - not same type, but both pointers,// one or the other is unnamed, and same element type, so assignable.goto okarg}case fint.kind&kindMask == kindInterface:ityp := (*interfacetype)(unsafe.Pointer(fint))if len(ityp.mhdr) == 0 {// ok - satisfies empty interfacegoto okarg}if iface := assertE2I2(ityp, *efaceOf(&obj)); iface.tab != nil {goto okarg}}throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())okarg:// compute size needed for return parametersnret := uintptr(0)for _, t := range ft.out() {nret = alignUp(nret, uintptr(t.align)) + uintptr(t.size)}nret = alignUp(nret, goarch.PtrSize)// make sure we have a finalizer goroutinecreatefing()systemstack(func() {if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {throw("runtime.SetFinalizer: finalizer already set")}})}

看代码，看不出什么。其端倪在注释中。注意如下注释：

// Finalizers are run in dependency order: if A points at B, both have

// finalizers, and they are otherwise unreachable, only the finalizer

// for A runs; once A is freed, the finalizer for B can run.

// If a cyclic structure includes a block with a finalizer, that

// cycle is not guaranteed to be garbage collected and the finalizer

// is not guaranteed to run, because there is no ordering that

// respects the dependencies.

这段英文翻译成中文如下：

Finalizers（终结器）按照依赖顺序运行：如果 A 指向 B，两者都有终结器，并且它们除此之外不可达，则仅运行 A 的终结器；一旦 A 被释放，可以运行 B 的终结器。如果一个循环结构包含一个具有终结器的块，则该循环体不能保证被垃圾回收并且终结器不能保证运行，因为没有符合依赖关系的排序方式。

这意思很明显了，析构函数会检查当前对象A是否有外部对象指向当前对象A。如果有外部对象指向当前对象A时，A的析构是无法执行的；如果有外部对象指向当前对象A时，A的析构才能执行。

代码中的a和b是循环依赖，当析构判断a和b时，都会有外部对象指向a和b，析构函数无法执行。析构无法执行，内存也无法回收。因此答案选A。

去掉析构函数后，a和b肯定会被释放的。不用析构函数去证明，那如何证明呢？用以下代码就可以证明，代码如下：

package mainimport ("fmt""runtime""time")type ListNode struct {Val  [1024 * 1024]boolNext *ListNode}func printAlloc() {var m runtime.MemStatsruntime.ReadMemStats(&m)fmt.Printf("%d KB\n", m.Alloc/1024)}func main0() {printAlloc()a := &ListNode{Val: [1024 * 1024]bool{true}}b := &ListNode{Val: [1024 * 1024]bool{false}}a.Next = bb.Next = a// runtime.SetFinalizer(a, func(obj *ListNode) {// fmt.Printf("a被删除--")// })printAlloc()}func main() {fmt.Print("开始")main0()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()fmt.Print("结束")printAlloc()}

根据运行结果，内存大小明显变小，说明a和b已经被回收了。

让我们再看看有析构函数的情况，运行结果是咋样的，如下：

package mainimport ("fmt""runtime""time")type ListNode struct {Val  [1024 * 1024]boolNext *ListNode}func printAlloc() {var m runtime.MemStatsruntime.ReadMemStats(&m)fmt.Printf("%d KB\n", m.Alloc/1024)}func main0() {printAlloc()a := &ListNode{Val: [1024 * 1024]bool{true}}b := &ListNode{Val: [1024 * 1024]bool{false}}a.Next = bb.Next = aruntime.SetFinalizer(a, func(obj *ListNode) {fmt.Printf("a被删除--")})printAlloc()}func main() {fmt.Print("开始")main0()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()time.Sleep(1 * time.Second)runtime.GC()fmt.Print("结束")printAlloc()}

根据运行结果，有析构函数的情况下，a和b确实是无法被回收。

总结

1.不要怀疑八股文的正确性，golang的垃圾回收确实是根可达算法。

2.不要用析构函数去测试无用对象被回收的情况，上面的例子也看到了，两对象的循环引用，析构函数的测试结果就是错误的。只能根据内存变化，看无用对象是否被回收。

3.在写代码的时候，能手动设置引用为nil，最好手动设置，这样能更好的避免内存泄漏。

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