1 // Copyright 2009 The Go Authors. All rights reserved.
2 // Use of this source code is governed by a BSD-style
3 // license that can be found in the LICENSE file.
5 // Garbage collector: finalizers and block profiling.
10 "runtime/internal/atomic"
11 "runtime/internal/sys"
15 // finblock is an array of finalizers to be executed. finblocks are
16 // arranged in a linked list for the finalizer queue.
18 // finblock is allocated from non-GC'd memory, so any heap pointers
19 // must be specially handled. GC currently assumes that the finalizer
20 // queue does not grow during marking (but it can shrink).
23 type finblock struct {
28 fin [(_FinBlockSize - 2*sys.PtrSize - 2*4) / unsafe.Sizeof(finalizer{})]finalizer
31 var finlock mutex // protects the following variables
32 var fing *g // goroutine that runs finalizers
33 var finq *finblock // list of finalizers that are to be executed
34 var finc *finblock // cache of free blocks
35 var finptrmask [_FinBlockSize / sys.PtrSize / 8]byte
38 var allfin *finblock // list of all blocks
40 // NOTE: Layout known to queuefinalizer.
41 type finalizer struct {
42 fn *funcval // function to call (may be a heap pointer)
43 arg unsafe.Pointer // ptr to object (may be a heap pointer)
44 nret uintptr // bytes of return values from fn
45 fint *_type // type of first argument of fn
46 ot *ptrtype // type of ptr to object (may be a heap pointer)
49 var finalizer1 = [...]byte{
50 // Each Finalizer is 5 words, ptr ptr INT ptr ptr (INT = uintptr here)
51 // Each byte describes 8 words.
52 // Need 8 Finalizers described by 5 bytes before pattern repeats:
53 // ptr ptr INT ptr ptr
54 // ptr ptr INT ptr ptr
55 // ptr ptr INT ptr ptr
56 // ptr ptr INT ptr ptr
57 // ptr ptr INT ptr ptr
58 // ptr ptr INT ptr ptr
59 // ptr ptr INT ptr ptr
60 // ptr ptr INT ptr ptr
63 // ptr ptr INT ptr ptr ptr ptr INT
64 // ptr ptr ptr ptr INT ptr ptr ptr
65 // ptr INT ptr ptr ptr ptr INT ptr
66 // ptr ptr ptr INT ptr ptr ptr ptr
67 // INT ptr ptr ptr ptr INT ptr ptr
69 // Assumptions about Finalizer layout checked below.
70 1<<0 | 1<<1 | 0<<2 | 1<<3 | 1<<4 | 1<<5 | 1<<6 | 0<<7,
71 1<<0 | 1<<1 | 1<<2 | 1<<3 | 0<<4 | 1<<5 | 1<<6 | 1<<7,
72 1<<0 | 0<<1 | 1<<2 | 1<<3 | 1<<4 | 1<<5 | 0<<6 | 1<<7,
73 1<<0 | 1<<1 | 1<<2 | 0<<3 | 1<<4 | 1<<5 | 1<<6 | 1<<7,
74 0<<0 | 1<<1 | 1<<2 | 1<<3 | 1<<4 | 0<<5 | 1<<6 | 1<<7,
77 func queuefinalizer(p unsafe.Pointer, fn *funcval, nret uintptr, fint *_type, ot *ptrtype) {
78 if gcphase != _GCoff {
79 // Currently we assume that the finalizer queue won't
80 // grow during marking so we don't have to rescan it
81 // during mark termination. If we ever need to lift
82 // this assumption, we can do it by adding the
83 // necessary barriers to queuefinalizer (which it may
84 // have automatically).
85 throw("queuefinalizer during GC")
89 if finq == nil || finq.cnt == uint32(len(finq.fin)) {
91 finc = (*finblock)(persistentalloc(_FinBlockSize, 0, &memstats.gc_sys))
94 if finptrmask[0] == 0 {
95 // Build pointer mask for Finalizer array in block.
96 // Check assumptions made in finalizer1 array above.
97 if (unsafe.Sizeof(finalizer{}) != 5*sys.PtrSize ||
98 unsafe.Offsetof(finalizer{}.fn) != 0 ||
99 unsafe.Offsetof(finalizer{}.arg) != sys.PtrSize ||
100 unsafe.Offsetof(finalizer{}.nret) != 2*sys.PtrSize ||
101 unsafe.Offsetof(finalizer{}.fint) != 3*sys.PtrSize ||
102 unsafe.Offsetof(finalizer{}.ot) != 4*sys.PtrSize) {
103 throw("finalizer out of sync")
105 for i := range finptrmask {
106 finptrmask[i] = finalizer1[i%len(finalizer1)]
115 f := &finq.fin[finq.cnt]
116 atomic.Xadd(&finq.cnt, +1) // Sync with markroots
127 func iterate_finq(callback func(*funcval, unsafe.Pointer, uintptr, *_type, *ptrtype)) {
128 for fb := allfin; fb != nil; fb = fb.alllink {
129 for i := uint32(0); i < fb.cnt; i++ {
131 callback(f.fn, f.arg, f.nret, f.fint, f.ot)
139 if fingwait && fingwake {
154 // start the finalizer goroutine exactly once
155 if fingCreate == 0 && atomic.Cas(&fingCreate, 0, 1) {
160 // This is the goroutine that runs all of the finalizers
175 goparkunlock(&finlock, "finalizer wait", traceEvGoBlock, 1)
183 for i := fb.cnt; i > 0; i-- {
186 framesz := unsafe.Sizeof((interface{})(nil)) + f.nret
187 if framecap < framesz {
188 // The frame does not contain pointers interesting for GC,
189 // all not yet finalized objects are stored in finq.
190 // If we do not mark it as FlagNoScan,
191 // the last finalized object is not collected.
192 frame = mallocgc(framesz, nil, true)
197 throw("missing type in runfinq")
199 // frame is effectively uninitialized
200 // memory. That means we have to clear
201 // it before writing to it to avoid
202 // confusing the write barrier.
203 *(*[2]uintptr)(frame) = [2]uintptr{}
204 switch f.fint.kind & kindMask {
206 // direct use of pointer
207 *(*unsafe.Pointer)(frame) = f.arg
209 ityp := (*interfacetype)(unsafe.Pointer(f.fint))
210 // set up with empty interface
211 (*eface)(frame)._type = &f.ot.typ
212 (*eface)(frame).data = f.arg
213 if len(ityp.mhdr) != 0 {
214 // convert to interface with methods
215 // this conversion is guaranteed to succeed - we checked in SetFinalizer
216 *(*iface)(frame) = assertE2I(ityp, *(*eface)(frame))
219 throw("bad kind in runfinq")
222 reflectcall(nil, unsafe.Pointer(f.fn), frame, uint32(framesz), uint32(framesz))
225 // Drop finalizer queue heap references
226 // before hiding them from markroot.
227 // This also ensures these will be
228 // clear if we reuse the finalizer.
232 atomic.Store(&fb.cnt, i-1)
244 // SetFinalizer sets the finalizer associated with obj to the provided
245 // finalizer function. When the garbage collector finds an unreachable block
246 // with an associated finalizer, it clears the association and runs
247 // finalizer(obj) in a separate goroutine. This makes obj reachable again,
248 // but now without an associated finalizer. Assuming that SetFinalizer
249 // is not called again, the next time the garbage collector sees
250 // that obj is unreachable, it will free obj.
252 // SetFinalizer(obj, nil) clears any finalizer associated with obj.
254 // The argument obj must be a pointer to an object allocated by calling
255 // new, by taking the address of a composite literal, or by taking the
256 // address of a local variable.
257 // The argument finalizer must be a function that takes a single argument
258 // to which obj's type can be assigned, and can have arbitrary ignored return
259 // values. If either of these is not true, SetFinalizer may abort the
262 // Finalizers are run in dependency order: if A points at B, both have
263 // finalizers, and they are otherwise unreachable, only the finalizer
264 // for A runs; once A is freed, the finalizer for B can run.
265 // If a cyclic structure includes a block with a finalizer, that
266 // cycle is not guaranteed to be garbage collected and the finalizer
267 // is not guaranteed to run, because there is no ordering that
268 // respects the dependencies.
270 // The finalizer for obj is scheduled to run at some arbitrary time after
271 // obj becomes unreachable.
272 // There is no guarantee that finalizers will run before a program exits,
273 // so typically they are useful only for releasing non-memory resources
274 // associated with an object during a long-running program.
275 // For example, an os.File object could use a finalizer to close the
276 // associated operating system file descriptor when a program discards
277 // an os.File without calling Close, but it would be a mistake
278 // to depend on a finalizer to flush an in-memory I/O buffer such as a
279 // bufio.Writer, because the buffer would not be flushed at program exit.
281 // It is not guaranteed that a finalizer will run if the size of *obj is
284 // It is not guaranteed that a finalizer will run for objects allocated
285 // in initializers for package-level variables. Such objects may be
286 // linker-allocated, not heap-allocated.
288 // A finalizer may run as soon as an object becomes unreachable.
289 // In order to use finalizers correctly, the program must ensure that
290 // the object is reachable until it is no longer required.
291 // Objects stored in global variables, or that can be found by tracing
292 // pointers from a global variable, are reachable. For other objects,
293 // pass the object to a call of the KeepAlive function to mark the
294 // last point in the function where the object must be reachable.
296 // For example, if p points to a struct that contains a file descriptor d,
297 // and p has a finalizer that closes that file descriptor, and if the last
298 // use of p in a function is a call to syscall.Write(p.d, buf, size), then
299 // p may be unreachable as soon as the program enters syscall.Write. The
300 // finalizer may run at that moment, closing p.d, causing syscall.Write
301 // to fail because it is writing to a closed file descriptor (or, worse,
302 // to an entirely different file descriptor opened by a different goroutine).
303 // To avoid this problem, call runtime.KeepAlive(p) after the call to
306 // A single goroutine runs all finalizers for a program, sequentially.
307 // If a finalizer must run for a long time, it should do so by starting
309 func SetFinalizer(obj interface{}, finalizer interface{}) {
311 // debug.sbrk never frees memory, so no finalizers run
312 // (and we don't have the data structures to record them).
318 throw("runtime.SetFinalizer: first argument is nil")
320 if etyp.kind&kindMask != kindPtr {
321 throw("runtime.SetFinalizer: first argument is " + etyp.string() + ", not pointer")
323 ot := (*ptrtype)(unsafe.Pointer(etyp))
325 throw("nil elem type!")
328 // find the containing object
329 base, _, _ := findObject(uintptr(e.data), 0, 0)
332 // 0-length objects are okay.
333 if e.data == unsafe.Pointer(&zerobase) {
337 // Global initializers might be linker-allocated.
338 // var Foo = &Object{}
340 // runtime.SetFinalizer(Foo, nil)
342 // The relevant segments are: noptrdata, data, bss, noptrbss.
343 // We cannot assume they are in any order or even contiguous,
344 // due to external linking.
345 for datap := &firstmoduledata; datap != nil; datap = datap.next {
346 if datap.noptrdata <= uintptr(e.data) && uintptr(e.data) < datap.enoptrdata ||
347 datap.data <= uintptr(e.data) && uintptr(e.data) < datap.edata ||
348 datap.bss <= uintptr(e.data) && uintptr(e.data) < datap.ebss ||
349 datap.noptrbss <= uintptr(e.data) && uintptr(e.data) < datap.enoptrbss {
353 throw("runtime.SetFinalizer: pointer not in allocated block")
356 if uintptr(e.data) != base {
357 // As an implementation detail we allow to set finalizers for an inner byte
358 // of an object if it could come from tiny alloc (see mallocgc for details).
359 if ot.elem == nil || ot.elem.kind&kindNoPointers == 0 || ot.elem.size >= maxTinySize {
360 throw("runtime.SetFinalizer: pointer not at beginning of allocated block")
364 f := efaceOf(&finalizer)
367 // switch to system stack and remove finalizer
369 removefinalizer(e.data)
374 if ftyp.kind&kindMask != kindFunc {
375 throw("runtime.SetFinalizer: second argument is " + ftyp.string() + ", not a function")
377 ft := (*functype)(unsafe.Pointer(ftyp))
379 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string() + " because dotdotdot")
382 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
389 case fint.kind&kindMask == kindPtr:
390 if (fint.uncommon() == nil || etyp.uncommon() == nil) && (*ptrtype)(unsafe.Pointer(fint)).elem == ot.elem {
391 // ok - not same type, but both pointers,
392 // one or the other is unnamed, and same element type, so assignable.
395 case fint.kind&kindMask == kindInterface:
396 ityp := (*interfacetype)(unsafe.Pointer(fint))
397 if len(ityp.mhdr) == 0 {
398 // ok - satisfies empty interface
401 if _, ok := assertE2I2(ityp, *efaceOf(&obj)); ok {
405 throw("runtime.SetFinalizer: cannot pass " + etyp.string() + " to finalizer " + ftyp.string())
407 // compute size needed for return parameters
409 for _, t := range ft.out() {
410 nret = round(nret, uintptr(t.align)) + uintptr(t.size)
412 nret = round(nret, sys.PtrSize)
414 // make sure we have a finalizer goroutine
418 if !addfinalizer(e.data, (*funcval)(f.data), nret, fint, ot) {
419 throw("runtime.SetFinalizer: finalizer already set")
424 // Mark KeepAlive as noinline so that it is easily detectable as an intrinsic.
427 // KeepAlive marks its argument as currently reachable.
428 // This ensures that the object is not freed, and its finalizer is not run,
429 // before the point in the program where KeepAlive is called.
431 // A very simplified example showing where KeepAlive is required:
432 // type File struct { d int }
433 // d, err := syscall.Open("/file/path", syscall.O_RDONLY, 0)
434 // // ... do something if err != nil ...
436 // runtime.SetFinalizer(p, func(p *File) { syscall.Close(p.d) })
438 // n, err := syscall.Read(p.d, buf[:])
439 // // Ensure p is not finalized until Read returns.
440 // runtime.KeepAlive(p)
441 // // No more uses of p after this point.
443 // Without the KeepAlive call, the finalizer could run at the start of
444 // syscall.Read, closing the file descriptor before syscall.Read makes
445 // the actual system call.
446 func KeepAlive(x interface{}) {
447 // Introduce a use of x that the compiler can't eliminate.
448 // This makes sure x is alive on entry. We need x to be alive
449 // on entry for "defer runtime.KeepAlive(x)"; see issue 21402.