关注公众号 jb51net

首页 > 脚本专栏 > Golang > Golang defer


作者:​ 谈笑风生间   ​



在defer出现的地方插入了指令CALL runtime.deferproc,在函数返回的地方插入了CALL runtime.deferreturn。goroutine的控制结构中,有一张表记录defer,调用runtime.deferproc时会将需要defer的表达式记录在表中,而在调用runtime.deferreturn的时候,则会依次从defer表中“出栈”并执行



defer信息会注册到链表,当前执行的 goroutine 持有这个链表的头指针,每个 goroutine 都有一个对应的结构体struct G,其中有一个字段指向这个defer链表头

type g struct {
	// Stack parameters.
	// stack describes the actual stack memory: [stack.lo, stack.hi).
	// stackguard0 is the stack pointer compared in the Go stack growth prologue.
	// It is stack.lo+StackGuard normally, but can be StackPreempt to trigger a preemption.
	// stackguard1 is the stack pointer compared in the C stack growth prologue.
	// It is stack.lo+StackGuard on g0 and gsignal stacks.
	// It is ~0 on other goroutine stacks, to trigger a call to morestackc (and crash).
	stack       stack   // offset known to runtime/cgo
	stackguard0 uintptr // offset known to liblink
	stackguard1 uintptr // offset known to liblink

	_panic       *_panic // innermost panic - offset known to liblink
    // _defer 这个字段指向defer链表头
	_defer       *_defer // innermost defer




// Create a new deferred function fn with siz bytes of arguments.
// The compiler turns a defer statement into a call to this.
func deferproc(siz int32, fn *funcval) { // arguments of fn follow fn
    // 获取当前goroutine
	gp := getg()
	if gp.m.curg != gp {
		// go code on the system stack can't defer
		throw("defer on system stack")
	// the arguments of fn are in a perilous state. The stack map
	// for deferproc does not describe them. So we can't let garbage
	// collection or stack copying trigger until we've copied them out
	// to somewhere safe. The memmove below does that.
	// Until the copy completes, we can only call nosplit routines.
    // 获取调用者指针
	sp := getcallersp()
    // 通过偏移获得参数
	argp := uintptr(unsafe.Pointer(&fn)) + unsafe.Sizeof(fn)
	callerpc := getcallerpc()

    // 创建defer结构体
	d := newdefer(siz)
	if d._panic != nil {
		throw("deferproc: d.panic != nil after newdefer")
    // 初始化
	d.link = gp._defer
	gp._defer = d
	d.fn = fn
	d.pc = callerpc
	d.sp = sp
	switch siz {
	case 0:
		// Do nothing.
	case sys.PtrSize:
		*(*uintptr)(deferArgs(d)) = *(*uintptr)(unsafe.Pointer(argp))
		memmove(deferArgs(d), unsafe.Pointer(argp), uintptr(siz))

	// deferproc returns 0 normally.
	// a deferred func that stops a panic
	// makes the deferproc return 1.
	// the code the compiler generates always
	// checks the return value and jumps to the
	// end of the function if deferproc returns != 0.
	// No code can go here - the C return register has
	// been set and must not be clobbered.
// 以下是_defer结构体
// A _defer holds an entry on the list of deferred calls.
// If you add a field here, add code to clear it in freedefer and deferProcStack
// This struct must match the code in cmd/compile/internal/gc/reflect.go:deferstruct
// and cmd/compile/internal/gc/ssa.go:(*state).call.
// Some defers will be allocated on the stack and some on the heap.
// All defers are logically part of the stack, so write barriers to
// initialize them are not required. All defers must be manually scanned,
// and for heap defers, marked.
type _defer struct {
    // siz 记录defer的参数和返回值共占多少字节
    // 会直接分配在_defer后面,在注册时保存参数,在执行完成时拷贝到调用者参数和返回值空间
	siz     int32 // includes both arguments and results
	// started 标记是否已经执行
    started bool
    // heap go1.13优化,标识是否为堆分配
	heap    bool
	// openDefer indicates that this _defer is for a frame with open-coded
	// defers. We have only one defer record for the entire frame (which may
	// currently have 0, 1, or more defers active).
    // openDefer 是否是open defer,通过这些信息可以找到未注册到链表的defer函数
	openDefer bool
    // sp 记录调用者栈指针,可以通过它判断自己注册的defer是否已经执行完了
	sp        uintptr  // sp at time of defer
    // pc deferproc的返回地址
	pc        uintptr  // pc at time of defer
    // fn 要注册的funcval
	fn        *funcval // can be nil for open-coded defers
    // _panic 指向当前的panic,表示这个defer是由这个panic触发的
	_panic    *_panic  // panic that is running defer
    // link 链到前一个注册的defer结构体
	link      *_defer

	// If openDefer is true, the fields below record values about the stack
	// frame and associated function that has the open-coded defer(s). sp
	// above will be the sp for the frame, and pc will be address of the
	// deferreturn call in the function.
    // 通过这些信息可以找到未注册到链表的defer函数
	fd   unsafe.Pointer // funcdata for the function associated with the frame
	varp uintptr        // value of varp for the stack frame
	// framepc is the current pc associated with the stack frame. Together,
	// with sp above (which is the sp associated with the stack frame),
	// framepc/sp can be used as pc/sp pair to continue a stack trace via
	// gentraceback().
	framepc uintptr


go会分配不同规格的_defer pool,执行时从空闲_defer中取一个出来用,没有合适的再进行堆分配。用完以后再放回空闲_defer pool。以避免频繁的堆分配和回收








// deferprocStack queues a new deferred function with a defer record on the stack.
// The defer record must have its siz and fn fields initialized.
// All other fields can contain junk.
// The defer record must be immediately followed in memory by
// the arguments of the defer.
// Nosplit because the arguments on the stack won't be scanned
// until the defer record is spliced into the gp._defer list.
func deferprocStack(d *_defer) {
    // 获得当前 goroutine
	gp := getg()
	if gp.m.curg != gp {
		// go code on the system stack can't defer
		throw("defer on system stack")
	// siz and fn are already set.
	// The other fields are junk on entry to deferprocStack and
	// are initialized here.
    // 初始化 _defer 信息
	d.started = false
	d.heap = false
	d.openDefer = false
	d.sp = getcallersp()
	d.pc = getcallerpc()
	d.framepc = 0
	d.varp = 0
	// The lines below implement:
	//   d.panic = nil
	//   d.fd = nil
	//   d.link = gp._defer
	//   gp._defer = d
	// But without write barriers. The first three are writes to
	// the stack so they don't need a write barrier, and furthermore
	// are to uninitialized memory, so they must not use a write barrier.
	// The fourth write does not require a write barrier because we
	// explicitly mark all the defer structures, so we don't need to
	// keep track of pointers to them with a write barrier.
	*(*uintptr)(unsafe.Pointer(&d._panic)) = 0
	*(*uintptr)(unsafe.Pointer(&d.fd)) = 0
	*(*uintptr)(unsafe.Pointer(&d.link)) = uintptr(unsafe.Pointer(gp._defer))
	*(*uintptr)(unsafe.Pointer(&gp._defer)) = uintptr(unsafe.Pointer(d))

	// No code can go here - the C return register has
	// been set and must not be clobbered.

到此这篇关于Golang的关键字defer的使用的文章就介绍到这了,更多相关 Golang关键字defer内容请搜索脚本之家以前的文章或继续浏览下面的相关文章希望大家以后多多支持脚本之家!