Go语言——channel详解

参考：

深入理解Golang Channel

channel和goroutine是go语言最具特色是结构，有必要仔细研究。

源码路径：go1.10\src\runtime\chan.go

struct

type hchan struct {
   qcount   uint           // total data in the queue
   dataqsiz uint           // size of the circular queue
   buf      unsafe.Pointer // points to an array of dataqsiz elements
   elemsize uint16
   closed   uint32
   elemtype *_type // element type
   sendx    uint   // send index
   recvx    uint   // receive index
   recvq    waitq  // list of recv waiters
   sendq    waitq  // list of send waiters

   lock mutex
}

• qcount：当前chann元素个数，len（）
• dataqsiz：chann容量，cap（）
• buf：元素缓冲区
• elemsize：元素大小
• closed：close标志位
• elemtype：元素类型
• sendx：下一个发送元素所在缓冲区索引
• recvx：下一个接收元素所在缓冲区索引
• recvq：接收G的等待队列
• sendq：发送G的等待队列

从上面可以看出，waitq非常重要

type waitq struct {
   first *sudog
   last  *sudog
}

type sudog struct {
    g *g

    isSelect bool
    next     *sudog
    prev     *sudog
    elem     unsafe.Pointer // data element (may point to stack)

    acquiretime int64
    releasetime int64
    ticket      uint32
    parent      *sudog // semaRoot binary tree
    waitlink    *sudog // g.waiting list or semaRoot
    waittail    *sudog // semaRoot
    c           *hchan // channel
}

waitq是一个queue，具有头尾指针。

sudog封装了等待chann接收/发送的G。

new

chan不能直接new，只能make，所以观察makechan方法。make的时候，会根据size判断是缓冲chan还是非缓冲chan，这个逻辑需要注意。

func makechan(t *chantype, size int) *hchan {
   elem := t.elem

   // Hchan does not contain pointers interesting for GC when elements stored in buf do not contain pointers.
   // buf points into the same allocation, elemtype is persistent.
   // SudoG's are referenced from their owning thread so they can't be collected.
   // TODO(dvyukov,rlh): Rethink when collector can move allocated objects.
   var c *hchan
   switch {
   case size == 0 || elem.size == 0:
      // Queue or element size is zero.
      c = (*hchan)(mallocgc(hchanSize, nil, true))
      // Race detector uses this location for synchronization.
      c.buf = unsafe.Pointer(c)
   case elem.kind&kindNoPointers != 0:
      // Elements do not contain pointers.
      // Allocate hchan and buf in one call.
      c = (*hchan)(mallocgc(hchanSize+uintptr(size)*elem.size, nil, true))
      c.buf = add(unsafe.Pointer(c), hchanSize)
   default:
      // Elements contain pointers.
      c = new(hchan)
      c.buf = mallocgc(uintptr(size)*elem.size, elem, true)
   }

   c.elemsize = uint16(elem.size)
   c.elemtype = elem
   c.dataqsiz = uint(size)
    
   return c
}

这里看到由于要对buf对象分配，所以没有提供new方法，只能make。除了对default情况比较容易理解，另外两只buf的内存分配都不是很理解=。=，先遗留。

send & revc

send & revc

send

recv