go slice切片到底是指针吗？为什么%p输出的切片是地址？

2年前 (2022) 程序员胖胖胖虎阿

351 0 0

我们先来看下slice的结构体

// runtime/slice.go
type slice struct {
    array unsafe.Pointer // 元素指针
    len   int // 长度 
    cap   int // 容量
}

我们先看一下创建slice的方法,我们用go1.11和go1.18做比较，go1.11返回的是结构体(slice),go1.18返回的是slice里面的array指针,然后在上层调用方再创建reflect.SliceHeader.这一改变是在cmd/compile: move slice construction to callers of makeslice

go1.11

// runtime/slice.go
func makeslice(et *_type, len, cap int) slice {
    // NOTE: The len > maxElements check here is not strictly necessary,
    // but it produces a 'len out of range' error instead of a 'cap out of range' error
    // when someone does make([]T, bignumber). 'cap out of range' is true too,
    // but since the cap is only being supplied implicitly, saying len is clearer.
    // See issue 4085.
    maxElements := maxSliceCap(et.size)
    if len < 0 || uintptr(len) > maxElements {
        panicmakeslicelen()
    }

    if cap < len || uintptr(cap) > maxElements {
        panicmakeslicecap()
    }

    p := mallocgc(et.size*uintptr(cap), et, true)
    return slice{p, len, cap}
}


// cmd/compile/internal/gc/walk.go

            // n escapes; set up a call to makeslice.
            // When len and cap can fit into int, use makeslice instead of
            // makeslice64, which is faster and shorter on 32 bit platforms.

            if t.Elem().NotInHeap() {
                yyerror("%v is go:notinheap; heap allocation disallowed", t.Elem())
            }

            len, cap := l, r

            fnname := "makeslice64"
            argtype := types.Types[TINT64]

            // Type checking guarantees that TIDEAL len/cap are positive and fit in an int.
            // The case of len or cap overflow when converting TUINT or TUINTPTR to TINT
            // will be handled by the negative range checks in makeslice during runtime.
            if (len.Type.IsKind(TIDEAL) || maxintval[len.Type.Etype].Cmp(maxintval[TUINT]) <= 0) &&
                (cap.Type.IsKind(TIDEAL) || maxintval[cap.Type.Etype].Cmp(maxintval[TUINT]) <= 0) {
                fnname = "makeslice"
                argtype = types.Types[TINT]
            }

            fn := syslook(fnname)
            fn = substArgTypes(fn, t.Elem()) // any-1
            n = mkcall1(fn, t, init, typename(t.Elem()), conv(len, argtype), conv(cap, argtype))

go1.18

// runtime/slice.go
func makeslice(et *_type, len, cap int) unsafe.Pointer {
    mem, overflow := math.MulUintptr(et.size, uintptr(cap))
    if overflow || mem > maxAlloc || len < 0 || len > cap {
        // NOTE: Produce a 'len out of range' error instead of a
        // 'cap out of range' error when someone does make([]T, bignumber).
        // 'cap out of range' is true too, but since the cap is only being
        // supplied implicitly, saying len is clearer.
        // See golang.org/issue/4085.
        mem, overflow := math.MulUintptr(et.size, uintptr(len))
        if overflow || mem > maxAlloc || len < 0 {
            panicmakeslicelen()
        }
        panicmakeslicecap()
    }

    return mallocgc(mem, et, true)
}

// cmd/compile/internal/walk/builtin.go
// walkMakeSlice walks an OMAKESLICE node.
func walkMakeSlice(n *ir.MakeExpr, init *ir.Nodes) ir.Node {
    l := n.Len
    r := n.Cap
    if r == nil {
        r = safeExpr(l, init)
        l = r
    }
    t := n.Type()
    if t.Elem().NotInHeap() {
        //....
    }

    // n escapes; set up a call to makeslice.
    // When len and cap can fit into int, use makeslice instead of
    // makeslice64, which is faster and shorter on 32 bit platforms.

    len, cap := l, r

    fnname := "makeslice64"
    argtype := types.Types[types.TINT64]

    // Type checking guarantees that TIDEAL len/cap are positive and fit in an int.
    // The case of len or cap overflow when converting TUINT or TUINTPTR to TINT
    // will be handled by the negative range checks in makeslice during runtime.
    if (len.Type().IsKind(types.TIDEAL) || len.Type().Size() <= types.Types[types.TUINT].Size()) &&
        (cap.Type().IsKind(types.TIDEAL) || cap.Type().Size() <= types.Types[types.TUINT].Size()) {
        fnname = "makeslice"
        argtype = types.Types[types.TINT]
    }
    fn := typecheck.LookupRuntime(fnname)
    ptr := mkcall1(fn, types.Types[types.TUNSAFEPTR], init, reflectdata.TypePtr(t.Elem()), typecheck.Conv(len, argtype), typecheck.Conv(cap, argtype))
    ptr.MarkNonNil()
    len = typecheck.Conv(len, types.Types[types.TINT])
    cap = typecheck.Conv(cap, types.Types[types.TINT])
    // 对比go1.11生成了数组指针后再初始化了len和cap
    // ir.NewSliceHeaderExpr主要就是生成了SliceHeader,参考下面的tcSliceHeader
    sh := ir.NewSliceHeaderExpr(base.Pos, t, ptr, len, cap)
    return walkExpr(typecheck.Expr(sh), init)
}

// cmd/compile/internal/typecheck/expr.go
// tcSliceHeader typechecks an OSLICEHEADER node.
func tcSliceHeader(n *ir.SliceHeaderExpr) ir.Node {
    // Errors here are Fatalf instead of Errorf because only the compiler
    // can construct an OSLICEHEADER node.
    // Components used in OSLICEHEADER that are supplied by parsed source code
    // have already been typechecked in e.g. OMAKESLICE earlier.
    t := n.Type()
    if t == nil {
        base.Fatalf("no type specified for OSLICEHEADER")
    }

    if !t.IsSlice() {
        base.Fatalf("invalid type %v for OSLICEHEADER", n.Type())
    }

    if n.Ptr == nil || n.Ptr.Type() == nil || !n.Ptr.Type().IsUnsafePtr() {
        base.Fatalf("need unsafe.Pointer for OSLICEHEADER")
    }

    n.Ptr = Expr(n.Ptr)
    n.Len = DefaultLit(Expr(n.Len), types.Types[types.TINT])
    n.Cap = DefaultLit(Expr(n.Cap), types.Types[types.TINT])

    ...
    return n
}


// SliceHeader is the runtime representation of a slice.
// SliceHeader就是slice的运行时表示
// ...
type SliceHeader struct {
    Data uintptr
    Len  int
    Cap  int

所以我们可以简单的理解成slice的创建返回的是slice结构体,而不是指针.所以在函数参数传递拷贝的时候拷贝的是slice结构体,而不是*slice,但是map不一样,map创建的时候是*hmap,创建的是一个指针.

// runtime/map.go
// makemap implements Go map creation for make(map[k]v, hint).
// If the compiler has determined that the map or the first bucket
// can be created on the stack, h and/or bucket may be non-nil.
// If h != nil, the map can be created directly in h.
// If h.buckets != nil, bucket pointed to can be used as the first bucket.
func makemap(t *maptype, hint int, h *hmap) *hmap {
    ...
    return h
}

但是我们很好奇,这里有两个问题:
1.并且%p能输出地址
2.为什么复制slice的时候会影响旧的值

问题1:
其实fmt.Printf()的%p并不只是指针地址，当为slice的时候是输出的是底层数组第0个元素的地址
go slice切片到底是指针吗？为什么%p输出的切片是地址？

//go1.18 fmt/print.go
func (p *pp) fmtPointer(value reflect.Value, verb rune) {
    var u uintptr
    switch value.Kind() {
    case reflect.Chan, reflect.Func, reflect.Map, reflect.Pointer, reflect.Slice, reflect.UnsafePointer:
        u = value.Pointer()
    default:
        p.badVerb(verb)
        return
    }
...
}

// reflect/value.go
func (v Value) Pointer() uintptr {
    k := v.kind()
    switch k {
    ...
    case Slice:
        //这里的slice返回的指针其实是`*SliceHeader.Data`,Data就是前面slice.array指针
        return (*SliceHeader)(v.ptr).Data
    }
    panic(&ValueError{"reflect.Value.Pointer", v.kind()})
}

type SliceHeader struct {
    Data uintptr
    Len  int
    Cap  int
}

问题2:
这是因为复制的时候并不是深拷贝,相当于slice.array还没有变,但是如果我们append足够大就会申请新的slice.array,如下

func main() {
    var oldSlice = []int64{1, 2, 3, 4, 5} // len:5,capacity:5
    var newSlice = oldSlice[1:3]          // len:2,capacity:4   (已经使用了两个位置，所以还空两位置可以append)
    fmt.Printf("%p\n", oldSlice)          //0xc420098000
    fmt.Printf("%p\n", newSlice)          //0xc420098008 可以看到newSlice的地址指向的是array[1]的地址，即他们底层使用的还是一个数组
    fmt.Printf("%v\n", oldSlice)          //[1 2 3 4 5]
    fmt.Printf("%v\n", newSlice)          //[2 3]

    newSlice[1] = 9              //更改后oldSlice、newSlice都改变了
    fmt.Printf("%v\n", oldSlice) // [1 2 9 4 5]
    fmt.Printf("%v\n", newSlice) // [2 9]

    newSlice = append(newSlice, 11, 12) //append 操作之后，oldSlice的len和capacity不变,newSlice的len变为4，capacity：4。因为这是对newSlice的操作
    fmt.Printf("%v\n", oldSlice)        //[1 2 9 11 12] //注意对newSlice做append操作之后，oldSlice[3],oldSlice[4]的值也发生了改变
    fmt.Printf("%v\n", newSlice)        //[2 9 11 12]

    newSlice = append(newSlice, 13, 14) // 因为newSlice的len已经等于capacity，所以再次append就会超过capacity值，
    // 此时，append函数内部会创建一个新的底层数组（是一个扩容过的数组），并将oldSlice指向的底层数组拷贝过去，然后在追加新的值。
    fmt.Printf("%p\n", oldSlice) //0xc420098000
    fmt.Printf("%p\n", newSlice) //0xc4200a0000
    fmt.Printf("%v\n", oldSlice) //[1 2 9 11 12]
    fmt.Printf("%v\n", newSlice) //[2 9 11 12 13 14]  它俩已经不再是指向同一个底层数组了
}

参考: