将项目附加到最大尺寸切片时附加行为

Question

I'm confused about the behavior when appending to a slice which already has the max possible size.

As far as my understanding goes a slice can either have a max size of int32 or int64 depending on your system.

When executing test := make([]struct{},math.MaxInt64+1) I get the error len argument too large in make([]struct {}) which is within my expectations. But when doing something like:

test := make([]struct{},math.MaxInt64)
for i:=0;i<100 ; i++ {
    test = append(test, struct{}{})
}
fmt.Println(len(test))

I expect the program to panic but surprisingly the code runs with out any problems and len returns an overflowed value with -9223372036854775709.

Could someone maybe elaborate the behavior?

(I'm using go1.11.2 linux/amd64)

Answer 1

The spec says about length and capacity of any slice:

At any time the following relationship holds:

0 <= len(s) <= cap(s)

This is clearly violated, as the length becomes negative, and thus less than 0. So this is a bug, already reported it and progress can be tracked here: https://github.com/golang/go/issues/29190

Ian Lance Taylor confirms this is a bug and not how it should work. The proper behavior would be a panic saying growslice: cap out of range, which should originate from slice.go / growslice() function (growslice() is called from append()).

If we slightly modify your example to this:

s := make([]struct{}, math.MaxInt32-2)
fmt.Println(len(s), cap(s))
for i := 0; i < 5; i++ {
    s = append(s, struct{}{})
    fmt.Println(len(s), cap(s))
}

And run it on the Go Playground:

2147483645 2147483645
2147483646 2147483646
2147483647 2147483647
-2147483648 2147483647
-2147483647 2147483647
-2147483646 2147483647

As we can see, the capacity stops growing once it reaches MaxInt32 on 32-bit architectures, and MaxInt64 on 64-bit architectures.