How Can I Inspect the Header of a Go Slice Safely and Unsafely?

Inspecting Slice Headers

In Go, slices are references to underlying arrays and their headers store information about the length and capacity of the slice, as well as a pointer to the underlying data. While the contents of a slice can be modified, its header typically remains the same.

Consider the following code snippet:

var buffer [256] byte

func SubtractOneFromLength(slice []byte) []byte {
    slice = slice[0 : len(slice)-1]
    return slice
}

func main() {
    slice := buffer[10:20]
    fmt.Println("Before: len(slice) =", len(slice))
    newSlice := SubtractOneFromLength(slice)
    fmt.Println("After:  len(slice) =", len(slice))
    fmt.Println("After:  len(newSlice) =", len(newSlice))
    newSlice2 := SubtractOneFromLength(newSlice)
    fmt.Println("After:  len(newSlice2) =", len(newSlice2))
}

When the function SubtractOneFromLength is called, it modifies the slice but not the slice header. To inspect the slice header, you can use the reflect.SliceHeader type.

type SliceHeader struct {
    Data uintptr
    Len  int
    Cap  int
}

The following unsafe operation demonstrates how to convert a slice pointer to a *reflect.SliceHeader:

sh := (*reflect.SliceHeader)(unsafe.Pointer(&newSlice2))

You can then print the contents of the header using the format string % v:

fmt.Printf("%+v", sh)

The Go Playground produces the following output:

&{Data:1792106 Len:8 Cap:246}

This output provides the memory address of the underlying data (Data), the length of the slice (Len), and its capacity (Cap).

Without unsafe:


Accessing the slice header information without unsafe is also possible:

• Use &newSlice2[0] for Data
• len(newSlice2) for Len
• cap(newSlice2) for Cap

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