问个问题 文件读写的，buf [] byte 里面不应该有值啊。。

stdio.go ```go package io import ( "fmt" "os" ) type FileReader struct { FileName string } func (file *FileReader) Read(buf []byte) (length int, err error) { fd, err := os.Open(file.FileName) fd.Read(buf) return len(buf), err } ``` stdio_test.go ```go package io import ( "fmt" "io" "testing" ) func TestFileReader(t *testing.T) { var fileReader io.Reader = &FileReader{"testfile.md"} var buf []byte = make([]byte, 1024) fmt.Printf("value buf [] byte after passed addr = %p \n", &buf) length, err := fileReader.Read(buf) if err == nil { fmt.Printf("length==%d,err==%v,file==%s \n", length, err, string(buf)) } else { fmt.Printf("length==%d,err==%v,file==%s \n", length, err, string(buf)) } } ``` builtin.go 关于 make的说明 ，返回的是一个value 类型而不是一个指针 ```go // The make built-in function allocates and initializes an object of type // slice, map, or chan (only). Like new, the first argument is a type, not a // value. Unlike new, make's return type is the same as the type of its // argument, not a pointer to it. func make(t Type, size ...IntegerType) Type ``` 输出结果： ``` value buf [] byte addr = 0xc00005a420 value buf [] byte after passed addr = 0xc00005a460 length==1024,err==<nil>,file==# this is Test file ``` 而golang 里面 对于 value 类型的值在函数调用时，会传递这个的值的副本 指针的值会变（输出会说明） 那么我想问的是最终buf里面会什么会有值 就是文档里面的文本 。。不应该是全0么？ go是怎么操作的？