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tailscale/util/cstruct/cstruct_example_test.go

76 lines
2.1 KiB
Go

// Copyright (c) 2022 Tailscale Inc & AUTHORS All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Only built on 64-bit platforms to avoid complexity
//go:build amd64 || arm64 || mips64le || ppc64le || riscv64
// +build amd64 arm64 mips64le ppc64le riscv64
package cstruct
import "fmt"
// This test provides a semi-realistic example of how you can
// use this package to decode a C structure.
func ExampleDecoder() {
// Our example C structure:
// struct mystruct {
// char *p;
// char c;
// /* implicit: char _pad[3]; */
// int x;
// };
//
// The Go structure definition:
type myStruct struct {
Ptr uintptr
Ch byte
Intval uint32
}
// Our "in-memory" version of the above structure
buf := []byte{
1, 2, 3, 4, 0, 0, 0, 0, // ptr
5, // ch
99, 99, 99, // padding
78, 6, 0, 0, // x
}
d := NewDecoder(buf)
// Decode the structure; if one of these function returns an error,
// then subsequent decoder functions will return the zero value.
var x myStruct
x.Ptr = d.Uintptr()
x.Ch = d.Byte()
x.Intval = d.Uint32()
// Note that per the Go language spec:
// [...] when evaluating the operands of an expression, assignment,
// or return statement, all function calls, method calls, and
// (channel) communication operations are evaluated in lexical
// left-to-right order
//
// Since each field is assigned via a function call, one could use the
// following snippet to decode the struct.
// x := myStruct{
// Ptr: d.Uintptr(),
// Ch: d.Byte(),
// Intval: d.Uint32(),
// }
//
// However, this means that reordering the fields in the initialization
// statementnormally a semantically identical operationwould change
// the way the structure is parsed. Thus we do it as above with
// explicit ordering.
// After finishing with the decoder, check errors
if err := d.Err(); err != nil {
panic(err)
}
// Print the decoder offset and structure
fmt.Printf("off=%d struct=%#v\n", d.Offset(), x)
// Output: off=16 struct=cstruct.myStruct{Ptr:0x4030201, Ch:0x5, Intval:0x64e}
}