mirror of https://go.googlesource.com/go
270 lines
12 KiB
Go
270 lines
12 KiB
Go
// Copyright 2009 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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/*
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Package unsafe contains operations that step around the type safety of Go programs.
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Packages that import unsafe may be non-portable and are not protected by the
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Go 1 compatibility guidelines.
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*/
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package unsafe
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// ArbitraryType is here for the purposes of documentation only and is not actually
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// part of the unsafe package. It represents the type of an arbitrary Go expression.
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type ArbitraryType int
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// IntegerType is here for the purposes of documentation only and is not actually
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// part of the unsafe package. It represents any arbitrary integer type.
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type IntegerType int
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// Pointer represents a pointer to an arbitrary type. There are four special operations
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// available for type Pointer that are not available for other types:
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// - A pointer value of any type can be converted to a Pointer.
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// - A Pointer can be converted to a pointer value of any type.
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// - A uintptr can be converted to a Pointer.
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// - A Pointer can be converted to a uintptr.
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//
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// Pointer therefore allows a program to defeat the type system and read and write
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// arbitrary memory. It should be used with extreme care.
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//
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// The following patterns involving Pointer are valid.
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// Code not using these patterns is likely to be invalid today
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// or to become invalid in the future.
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// Even the valid patterns below come with important caveats.
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//
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// Running "go vet" can help find uses of Pointer that do not conform to these patterns,
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// but silence from "go vet" is not a guarantee that the code is valid.
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//
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// (1) Conversion of a *T1 to Pointer to *T2.
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//
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// Provided that T2 is no larger than T1 and that the two share an equivalent
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// memory layout, this conversion allows reinterpreting data of one type as
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// data of another type. An example is the implementation of
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// math.Float64bits:
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//
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// func Float64bits(f float64) uint64 {
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// return *(*uint64)(unsafe.Pointer(&f))
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// }
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//
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// (2) Conversion of a Pointer to a uintptr (but not back to Pointer).
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//
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// Converting a Pointer to a uintptr produces the memory address of the value
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// pointed at, as an integer. The usual use for such a uintptr is to print it.
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//
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// Conversion of a uintptr back to Pointer is not valid in general.
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//
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// A uintptr is an integer, not a reference.
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// Converting a Pointer to a uintptr creates an integer value
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// with no pointer semantics.
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// Even if a uintptr holds the address of some object,
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// the garbage collector will not update that uintptr's value
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// if the object moves, nor will that uintptr keep the object
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// from being reclaimed.
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//
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// The remaining patterns enumerate the only valid conversions
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// from uintptr to Pointer.
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//
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// (3) Conversion of a Pointer to a uintptr and back, with arithmetic.
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//
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// If p points into an allocated object, it can be advanced through the object
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// by conversion to uintptr, addition of an offset, and conversion back to Pointer.
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//
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// p = unsafe.Pointer(uintptr(p) + offset)
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//
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// The most common use of this pattern is to access fields in a struct
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// or elements of an array:
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//
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// // equivalent to f := unsafe.Pointer(&s.f)
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// f := unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f))
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//
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// // equivalent to e := unsafe.Pointer(&x[i])
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// e := unsafe.Pointer(uintptr(unsafe.Pointer(&x[0])) + i*unsafe.Sizeof(x[0]))
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//
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// It is valid both to add and to subtract offsets from a pointer in this way.
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// It is also valid to use &^ to round pointers, usually for alignment.
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// In all cases, the result must continue to point into the original allocated object.
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//
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// Unlike in C, it is not valid to advance a pointer just beyond the end of
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// its original allocation:
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//
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// // INVALID: end points outside allocated space.
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// var s thing
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// end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s))
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//
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// // INVALID: end points outside allocated space.
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// b := make([]byte, n)
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// end = unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(n))
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//
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// Note that both conversions must appear in the same expression, with only
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// the intervening arithmetic between them:
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//
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// // INVALID: uintptr cannot be stored in variable
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// // before conversion back to Pointer.
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// u := uintptr(p)
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// p = unsafe.Pointer(u + offset)
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//
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// Note that the pointer must point into an allocated object, so it may not be nil.
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//
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// // INVALID: conversion of nil pointer
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// u := unsafe.Pointer(nil)
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// p := unsafe.Pointer(uintptr(u) + offset)
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//
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// (4) Conversion of a Pointer to a uintptr when calling [syscall.Syscall].
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//
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// The Syscall functions in package syscall pass their uintptr arguments directly
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// to the operating system, which then may, depending on the details of the call,
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// reinterpret some of them as pointers.
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// That is, the system call implementation is implicitly converting certain arguments
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// back from uintptr to pointer.
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//
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// If a pointer argument must be converted to uintptr for use as an argument,
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// that conversion must appear in the call expression itself:
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//
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// syscall.Syscall(SYS_READ, uintptr(fd), uintptr(unsafe.Pointer(p)), uintptr(n))
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//
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// The compiler handles a Pointer converted to a uintptr in the argument list of
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// a call to a function implemented in assembly by arranging that the referenced
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// allocated object, if any, is retained and not moved until the call completes,
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// even though from the types alone it would appear that the object is no longer
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// needed during the call.
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//
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// For the compiler to recognize this pattern,
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// the conversion must appear in the argument list:
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//
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// // INVALID: uintptr cannot be stored in variable
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// // before implicit conversion back to Pointer during system call.
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// u := uintptr(unsafe.Pointer(p))
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// syscall.Syscall(SYS_READ, uintptr(fd), u, uintptr(n))
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//
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// (5) Conversion of the result of [reflect.Value.Pointer] or [reflect.Value.UnsafeAddr]
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// from uintptr to Pointer.
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//
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// Package reflect's Value methods named Pointer and UnsafeAddr return type uintptr
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// instead of unsafe.Pointer to keep callers from changing the result to an arbitrary
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// type without first importing "unsafe". However, this means that the result is
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// fragile and must be converted to Pointer immediately after making the call,
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// in the same expression:
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//
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// p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer()))
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//
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// As in the cases above, it is invalid to store the result before the conversion:
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//
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// // INVALID: uintptr cannot be stored in variable
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// // before conversion back to Pointer.
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// u := reflect.ValueOf(new(int)).Pointer()
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// p := (*int)(unsafe.Pointer(u))
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//
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// (6) Conversion of a [reflect.SliceHeader] or [reflect.StringHeader] Data field to or from Pointer.
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//
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// As in the previous case, the reflect data structures SliceHeader and StringHeader
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// declare the field Data as a uintptr to keep callers from changing the result to
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// an arbitrary type without first importing "unsafe". However, this means that
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// SliceHeader and StringHeader are only valid when interpreting the content
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// of an actual slice or string value.
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//
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// var s string
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// hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
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// hdr.Data = uintptr(unsafe.Pointer(p)) // case 6 (this case)
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// hdr.Len = n
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//
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// In this usage hdr.Data is really an alternate way to refer to the underlying
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// pointer in the string header, not a uintptr variable itself.
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//
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// In general, [reflect.SliceHeader] and [reflect.StringHeader] should be used
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// only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual
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// slices or strings, never as plain structs.
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// A program should not declare or allocate variables of these struct types.
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//
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// // INVALID: a directly-declared header will not hold Data as a reference.
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// var hdr reflect.StringHeader
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// hdr.Data = uintptr(unsafe.Pointer(p))
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// hdr.Len = n
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// s := *(*string)(unsafe.Pointer(&hdr)) // p possibly already lost
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type Pointer *ArbitraryType
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// Sizeof takes an expression x of any type and returns the size in bytes
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// of a hypothetical variable v as if v was declared via var v = x.
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// The size does not include any memory possibly referenced by x.
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// For instance, if x is a slice, Sizeof returns the size of the slice
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// descriptor, not the size of the memory referenced by the slice.
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// For a struct, the size includes any padding introduced by field alignment.
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// The return value of Sizeof is a Go constant if the type of the argument x
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// does not have variable size.
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// (A type has variable size if it is a type parameter or if it is an array
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// or struct type with elements of variable size).
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func Sizeof(x ArbitraryType) uintptr
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// Offsetof returns the offset within the struct of the field represented by x,
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// which must be of the form structValue.field. In other words, it returns the
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// number of bytes between the start of the struct and the start of the field.
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// The return value of Offsetof is a Go constant if the type of the argument x
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// does not have variable size.
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// (See the description of [Sizeof] for a definition of variable sized types.)
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func Offsetof(x ArbitraryType) uintptr
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// Alignof takes an expression x of any type and returns the required alignment
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// of a hypothetical variable v as if v was declared via var v = x.
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// It is the largest value m such that the address of v is always zero mod m.
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// It is the same as the value returned by [reflect.TypeOf](x).Align().
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// As a special case, if a variable s is of struct type and f is a field
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// within that struct, then Alignof(s.f) will return the required alignment
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// of a field of that type within a struct. This case is the same as the
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// value returned by [reflect.TypeOf](s.f).FieldAlign().
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// The return value of Alignof is a Go constant if the type of the argument
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// does not have variable size.
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// (See the description of [Sizeof] for a definition of variable sized types.)
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func Alignof(x ArbitraryType) uintptr
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// The function Add adds len to ptr and returns the updated pointer
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// [Pointer](uintptr(ptr) + uintptr(len)).
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// The len argument must be of integer type or an untyped constant.
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// A constant len argument must be representable by a value of type int;
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// if it is an untyped constant it is given type int.
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// The rules for valid uses of Pointer still apply.
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func Add(ptr Pointer, len IntegerType) Pointer
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// The function Slice returns a slice whose underlying array starts at ptr
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// and whose length and capacity are len.
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// Slice(ptr, len) is equivalent to
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//
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// (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]
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//
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// except that, as a special case, if ptr is nil and len is zero,
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// Slice returns nil.
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//
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// The len argument must be of integer type or an untyped constant.
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// A constant len argument must be non-negative and representable by a value of type int;
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// if it is an untyped constant it is given type int.
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// At run time, if len is negative, or if ptr is nil and len is not zero,
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// a run-time panic occurs.
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func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType
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// SliceData returns a pointer to the underlying array of the argument
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// slice.
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// - If cap(slice) > 0, SliceData returns &slice[:1][0].
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// - If slice == nil, SliceData returns nil.
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// - Otherwise, SliceData returns a non-nil pointer to an
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// unspecified memory address.
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func SliceData(slice []ArbitraryType) *ArbitraryType
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// String returns a string value whose underlying bytes
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// start at ptr and whose length is len.
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//
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// The len argument must be of integer type or an untyped constant.
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// A constant len argument must be non-negative and representable by a value of type int;
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// if it is an untyped constant it is given type int.
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// At run time, if len is negative, or if ptr is nil and len is not zero,
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// a run-time panic occurs.
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//
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// Since Go strings are immutable, the bytes passed to String
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// must not be modified afterwards.
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func String(ptr *byte, len IntegerType) string
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// StringData returns a pointer to the underlying bytes of str.
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// For an empty string the return value is unspecified, and may be nil.
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//
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// Since Go strings are immutable, the bytes returned by StringData
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// must not be modified.
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func StringData(str string) *byte
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