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Diffstat (limited to 'src/unsafe/unsafe.go')
| -rw-r--r-- | src/unsafe/unsafe.go | 205 |
1 files changed, 205 insertions, 0 deletions
diff --git a/src/unsafe/unsafe.go b/src/unsafe/unsafe.go new file mode 100644 index 0000000..272761d --- /dev/null +++ b/src/unsafe/unsafe.go @@ -0,0 +1,205 @@ +// Copyright 2009 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +/* + Package unsafe contains operations that step around the type safety of Go programs. + + Packages that import unsafe may be non-portable and are not protected by the + Go 1 compatibility guidelines. +*/ +package unsafe + +// ArbitraryType is here for the purposes of documentation only and is not actually +// part of the unsafe package. It represents the type of an arbitrary Go expression. +type ArbitraryType int + +// Pointer represents a pointer to an arbitrary type. There are four special operations +// available for type Pointer that are not available for other types: +// - A pointer value of any type can be converted to a Pointer. +// - A Pointer can be converted to a pointer value of any type. +// - A uintptr can be converted to a Pointer. +// - A Pointer can be converted to a uintptr. +// Pointer therefore allows a program to defeat the type system and read and write +// arbitrary memory. It should be used with extreme care. +// +// The following patterns involving Pointer are valid. +// Code not using these patterns is likely to be invalid today +// or to become invalid in the future. +// Even the valid patterns below come with important caveats. +// +// Running "go vet" can help find uses of Pointer that do not conform to these patterns, +// but silence from "go vet" is not a guarantee that the code is valid. +// +// (1) Conversion of a *T1 to Pointer to *T2. +// +// Provided that T2 is no larger than T1 and that the two share an equivalent +// memory layout, this conversion allows reinterpreting data of one type as +// data of another type. An example is the implementation of +// math.Float64bits: +// +// func Float64bits(f float64) uint64 { +// return *(*uint64)(unsafe.Pointer(&f)) +// } +// +// (2) Conversion of a Pointer to a uintptr (but not back to Pointer). +// +// Converting a Pointer to a uintptr produces the memory address of the value +// pointed at, as an integer. The usual use for such a uintptr is to print it. +// +// Conversion of a uintptr back to Pointer is not valid in general. +// +// A uintptr is an integer, not a reference. +// Converting a Pointer to a uintptr creates an integer value +// with no pointer semantics. +// Even if a uintptr holds the address of some object, +// the garbage collector will not update that uintptr's value +// if the object moves, nor will that uintptr keep the object +// from being reclaimed. +// +// The remaining patterns enumerate the only valid conversions +// from uintptr to Pointer. +// +// (3) Conversion of a Pointer to a uintptr and back, with arithmetic. +// +// If p points into an allocated object, it can be advanced through the object +// by conversion to uintptr, addition of an offset, and conversion back to Pointer. +// +// p = unsafe.Pointer(uintptr(p) + offset) +// +// The most common use of this pattern is to access fields in a struct +// or elements of an array: +// +// // equivalent to f := unsafe.Pointer(&s.f) +// f := unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f)) +// +// // equivalent to e := unsafe.Pointer(&x[i]) +// e := unsafe.Pointer(uintptr(unsafe.Pointer(&x[0])) + i*unsafe.Sizeof(x[0])) +// +// It is valid both to add and to subtract offsets from a pointer in this way. +// It is also valid to use &^ to round pointers, usually for alignment. +// In all cases, the result must continue to point into the original allocated object. +// +// Unlike in C, it is not valid to advance a pointer just beyond the end of +// its original allocation: +// +// // INVALID: end points outside allocated space. +// var s thing +// end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s)) +// +// // INVALID: end points outside allocated space. +// b := make([]byte, n) +// end = unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(n)) +// +// Note that both conversions must appear in the same expression, with only +// the intervening arithmetic between them: +// +// // INVALID: uintptr cannot be stored in variable +// // before conversion back to Pointer. +// u := uintptr(p) +// p = unsafe.Pointer(u + offset) +// +// Note that the pointer must point into an allocated object, so it may not be nil. +// +// // INVALID: conversion of nil pointer +// u := unsafe.Pointer(nil) +// p := unsafe.Pointer(uintptr(u) + offset) +// +// (4) Conversion of a Pointer to a uintptr when calling syscall.Syscall. +// +// The Syscall functions in package syscall pass their uintptr arguments directly +// to the operating system, which then may, depending on the details of the call, +// reinterpret some of them as pointers. +// That is, the system call implementation is implicitly converting certain arguments +// back from uintptr to pointer. +// +// If a pointer argument must be converted to uintptr for use as an argument, +// that conversion must appear in the call expression itself: +// +// syscall.Syscall(SYS_READ, uintptr(fd), uintptr(unsafe.Pointer(p)), uintptr(n)) +// +// The compiler handles a Pointer converted to a uintptr in the argument list of +// a call to a function implemented in assembly by arranging that the referenced +// allocated object, if any, is retained and not moved until the call completes, +// even though from the types alone it would appear that the object is no longer +// needed during the call. +// +// For the compiler to recognize this pattern, +// the conversion must appear in the argument list: +// +// // INVALID: uintptr cannot be stored in variable +// // before implicit conversion back to Pointer during system call. +// u := uintptr(unsafe.Pointer(p)) +// syscall.Syscall(SYS_READ, uintptr(fd), u, uintptr(n)) +// +// (5) Conversion of the result of reflect.Value.Pointer or reflect.Value.UnsafeAddr +// from uintptr to Pointer. +// +// Package reflect's Value methods named Pointer and UnsafeAddr return type uintptr +// instead of unsafe.Pointer to keep callers from changing the result to an arbitrary +// type without first importing "unsafe". However, this means that the result is +// fragile and must be converted to Pointer immediately after making the call, +// in the same expression: +// +// p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer())) +// +// As in the cases above, it is invalid to store the result before the conversion: +// +// // INVALID: uintptr cannot be stored in variable +// // before conversion back to Pointer. +// u := reflect.ValueOf(new(int)).Pointer() +// p := (*int)(unsafe.Pointer(u)) +// +// (6) Conversion of a reflect.SliceHeader or reflect.StringHeader Data field to or from Pointer. +// +// As in the previous case, the reflect data structures SliceHeader and StringHeader +// declare the field Data as a uintptr to keep callers from changing the result to +// an arbitrary type without first importing "unsafe". However, this means that +// SliceHeader and StringHeader are only valid when interpreting the content +// of an actual slice or string value. +// +// var s string +// hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1 +// hdr.Data = uintptr(unsafe.Pointer(p)) // case 6 (this case) +// hdr.Len = n +// +// In this usage hdr.Data is really an alternate way to refer to the underlying +// pointer in the string header, not a uintptr variable itself. +// +// In general, reflect.SliceHeader and reflect.StringHeader should be used +// only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual +// slices or strings, never as plain structs. +// A program should not declare or allocate variables of these struct types. +// +// // INVALID: a directly-declared header will not hold Data as a reference. +// var hdr reflect.StringHeader +// hdr.Data = uintptr(unsafe.Pointer(p)) +// hdr.Len = n +// s := *(*string)(unsafe.Pointer(&hdr)) // p possibly already lost +// +type Pointer *ArbitraryType + +// Sizeof takes an expression x of any type and returns the size in bytes +// of a hypothetical variable v as if v was declared via var v = x. +// The size does not include any memory possibly referenced by x. +// For instance, if x is a slice, Sizeof returns the size of the slice +// descriptor, not the size of the memory referenced by the slice. +// The return value of Sizeof is a Go constant. +func Sizeof(x ArbitraryType) uintptr + +// Offsetof returns the offset within the struct of the field represented by x, +// which must be of the form structValue.field. In other words, it returns the +// number of bytes between the start of the struct and the start of the field. +// The return value of Offsetof is a Go constant. +func Offsetof(x ArbitraryType) uintptr + +// Alignof takes an expression x of any type and returns the required alignment +// of a hypothetical variable v as if v was declared via var v = x. +// It is the largest value m such that the address of v is always zero mod m. +// It is the same as the value returned by reflect.TypeOf(x).Align(). +// As a special case, if a variable s is of struct type and f is a field +// within that struct, then Alignof(s.f) will return the required alignment +// of a field of that type within a struct. This case is the same as the +// value returned by reflect.TypeOf(s.f).FieldAlign(). +// The return value of Alignof is a Go constant. +func Alignof(x ArbitraryType) uintptr |