// 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 // IntegerType is here for the purposes of documentation only and is not actually // part of the unsafe package. It represents any arbitrary integer type. type IntegerType 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. // For a struct, the size includes any padding introduced by field alignment. // The return value of Sizeof is a Go constant if the type of the argument x // does not have variable size. // (A type has variable size if it is a type parameter or if it is an array // or struct type with elements of variable size). 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 if the type of the argument x // does not have variable size. // (See the description of [Sizeof] for a definition of variable sized types.) 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 if the type of the argument // does not have variable size. // (See the description of [Sizeof] for a definition of variable sized types.) func Alignof(x ArbitraryType) uintptr // The function Add adds len to ptr and returns the updated pointer // Pointer(uintptr(ptr) + uintptr(len)). // The len argument must be of integer type or an untyped constant. // A constant len argument must be representable by a value of type int; // if it is an untyped constant it is given type int. // The rules for valid uses of Pointer still apply. func Add(ptr Pointer, len IntegerType) Pointer // The function Slice returns a slice whose underlying array starts at ptr // and whose length and capacity are len. // Slice(ptr, len) is equivalent to // // (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:] // // except that, as a special case, if ptr is nil and len is zero, // Slice returns nil. // // The len argument must be of integer type or an untyped constant. // A constant len argument must be non-negative and representable by a value of type int; // if it is an untyped constant it is given type int. // At run time, if len is negative, or if ptr is nil and len is not zero, // a run-time panic occurs. func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType