1 // Copyright 2009 The Go Authors. All rights reserved. 2 // Use of this source code is governed by a BSD-style 3 // license that can be found in the LICENSE file. 4 5 /* 6 Package unsafe contains operations that step around the type safety of Go programs. 7 8 Packages that import unsafe may be non-portable and are not protected by the 9 Go 1 compatibility guidelines. 10 */ 11 package unsafe 12 13 // ArbitraryType is here for the purposes of documentation only and is not actually 14 // part of the unsafe package. It represents the type of an arbitrary Go expression. 15 type ArbitraryType int 16 17 // IntegerType is here for the purposes of documentation only and is not actually 18 // part of the unsafe package. It represents any arbitrary integer type. 19 type IntegerType int 20 21 // Pointer represents a pointer to an arbitrary type. There are four special operations 22 // available for type Pointer that are not available for other types: 23 // - A pointer value of any type can be converted to a Pointer. 24 // - A Pointer can be converted to a pointer value of any type. 25 // - A uintptr can be converted to a Pointer. 26 // - A Pointer can be converted to a uintptr. 27 // Pointer therefore allows a program to defeat the type system and read and write 28 // arbitrary memory. It should be used with extreme care. 29 // 30 // The following patterns involving Pointer are valid. 31 // Code not using these patterns is likely to be invalid today 32 // or to become invalid in the future. 33 // Even the valid patterns below come with important caveats. 34 // 35 // Running "go vet" can help find uses of Pointer that do not conform to these patterns, 36 // but silence from "go vet" is not a guarantee that the code is valid. 37 // 38 // (1) Conversion of a *T1 to Pointer to *T2. 39 // 40 // Provided that T2 is no larger than T1 and that the two share an equivalent 41 // memory layout, this conversion allows reinterpreting data of one type as 42 // data of another type. An example is the implementation of 43 // math.Float64bits: 44 // 45 // func Float64bits(f float64) uint64 { 46 // return *(*uint64)(unsafe.Pointer(&f)) 47 // } 48 // 49 // (2) Conversion of a Pointer to a uintptr (but not back to Pointer). 50 // 51 // Converting a Pointer to a uintptr produces the memory address of the value 52 // pointed at, as an integer. The usual use for such a uintptr is to print it. 53 // 54 // Conversion of a uintptr back to Pointer is not valid in general. 55 // 56 // A uintptr is an integer, not a reference. 57 // Converting a Pointer to a uintptr creates an integer value 58 // with no pointer semantics. 59 // Even if a uintptr holds the address of some object, 60 // the garbage collector will not update that uintptr's value 61 // if the object moves, nor will that uintptr keep the object 62 // from being reclaimed. 63 // 64 // The remaining patterns enumerate the only valid conversions 65 // from uintptr to Pointer. 66 // 67 // (3) Conversion of a Pointer to a uintptr and back, with arithmetic. 68 // 69 // If p points into an allocated object, it can be advanced through the object 70 // by conversion to uintptr, addition of an offset, and conversion back to Pointer. 71 // 72 // p = unsafe.Pointer(uintptr(p) + offset) 73 // 74 // The most common use of this pattern is to access fields in a struct 75 // or elements of an array: 76 // 77 // // equivalent to f := unsafe.Pointer(&s.f) 78 // f := unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f)) 79 // 80 // // equivalent to e := unsafe.Pointer(&x[i]) 81 // e := unsafe.Pointer(uintptr(unsafe.Pointer(&x[0])) + i*unsafe.Sizeof(x[0])) 82 // 83 // It is valid both to add and to subtract offsets from a pointer in this way. 84 // It is also valid to use &^ to round pointers, usually for alignment. 85 // In all cases, the result must continue to point into the original allocated object. 86 // 87 // Unlike in C, it is not valid to advance a pointer just beyond the end of 88 // its original allocation: 89 // 90 // // INVALID: end points outside allocated space. 91 // var s thing 92 // end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s)) 93 // 94 // // INVALID: end points outside allocated space. 95 // b := make([]byte, n) 96 // end = unsafe.Pointer(uintptr(unsafe.Pointer(&b[0])) + uintptr(n)) 97 // 98 // Note that both conversions must appear in the same expression, with only 99 // the intervening arithmetic between them: 100 // 101 // // INVALID: uintptr cannot be stored in variable 102 // // before conversion back to Pointer. 103 // u := uintptr(p) 104 // p = unsafe.Pointer(u + offset) 105 // 106 // Note that the pointer must point into an allocated object, so it may not be nil. 107 // 108 // // INVALID: conversion of nil pointer 109 // u := unsafe.Pointer(nil) 110 // p := unsafe.Pointer(uintptr(u) + offset) 111 // 112 // (4) Conversion of a Pointer to a uintptr when calling syscall.Syscall. 113 // 114 // The Syscall functions in package syscall pass their uintptr arguments directly 115 // to the operating system, which then may, depending on the details of the call, 116 // reinterpret some of them as pointers. 117 // That is, the system call implementation is implicitly converting certain arguments 118 // back from uintptr to pointer. 119 // 120 // If a pointer argument must be converted to uintptr for use as an argument, 121 // that conversion must appear in the call expression itself: 122 // 123 // syscall.Syscall(SYS_READ, uintptr(fd), uintptr(unsafe.Pointer(p)), uintptr(n)) 124 // 125 // The compiler handles a Pointer converted to a uintptr in the argument list of 126 // a call to a function implemented in assembly by arranging that the referenced 127 // allocated object, if any, is retained and not moved until the call completes, 128 // even though from the types alone it would appear that the object is no longer 129 // needed during the call. 130 // 131 // For the compiler to recognize this pattern, 132 // the conversion must appear in the argument list: 133 // 134 // // INVALID: uintptr cannot be stored in variable 135 // // before implicit conversion back to Pointer during system call. 136 // u := uintptr(unsafe.Pointer(p)) 137 // syscall.Syscall(SYS_READ, uintptr(fd), u, uintptr(n)) 138 // 139 // (5) Conversion of the result of reflect.Value.Pointer or reflect.Value.UnsafeAddr 140 // from uintptr to Pointer. 141 // 142 // Package reflect's Value methods named Pointer and UnsafeAddr return type uintptr 143 // instead of unsafe.Pointer to keep callers from changing the result to an arbitrary 144 // type without first importing "unsafe". However, this means that the result is 145 // fragile and must be converted to Pointer immediately after making the call, 146 // in the same expression: 147 // 148 // p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer())) 149 // 150 // As in the cases above, it is invalid to store the result before the conversion: 151 // 152 // // INVALID: uintptr cannot be stored in variable 153 // // before conversion back to Pointer. 154 // u := reflect.ValueOf(new(int)).Pointer() 155 // p := (*int)(unsafe.Pointer(u)) 156 // 157 // (6) Conversion of a reflect.SliceHeader or reflect.StringHeader Data field to or from Pointer. 158 // 159 // As in the previous case, the reflect data structures SliceHeader and StringHeader 160 // declare the field Data as a uintptr to keep callers from changing the result to 161 // an arbitrary type without first importing "unsafe". However, this means that 162 // SliceHeader and StringHeader are only valid when interpreting the content 163 // of an actual slice or string value. 164 // 165 // var s string 166 // hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1 167 // hdr.Data = uintptr(unsafe.Pointer(p)) // case 6 (this case) 168 // hdr.Len = n 169 // 170 // In this usage hdr.Data is really an alternate way to refer to the underlying 171 // pointer in the string header, not a uintptr variable itself. 172 // 173 // In general, reflect.SliceHeader and reflect.StringHeader should be used 174 // only as *reflect.SliceHeader and *reflect.StringHeader pointing at actual 175 // slices or strings, never as plain structs. 176 // A program should not declare or allocate variables of these struct types. 177 // 178 // // INVALID: a directly-declared header will not hold Data as a reference. 179 // var hdr reflect.StringHeader 180 // hdr.Data = uintptr(unsafe.Pointer(p)) 181 // hdr.Len = n 182 // s := *(*string)(unsafe.Pointer(&hdr)) // p possibly already lost 183 // 184 type Pointer *ArbitraryType 185 186 // Sizeof takes an expression x of any type and returns the size in bytes 187 // of a hypothetical variable v as if v was declared via var v = x. 188 // The size does not include any memory possibly referenced by x. 189 // For instance, if x is a slice, Sizeof returns the size of the slice 190 // descriptor, not the size of the memory referenced by the slice. 191 // The return value of Sizeof is a Go constant. 192 func Sizeof(x ArbitraryType) uintptr 193 194 // Offsetof returns the offset within the struct of the field represented by x, 195 // which must be of the form structValue.field. In other words, it returns the 196 // number of bytes between the start of the struct and the start of the field. 197 // The return value of Offsetof is a Go constant. 198 func Offsetof(x ArbitraryType) uintptr 199 200 // Alignof takes an expression x of any type and returns the required alignment 201 // of a hypothetical variable v as if v was declared via var v = x. 202 // It is the largest value m such that the address of v is always zero mod m. 203 // It is the same as the value returned by reflect.TypeOf(x).Align(). 204 // As a special case, if a variable s is of struct type and f is a field 205 // within that struct, then Alignof(s.f) will return the required alignment 206 // of a field of that type within a struct. This case is the same as the 207 // value returned by reflect.TypeOf(s.f).FieldAlign(). 208 // The return value of Alignof is a Go constant. 209 func Alignof(x ArbitraryType) uintptr 210 211 // The function Add adds len to ptr and returns the updated pointer 212 // Pointer(uintptr(ptr) + uintptr(len)). 213 // The len argument must be of integer type or an untyped constant. 214 // A constant len argument must be representable by a value of type int; 215 // if it is an untyped constant it is given type int. 216 // The rules for valid uses of Pointer still apply. 217 func Add(ptr Pointer, len IntegerType) Pointer 218 219 // The function Slice returns a slice whose underlying array starts at ptr 220 // and whose length and capacity are len. 221 // Slice(ptr, len) is equivalent to 222 // 223 // (*[len]ArbitraryType)(unsafe.Pointer(ptr))[:] 224 // 225 // except that, as a special case, if ptr is nil and len is zero, 226 // Slice returns nil. 227 // 228 // The len argument must be of integer type or an untyped constant. 229 // A constant len argument must be non-negative and representable by a value of type int; 230 // if it is an untyped constant it is given type int. 231 // At run time, if len is negative, or if ptr is nil and len is not zero, 232 // a run-time panic occurs. 233 func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType 234