| // 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) |
| // |
| // (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 slice 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. |
| 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. |
| 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(). |
| func Alignof(x ArbitraryType) uintptr |
