Commit 2a09a68d authored by Russ Cox's avatar Russ Cox

unsafe: document valid uses of Pointer

Add docs for valid uses of Pointer.
Then document change made for #13372 in CL 18584.

Fixes #8994.

Change-Id: Ifba71e5aeafd11f684aed0b7ddacf3c8ec07c580
Reviewed-on: https://go-review.googlesource.com/18640Reviewed-by: default avatarAlan Donovan <adonovan@google.com>
Reviewed-by: default avatarRobert Griesemer <gri@golang.org>
Reviewed-by: default avatarIan Lance Taylor <iant@golang.org>
Reviewed-by: default avatarRob Pike <r@golang.org>
Reviewed-by: default avatarAustin Clements <austin@google.com>
Reviewed-by: default avatarRick Hudson <rlh@golang.org>
parent 1b6d55ac
......@@ -15,13 +15,160 @@ package unsafe
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.
// 1) A pointer value of any type can be converted to a Pointer.
// 2) A Pointer can be converted to a pointer value of any type.
// 3) A uintptr can be converted to a Pointer.
// 4) A Pointer can be converted to a uintptr.
// 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 uintptr.
//
// 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,
// but 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 = uintptr(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 = uintptr(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
......
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