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: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/unsafe/unsafe.go

@@ -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