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unsafe: document when Sizeof/Offsetof/Alignof are not constant 

They are not constant if their arguments have types that are variable size.

Fixes #53921

Change-Id: I2d46754c27f55a281331c099a11ea3cd85ec4e89
Reviewed-on: https://go-review.googlesource.com/c/go/+/417917
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Rob Pike <r@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
This commit is contained in:
Ian Lance Taylor 2022-07-16 15:51:25 -07:00 committed by Gopher Robot
parent 076c3d7f07
commit 2d655fb15a
1 changed files with 10 additions and 3 deletions
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13
src/unsafe/unsafe.go
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@ -189,13 +189,18 @@ type Pointer *ArbitraryType
// For instance, if x is a slice, Sizeof returns the size of the slice // 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. // descriptor, not the size of the memory referenced by the slice.
// For a struct, the size includes any padding introduced by field alignment. // For a struct, the size includes any padding introduced by field alignment.
// The return value of Sizeof is a Go constant. // 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 func Sizeof(x ArbitraryType) uintptr
// Offsetof returns the offset within the struct of the field represented by x, // 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 // 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. // number of bytes between the start of the struct and the start of the field.
// The return value of Offsetof is a Go constant. // 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 func Offsetof(x ArbitraryType) uintptr
// Alignof takes an expression x of any type and returns the required alignment // Alignof takes an expression x of any type and returns the required alignment
@ -206,7 +211,9 @@ func Offsetof(x ArbitraryType) uintptr
// within that struct, then Alignof(s.f) will return the required alignment // 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 // of a field of that type within a struct. This case is the same as the
// value returned by reflect.TypeOf(s.f).FieldAlign(). // value returned by reflect.TypeOf(s.f).FieldAlign().
// The return value of Alignof is a Go constant. // 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 func Alignof(x ArbitraryType) uintptr
// The function Add adds len to ptr and returns the updated pointer // The function Add adds len to ptr and returns the updated pointer