version: 1.10

package unsafe

import "unsafe"

Overview

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.

Index

Package files

unsafe.go

func Alignof

  1. func Alignof(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 Offsetof

  1. func Offsetof(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 Sizeof

  1. func Sizeof(x ArbitraryType) uintptr

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.

type ArbitraryType

  1. type ArbitraryType int

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 Pointer

  1. type Pointer *ArbitraryType

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.

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:

  1. func Float64bits(f float64) uint64 {
  2.     return *(*uint64)(unsafe.Pointer(&f))
  3. }

(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.

  1. 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:

  1. // equivalent to f := unsafe.Pointer(&s.f)
  2. f := unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f))
  4. // equivalent to e := unsafe.Pointer(&x[i])
  5. 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:

  1. // INVALID: end points outside allocated space.
  2. var s thing
  3. end = unsafe.Pointer(uintptr(unsafe.Pointer(&s)) + unsafe.Sizeof(s))
  5. // INVALID: end points outside allocated space.
  6. b := make([]byte, n)
  7. 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:

  1. // INVALID: uintptr cannot be stored in variable
  2. // before conversion back to Pointer.
  3. u := uintptr(p)
  4. 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:

  1. 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:

  1. // INVALID: uintptr cannot be stored in variable
  2. // before implicit conversion back to Pointer during system call.
  3. u := uintptr(unsafe.Pointer(p))
  4. 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:

  1. p := (*int)(unsafe.Pointer(reflect.ValueOf(new(int)).Pointer()))

As in the cases above, it is invalid to store the result before the conversion:

  1. // INVALID: uintptr cannot be stored in variable
  2. // before conversion back to Pointer.
  3. u := reflect.ValueOf(new(int)).Pointer()
  4. 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.

  1. var s string
  2. hdr := (*reflect.StringHeader)(unsafe.Pointer(&s)) // case 1
  3. hdr.Data = uintptr(unsafe.Pointer(p))              // case 6 (this case)
  4. 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.

  1. // INVALID: a directly-declared header will not hold Data as a reference.
  2. var hdr reflect.StringHeader
  3. hdr.Data = uintptr(unsafe.Pointer(p))
  4. hdr.Len = n
  5. s := *(*string)(unsafe.Pointer(&hdr)) // p possibly already lost