In
computer science
, a
type punning
is any programming technique that subverts or circumvents the
type system
of a
programming language
in order to achieve an effect that would be difficult or impossible to achieve within the bounds of the formal language.
In
C
and
C++
, constructs such as
pointer
type conversion
and
union
? C++ adds
reference
type conversion and
reinterpret_cast
to this list ? are provided in order to permit many kinds of type punning, although some kinds are not actually supported by the standard language.
In the
Pascal
programming language, the use of
records
with
variants
may be used to treat a particular data type in more than one manner, or in a manner not normally permitted.
Sockets example
[
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]
One classic example of type punning is found in the
Berkeley sockets
interface. The function to bind an opened but uninitialized socket to an
IP address
is declared as follows:
int
bind
(
int
sockfd
,
struct
sockaddr
*
my_addr
,
socklen_t
addrlen
);
The
bind
function is usually called as follows:
struct
sockaddr_in
sa
=
{
0
};
int
sockfd
=
...;
sa
.
sin_family
=
AF_INET
;
sa
.
sin_port
=
htons
(
port
);
bind
(
sockfd
,
(
struct
sockaddr
*
)
&
sa
,
sizeof
sa
);
The Berkeley sockets library fundamentally relies on the fact that in
C
, a pointer to
struct sockaddr_in
is freely convertible to a pointer to
struct sockaddr
; and, in addition, that the two structure types share the same memory layout. Therefore, a reference to the structure field
my_addr->sin_family
(where
my_addr
is of type
struct sockaddr*
) will actually refer to the field
sa.sin_family
(where
sa
is of type
struct sockaddr_in
). In other words, the sockets library uses type punning to implement a rudimentary form of
polymorphism
or
inheritance
.
Often seen in the programming world is the use of "padded" data structures to allow for the storage of different kinds of values in what is effectively the same storage space. This is often seen when two structures are used in mutual exclusivity for optimization.
Floating-point example
[
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]
Not all examples of type punning involve structures, as the previous example did. Suppose we want to determine whether a
floating-point
number is negative. We could write:
bool
is_negative
(
float
x
)
{
return
x
<
0.0f
;
}
However, supposing that floating-point comparisons are expensive, and also supposing that
float
is represented according to the
IEEE floating-point standard
, and integers are 32 bits wide, we could engage in type punning to extract the
sign bit
of the floating-point number using only integer operations:
bool
is_negative
(
float
x
)
{
int
*
i
=
(
int
*
)
&
x
;
return
*
i
<
0
;
}
Note that the behaviour will not be exactly the same: in the special case of
x
being
negative zero
, the first implementation yields
false
while the second yields
true
. Also, the first implementation will return
false
for any
NaN
value, but the latter might return
true
for NaN values with the sign bit set.
This kind of type punning is more dangerous than most. Whereas the former example relied only on guarantees made by the C programming language about structure layout and pointer convertibility, the latter example relies on assumptions about a particular system's hardware. Some situations, such as
time-critical
code that the compiler otherwise fails to
optimize
, may require dangerous code. In these cases, documenting all such assumptions in
comments
, and introducing
static assertions
to verify portability expectations, helps to keep the code
maintainable
.
Practical examples of floating-point punning include
fast inverse square root
popularized by
Quake III
, fast FP comparison as integers,
[1]
and finding neighboring values by incrementing as an integer (implementing
nextafter
).
[2]
By language
[
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]
C and C++
[
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]
In addition to the assumption about bit-representation of floating-point numbers, the above floating-point type-punning example also violates the C language's constraints on how objects are accessed:
[3]
the declared type of
x
is
float
but it is read through an expression of type
unsigned int
. On many common platforms, this use of pointer punning can create problems if different pointers are
aligned in machine-specific ways
. Furthermore, pointers of different sizes can
alias accesses to the same memory
, causing problems that are unchecked by the compiler. Even when data size and pointer representation match, however, compilers can rely on the non-aliasing constraints to perform optimizations that would be unsafe in the presence of disallowed aliasing.
Use of pointers
[
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]
A naive attempt at type-punning can be achieved by using pointers: (The following running example assumes IEEE-754 bit-representation for type
float
.)
bool
is_negative
(
float
x
)
{
int32_t
i
=
*
(
int32_t
*
)
&
x
;
// In C++ this is equivalent to: int32_t i = *reinterpret_cast<int32_t*>(&x);
return
i
<
0
;
}
The C standard's aliasing rules state that an object shall have its stored value accessed only by an lvalue expression of a compatible type.
[4]
The types
float
and
int32_t
are not compatible, therefore this code's behavior is
undefined
. Although on GCC and LLVM this particular program compiles and runs as expected, more complicated examples may interact with assumptions made by
strict aliasing
and lead to unwanted behavior. The option
-fno-strict-aliasing
will ensure correct behavior of code using this form of type-punning, although using other forms of type punning is recommended.
[5]
Use of
union
[
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]
In C, but not in C++, it is sometimes possible to perform type punning via a
union
.
bool
is_negative
(
float
x
)
{
union
{
int
i
;
float
d
;
}
my_union
;
my_union
.
d
=
x
;
return
my_union
.
i
<
0
;
}
Accessing
my_union.i
after most recently writing to the other member,
my_union.d
, is an allowed form of type-punning in C,
[6]
provided that the member read is not larger than the one whose value was set (otherwise the read has
unspecified behavior
[7]
). The same is syntactically valid but has
undefined behavior
in C++,
[8]
however, where only the last-written member of a
union
is considered to have any value at all.
For another example of type punning, see
Stride of an array
.
Use of
bit_cast
[
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]
In
C++20
, the
std::bit_cast
function allows type punning with no undefined behavior. It also allows the function be labeled
constexpr
.
constexpr
bool
is_negative
(
float
x
)
noexcept
{
static_assert
(
std
::
numeric_limits
<
float
>::
is_iec559
);
// (enable only on IEEE 754)
auto
i
=
std
::
bit_cast
<
std
::
int32_t
>
(
x
);
return
i
<
0
;
}
Pascal
[
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]
A variant record permits treating a data type as multiple kinds of data depending on which variant is being referenced. In the following example,
integer
is presumed to be 16 bit, while
longint
and
real
are presumed to be 32, while character is presumed to be 8 bit:
type
VariantRecord
=
record
case
RecType
:
LongInt
of
1
:
(
I
:
array
[
1
..
2
]
of
Integer
)
;
(* not show here: there can be several variables in a variant record's case statement *)
2
:
(
L
:
LongInt
)
;
3
:
(
R
:
Real
)
;
4
:
(
C
:
array
[
1
..
4
]
of
Char
)
;
end
;
var
V
:
VariantRecord
;
K
:
Integer
;
LA
:
LongInt
;
RA
:
Real
;
Ch
:
Character
;
V
.
I
[
1
]
:=
1
;
Ch
:=
V
.
C
[
1
]
;
(* this would extract the first byte of V.I *)
V
.
R
:=
8.3
;
LA
:=
V
.
L
;
(* this would store a Real into an Integer *)
In Pascal, copying a real to an integer converts it to the truncated value. This method would translate the binary value of the floating-point number into whatever it is as a long integer (32 bit), which will not be the same and may be incompatible with the long integer value on some systems.
These examples could be used to create strange conversions, although, in some cases, there may be legitimate uses for these types of constructs, such as for determining locations of particular pieces of data. In the following example a pointer and a longint are both presumed to be 32 bit:
type
PA
=
^
Arec
;
Arec
=
record
case
RT
:
LongInt
of
1
:
(
P
:
PA
)
;
2
:
(
L
:
LongInt
)
;
end
;
var
PP
:
PA
;
K
:
LongInt
;
New
(
PP
)
;
PP
^.
P
:=
PP
;
WriteLn
(
'Variable PP is located at address '
,
Hex
(
PP
^.
L
))
;
Where "new" is the standard routine in Pascal for allocating memory for a pointer, and "hex" is presumably a routine to print the hexadecimal string describing the value of an integer. This would allow the display of the address of a pointer, something which is not normally permitted. (Pointers cannot be read or written, only assigned.) Assigning a value to an integer variant of a pointer would allow examining or writing to any location in system memory:
PP
^.
L
:=
0
;
PP
:=
PP
^.
P
;
(* PP now points to address 0 *)
K
:=
PP
^.
L
;
(* K contains the value of word 0 *)
WriteLn
(
'Word 0 of this machine contains '
,
K
)
;
This construct may cause a program check or protection violation if address 0 is protected against reading on the machine the program is running upon or the operating system it is running under.
The reinterpret cast technique from C/C++ also works in Pascal. This can be useful, when eg. reading dwords from a byte stream, and we want to treat them as float. Here is a working example, where we reinterpret-cast a dword to a float:
type
pReal
=
^
Real
;
var
DW
:
DWord
;
F
:
Real
;
F
:=
pReal
(
@
DW
)
^;
In
C#
(and other .NET languages), type punning is a little harder to achieve because of the type system, but can be done nonetheless, using pointers or struct unions.
Pointers
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]
C# only allows pointers to so-called native types, i.e. any primitive type (except
string
), enum, array or struct that is composed only of other native types. Note that pointers are only allowed in code blocks marked 'unsafe'.
float
pi
=
3.14159
;
uint
piAsRawData
=
*
(
uint
*
)
&
pi
;
Struct unions
[
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]
Struct unions are allowed without any notion of 'unsafe' code, but they do require the definition of a new type.
[StructLayout(LayoutKind.Explicit)]
struct
FloatAndUIntUnion
{
[FieldOffset(0)]
public
float
DataAsFloat
;
[FieldOffset(0)]
public
uint
DataAsUInt
;
}
// ...
FloatAndUIntUnion
union
;
union
.
DataAsFloat
=
3.14159
;
uint
piAsRawData
=
union
.
DataAsUInt
;
Raw CIL code
[
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]
Raw
CIL
can be used instead of C#, because it doesn't have most of the type limitations. This allows one to, for example, combine two enum values of a generic type:
TEnum
a
=
...;
TEnum
b
=
...;
TEnum
combined
=
a
|
b
;
// illegal
This can be circumvented by the following CIL code:
.
method
public
static
hidebysig
!!
TEnum
CombineEnums
<
valuetype
.
ctor
([
mscorlib
]
System
.
ValueType
)
TEnum
>
(
!!
TEnum
a
,
!!
TEnum
b
)
cil
managed
{
.
maxstack
2
ldarg
.
0
ldarg
.
1
or
// this will not cause an overflow, because a and b have the same type, and therefore the same size.
ret
}
The
cpblk
CIL opcode allows for some other tricks, such as converting a struct to a byte array:
.
method
public
static
hidebysig
uint8
[]
ToByteArray
<
valuetype
.
ctor
([
mscorlib
]
System
.
ValueType
)
T
>
(
!!
T
&
v
// 'ref T' in C#
)
cil
managed
{
.
locals
init
(
[0]
uint8
[]
)
.
maxstack
3
// create a new byte array with length sizeof(T) and store it in local 0
sizeof
!!
T
newarr
uint8
dup
// keep a copy on the stack for later (1)
stloc
.
0
ldc
.
i4
.
0
ldelema
uint8
// memcpy(local 0, &v, sizeof(T));
// <the array is still on the stack, see (1)>
ldarg
.
0
// this is the *address* of 'v', because its type is '!!T&'
sizeof
!!
T
cpblk
ldloc
.
0
ret
}
References
[
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]
- ^
Herf, Michael (December 2001).
"radix tricks"
.
stereopsis : graphics
.
- ^
"Stupid Float Tricks"
.
Random ASCII - tech blog of Bruce Dawson
. 24 January 2012.
- ^
ISO/IEC 9899:1999 s6.5/7
- ^
"§ 6.5/7"
(PDF)
,
ISO/IEC 9899:2018
, 2018, p. 55, archived from
the original
(PDF)
on 2018-12-30,
An object shall have its stored value accessed only by an lvalue expression that has one of the following types: [...]
- ^
"GCC Bugs - GNU Project"
.
gcc.gnu.org
.
- ^
"§ 6.5.2.3/3, footnote 97"
(PDF)
,
ISO/IEC 9899:2018
, 2018, p. 59, archived from
the original
(PDF)
on 2018-12-30,
If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (
a process sometimes called "type punning"
).
This might be a trap representation.
- ^
"§ J.1/1, bullet 11"
(PDF)
,
ISO/IEC 9899:2018
, 2018, p. 403, archived from
the original
(PDF)
on 2018-12-30,
The following are unspecified: … The values of bytes that correspond to union members
other than the one last stored into
(6.2.6.1).
- ^
ISO/IEC 14882:2011 Section 9.5
External links
[
edit
]
- Section
of the
GCC
manual on
-fstrict-aliasing
, which defeats some type punning
- Defect Report 257
to the
C99
standard, incidentally defining "type punning" in terms of
union
, and discussing the issues surrounding the implementation-defined behavior of the last example above
- Defect Report 283
on the use of unions for type punning