c/language/object

C programs create, destroy, access, and manipulate objects.

An object, in C, is region of in the execution environment, the contents of which can represent values (a value is the meaning of the contents of an object, when interpreted as having a specific ).

Every object has
 * size (can be determined with )
 * alignment requirement
 * (automatic, static, allocated, thread-local)
 * (equal to storage duration or temporary)
 * effective type (see below)
 * value (which may be indeterminate)
 * optionally, an that denotes this object

Objects are created by, allocation functions, s, s, and by non-lvalue expressions that return.

Object representation
Except for, objects are composed of contiguous sequences of one or more bytes, each consisting of char_bit bits, and can be copied with memcpy into an object of type , where is the size of the object. The contents of the resulting array are known as object representation.

If two objects have the same object representation, they compare equal (except if they are floating-point NaNs). The opposite is not true: two objects that compare equal may have different object representations because not every bit of the object representation needs to participate in the value. Such bits may be used for padding to satisfy alignment requirement, for parity checks, to indicate trap representations, etc.

If an object representation does not represent any value of the object type, it is known as trap representation. Accessing a trap representation in any way other than reading it through an lvalue expression of character type is undefined behavior. The value of a structure or union is never a trap representation even if any particular member is one.

For the objects of type char, signed char, and unsigned char, every bit of the object representation is required to participate in the value representation and each possible bit pattern represents a distinct value (no padding, trap bits, or multiple representations allowed).

When objects of (short, int, long, long long) occupy multiple bytes, the use of those bytes is implementation-defined, but the two dominant implementations are big-endian (POWER, Sparc, Itanium) and little-endian (x86, x86_64): a big-endian platform stores the most significant byte at the lowest address of the region of storage occupied by the integer, a little-endian platform stores the least significant byte at the lowest address. See Endianness for detail. See also example below.

Although most implementations do not allow trap representations, padding bits, or multiple representations for integer types, there are exceptions; for example a value of an integer type on Itanium may be a trap representation.

Effective type
Every object has an effective type, which determines which accesses are valid and which violate the strict aliasing rules.

If the object was created by a, the declared type of that object is the object's effective type.

If the object was created by an allocation function (including realloc), it has no declared type. Such object acquires an effective type as follows:
 * The first write to that object through an lvalue that has a type other than character type, at which time the type of that lvalue becomes this object's effective type for that write and all subsequent reads.
 * memcpy or memmove copy another object into that object, or copy another object into that object as an array of character type, at which time the effective type of the source object (if it had one) becomes the effective type of this object for that write and all subsequent reads.
 * Any other access to the object with no declared type, the effective type is the type of the lvalue used for the access.

Strict aliasing
Given an object with effective type T1, using an lvalue expression (typically, dereferencing a pointer) of a different type T2 is undefined behavior, unless:
 * T2 and T1 are.
 * T2 is cvr-qualified version of a type that is with T1.
 * T2 is a signed or unsigned version of a type that is with T1.
 * T2 is an aggregate type or union type type that includes one of the aforementioned types among its members (including, recursively, a member of a subaggregate or contained union).
 * T2 is a character type (char, signed char, or unsigned char).

These rules control whether a function that receives two pointers must re-read one after writing through another:

Note that can be used to indicate that two pointers do not alias even if the rules above permit them to be.

Note that type-punning may also be performed through the inactive member of a.

Alignment
Every complete has a property called alignment requirement, which is an integer value of type size_t representing the number of bytes between successive addresses at which objects of this type can be allocated. The valid alignment values are non-negative integral powers of two.

In order to satisfy alignment requirements of all members of a struct, padding may be inserted after some of its members.

Each object type imposes its alignment requirement on every object of that type. The strictest (largest) fundamental alignment of any type is the alignment of max_align_t. The weakest (smallest) alignment is the alignment of the types, , and , and equals 1.