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Value categories

Each expression in C (an operator with its arguments, a function call, a constant, a variable name, etc) is characterized by two independent properties: a type and a value category.

Every expression belongs to one of three value categories: lvalue, non-lvalue object (rvalue), and function designator.

Lvalue expressions

Lvalue expression is any expression with object type other than the type void, which potentially designates an object (the behavior is undefined if an lvalue does not actually designate an object when it is evaluated). In other words, lvalue expression evaluates to the object identity. The name of this value category ("left value") is historic and reflects the use of lvalue expressions as the left-hand operand of the assignment operator in the CPL programming language.

Lvalue expressions can be used in the following lvalue contexts:

as the operand of the address-of operator (except if the lvalue designates a bit field or was declared register).
as the operand of the pre/post increment and decrement operators.
as the left-hand operand of the member access (dot) operator.
as the left-hand operand of the assignment and compound assignment operators.

If an lvalue expression is used in any context other than sizeof, _Alignof, or the operators listed above, non-array lvalues of any complete type undergo lvalue conversion, which models the memory load of the value of the object from its location. Similarly, array lvalues undergo array-to-pointer conversion when used in any context other than sizeof, _Alignof, address-of operator, or array initialization from a string literal.

The semantics of const/volatile/restrict-qualifiers and atomic types apply to lvalues only (lvalue conversion strips the qualifiers and removes atomicity).

The following expressions are lvalues:

identifiers, including function parameters, provided they were declared as designating objects (not functions or enumeration constants)
string literals
(C99) compound literals
parenthesized expression if the unparenthesized expression is an lvalue
the result of a member access (dot) operator if its left-hand argument is lvalue
the result of a member access through pointer -> operator
the result of the indirection (unary *) operator applied to a pointer to object
the result of the subscription operator ([])

Modifiable lvalue expressions

A modifiable lvalue is any lvalue expression of complete, non-array type which is not const-qualified, and, if it's a struct/union, has no members that are const-qualified, recursively.

Only modifiable lvalue expressions may be used as arguments to increment/decrement, and as left-hand arguments of assignment and compound assignment operators.

Non-lvalue object expressions

Colloquially known as rvalues, non-lvalue object expressions are the expressions of object types that do not designate objects, but rather values that have no object identity or storage location. The address of a non-lvalue object expression cannot be taken.

The following expressions are non-lvalue object expressions:

integer, character, and floating constants
all operators not specified to return lvalues, including

any function call expression
any cast expression (note that compound literals, which look similar, are lvalues)
member access operator (dot) applied to a non-lvalue structure/union, f().x, (x,s1).a, (s1=s2).m
all arithmetic, relational, logical, and bitwise operators
increment and decrement operators (note: pre- forms are lvalues in C++)
assignment and compound assignment operators (note: they are lvalues in C++)
the conditional operator (note: may be lvalue in C++)
the comma operator (note: may be lvalue in C++)
the address-of operator, even it if is neutralized by being applied to the result of the unary * operator

As a special case, expressions of type void are assumed to be non-lvalue object expressions that yield a value which has no representation and requires no storage.

Note that a struct/union rvalue that has a member (possibly nested) of array type does in fact designate an object with temporary lifetime. This object can be accessed through lvalue expressions that form by indexing the array member or by indirection through the pointer obtained by array-to-pointer conversion of the array member.

Function designator expression

A function designator (the identifier introduced by a function declaration) is an expression of function type. When used in any context other than the address-of operator, sizeof, and _Alignof (the last two generate compile errors when applied to functions), the function designator is always converted to a non-lvalue pointer to function. Note that the function-call operator is defined for pointers to functions and not for function designators themselves.

References

C11 standard (ISO/IEC 9899:2011):

6.3.2.1 Lvalues, arrays, and function designators (p: 54-55)

C99 standard (ISO/IEC 9899:1999):

6.3.2.1 Lvalues, arrays, and function designators (p: 46)

C89/C90 standard (ISO/IEC 9899:1990):