Yes, it’s legal. From the C99 draft standard:
§6.5.2.1, paragraph 2:
A postfix expression followed by an expression in square brackets
[]
is a subscripted
designation of an element of an array object. The definition of the subscript operator[]
is thatE1[E2]
is identical to(*((E1)+(E2)))
. Because of the conversion rules that
apply to the binary+
operator, ifE1
is an array object (equivalently, a pointer to the
initial element of an array object) andE2
is an integer,E1[E2]
designates theE2
-th
element ofE1
(counting from zero).
§6.5.3.2, paragraph 3 (emphasis mine):
The unary
&
operator yields the address of its operand. If the operand has type ‘‘type’’,
the result has type ‘‘pointer to type’’. If the operand is the result of a unary*
operator,
neither that operator nor the&
operator is evaluated and the result is as if both were
omitted, except that the constraints on the operators still apply and the result is not an
lvalue. Similarly, if the operand is the result of a[]
operator, neither the & operator nor the unary*
that is implied by the[]
is evaluated and the result is as if the&
operator
were removed and the[]
operator were changed to a+
operator. Otherwise, the result is
a pointer to the object or function designated by its operand.
§6.5.6, paragraph 8:
When an expression that has integer type is added to or subtracted from a pointer, the
result has the type of the pointer operand. If the pointer operand points to an element of
an array object, and the array is large enough, the result points to an element offset from
the original element such that the difference of the subscripts of the resulting and original
array elements equals the integer expression. In other words, if the expressionP
points to
thei
-th element of an array object, the expressions(P)+N
(equivalently,N+(P)
) and
(P)-N
(whereN
has the valuen
) point to, respectively, thei+n
-th andi−n
-th elements of
the array object, provided they exist. Moreover, if the expressionP
points to the last
element of an array object, the expression(P)+1
points one past the last element of the
array object, and if the expressionQ
points one past the last element of an array object,
the expression(Q)-1
points to the last element of the array object. If both the pointer
operand and the result point to elements of the same array object, or one past the last
element of the array object, the evaluation shall not produce an overflow; otherwise, the
behavior is undefined. If the result points one past the last element of the array object, it
shall not be used as the operand of a unary*
operator that is evaluated.
Note that the standard explicitly allows pointers to point one element past the end of the array, provided that they are not dereferenced. By 6.5.2.1 and 6.5.3.2, the expression &array[5]
is equivalent to &*(array + 5)
, which is equivalent to (array+5)
, which points one past the end of the array. This does not result in a dereference (by 6.5.3.2), so it is legal.