Using sizeof() in array declarations in C89
In ANSI C89 a.k.a. ISO C90, the sizeof
operator yields an integer constant, which is suitable for array dimensions. Function calls, for example, are not.
I'd like to add another remark, since I believe the code as-is has a problem that might get overlooked.
If the other_array
is declared as
double other_array[sizeof(array)];
it will neither have the same number of elements, nor the same size (that would only be true for array of char
) as array[]
. If the intent is to declare a second array with the same number of elements (regardless of type), use this:
double other_array[sizeof(array)/sizeof(*array)];
That is because result of sizeof
operator is constant expression, so it does not qualify for VLA, just like the following declaration:
int other_array[5];
cannot be variable length array either. From C11 (N1570) §6.6/p6 Constant expressions (emphasis mine going forward):
An integer constant expression117) shall have integer type and shall only have operands that are integer constants, enumeration constants, character constants,
sizeof
expressions whose results are integer constants,_Alignof
expressions, and floating constants that are the immediate operands of casts.
For sake of completeness, the sizeof
operator does not always results into constant expression, though this only affects post-C89 standards (in C11 VLAs were made optional). Referring to §6.5.3.4/p2 The sizeof
and _Alignof
operators:
If the type of the operand is a variable length array type, the operand is evaluated; otherwise, the operand is not evaluated and the result is an integer constant.
First, let's see the criteria for an array (not being) a VLA. C11
doc, chapter §6.7.6.2,
[...] If the size is an integer constant expression and the element type has a known constant size, the array type is not a variable length array type; [...]
Coming to your case, sizeof
is a compile-time operator, so it produces a value that is considered compile time constant expression. An array definition, whose size is specified as a compile time constant expression is not a VLA. So, in your code,
int other_array[sizeof(array)]
is not a VLA.
Regarding the sizeof
operator result, from C11
, chapter §6.5.3.4, (emphasis mine)
The
sizeof
operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. [...] otherwise, the operand is not evaluated and the result is an integer constant.