Flexible array member

C struct data types may end with a flexible Array [data structure|array] member with no specified size:

typedef struct DoubleArray;

Typically, such structures serve as the header in a larger, variable memory allocation:

DoubleArray* darray =
malloc +... * sizeof; // see section on size and padding
darray->len =...;
for

Effect on struct size and padding

The sizeof operator on such a struct gives the size of the structure as if the flexible array member were empty. This may include padding added to accommodate the flexible member; the compiler is also free to re-use such padding as part of the array itself.
It is common to allocate sizeof + array_len * sizeof bytes.
This is not wrong, but it may allocate a few more bytes than necessary: the compiler may be re-purposing some of the padding that is included in sizeof. Should this be a concern, macros are available to compute the minimum size while ensuring that the compiler's padding is not disrupted.
As the array may start in the padding before the end of the structure, its content should always be accessed via indexing or offsetof, not sizeof.

Availability

Flexible array members were officially standardized in C99. In practice, compilers provided them well before C99 was standardized.
Flexible array members are not officially part of C++, but language extensions are widely available.