Flexible array member

C struct data types may end with a flexible array member  with no specified size:

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

Effect on struct size and padding
The  operator on such a   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  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. 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  , not.

Availability
Flexible array members were officially standardized in C99. In practice, compilers (e.g., GCC, MSVC ) provided them well before C99 was standardized.

Flexible array members are not officially part of C++, but language extensions are widely available.