Flexible array members

A flexible array member is permitted as the last element of a structure even though it has incomplete type, provided that the structure has more than one named member. A flexible array member is a C99 feature and can be used to access a variable-length object. It is declared with an empty index, as follows:

array_identifier[ ];

For example, b is a flexible array member of Foo.

struct Foo{
   int a;
   int b[];
};

Since a flexible array member has incomplete type, you cannot apply the sizeof operator to a flexible array.

Any structure containing a flexible array member cannot be a member of another structure or array.

IBM extension

ILE C/C++ extends Standard C and C++ to ease the restrictions on flexible arrays and allow the following:

Flexible array members can be declared in any part of a structure, not just as the last member.
The type of any member that follows the flexible array member must be compatible with the type of the flexible array member.

The type of any member following the flexible array member is not required to be compatible with the type of the flexible array member; however, a warning is issued in this case.
Structures containing flexible array members can be members of other structures.
Flexible array members can be statically initialized.

In the following example:

struct Foo{
   int a;
   int b[];
}; 

struct Foo foo1 = { 55, {6, 8, 10} };
struct Foo foo2 = { 55, {15, 6, 14, 90} };

foo1

creates an array b of 3 elements, which are initialized to 6, 8, and 10; while foo2 creates an array of 4 elements, which are initialized to 15, 6, 14, and 90.

Flexible array members can only be initialized if they are contained in the outermost part of nested structures. Members of inner structures cannot be initialized.

End of IBM extension

Related information

Variable length arrays (C++ only)

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