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Flexible array members are a special type of array where the last element of a structure with more than one named member has an incomplete array type; that is, the size of the array is not specified explicitly within the structure. This "struct hack" was widely used in practice and supported by a variety of compilers. Consequently, a variety of different syntaxes have been used for declaring flexible array members. For C99-compliant implementations, use the syntax guaranteed valid by C99 \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. |
Flexible array members are defined in Section 6.7.2.1, paragraph 16: "Structure and Union Specifiers", says:16 of the C99 standard as follows,
As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In most situations, the flexible array member is ignored. In In particular, the size of the structure is as if the flexible array member were omitted except that it may have more trailing padding than the omission would imply. However, when a . (or ->) operator has a left operand that is (a pointer to) a structure with a flexible array member and the right operand names that member, it behaves as if that member were replaced with the longest array (with the same element type) that would not make the structure larger than the object being accessed; the offset of the array shall remain that of the flexible array member, even if this would differ from that of the replacement array. If this array would have no elements, it behaves as if it had one element but the behavior is undefined if any attempt is made to access that element or to generate a pointer one past it.
Structures with flexible array members can be used to produce code with defined behavior. However, some restrictions apply:
- The incomplete array type must be the last element within the structure.
- There cannot be an array of structures that contain flexible array members.
- Structures that contain a flexible array member cannot be used as a member in the middle of another structure.
Noncompliant Code Example
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