Flexible array members is a C feature introduced in C99 whereby one can declare the last element to be an array of unspecified size.
Quoted from N1570, 6.7.2.1 Structure and union specifiers:
18 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 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.
20 EXAMPLE 2 After the declaration:
struct s { int n; double d[]; };the structure struct s has a flexible array member d. A typical way to use this is:
int m = /* some value */; struct s *p = malloc(sizeof (struct s) + sizeof (double [m]));and assuming that the call to malloc succeeds, the object pointed to by p behaves, for most purposes, as if p had been declared as:
struct { int n; double d[m]; } *p;(there are circumstances in which this equivalence is broken; in particular, the offsets of member d might not be the same).
