一个结构体中未定义的数组声明

typedef struct s
{
  int arr[];
} s;

这是称为灵活阵列的C99功能,其主要特点是允许 answer to another question on flexible array members和07在这个 answer to another question on flexible array members中列出了使用灵活数组超过指针的好处.第6.7.2.1节结构和联盟规范第16段中的 draft C99 standard说：

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. […]

所以如果你有一个s *,那么除了结构体所需的空间之外,你还可以为数组分配空间,通常你会在结构中有其他成员：

s *s1 = malloc( sizeof(struct s) + n*sizeof(int) ) ;

该标准草案在第17段中实际上有一个指导性的例子：

EXAMPLE 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).