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 @L_696_2@和07在这个 @L_696_2@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：

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).