C语言struct中的长度可变数组(Flexible array member)

C_struct中的长度可变数组(Flexible array member)

Flexible array member is a feature introduced in the C99 standard of the C programming language (in particular, in section §6.7.2.1, item 16, page 103). It is a member of a struct, which is an array without a given dimension, and it must be the last member of such a struct, as in the following example:

struct vectord {
    uint8_t  len;
    double   arr[]; // the flexible array member must be last
};

• arr[]不占用结构体的存储空间，sizeof(strcut vectord)的值为1
• 变长数组必须是结构体的最后一个成员
• 结构体变量相邻的连续存储空间是arr[]数组的内容
• gcc中使用0长度的数组arr[0]来表示变长数组。

struct vectord *allocate_vectord (size_t len) {
   struct vectord *vec = malloc(offsetof(struct vectord, arr) + len * sizeof(vec->arr[0]));

   if (!vec) {
       perror("malloc vectord failed");
       exit(EXIT_FAILURE);
   }

   vec->len = len;

   for (size_t i = 0; i < len; i++)
       vec->arr[i] = 0;

   return vec;
}