C語言struct中的長度可變數組(Flexible array member)

C_struct中的長度可變數組(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[]不佔用結構體的存儲空間，sizeof(strcut vectord)的值爲1
• 變長數組必須是結構體的最後一個成員
• 結構體變量相鄰的連續存儲空間是arr[]數組的內容
• gcc中使用0長度的數組arr[0]來表示變長數組。

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;
}