sizeof 中的数组[]中为空，不计大小？ Zero-length arrays

peixiuhui

1
  2
  3 #include <stdio.h>
  4
  5 struct gpio_led_data {
  6     unsigned gpio;
  7     unsigned char new_level;
  8     unsigned char can_sleep;
  9     unsigned char active_low;
10     unsigned char blinking;
11 };
12
13 struct gpio_led_priv {
14     int num_leds;
15     struct gpio_led_data leds[];
16 };
17
18 int main(int argc, char *argv[])
19 {
20     printf("sizeof:=%d\n",sizeof(struct gpio_led_priv));
21 }

为什么 输出 sizeof:＝4 ？？？

qlb1234

你在提問前，有試過

1. struct gpio_led_priv {
   
2.     int num_leds;
   
3.     struct gpio_led_data leds[1];
   
4. };

复制代码

嗎？

dr2001

C99/N1256 6.7.2.1-16 flexible array member.

https://gcc.gnu.org/onlinedocs/g ... th.html#Zero-Length

所以，结果是对的。

这种声明直接用是方便的，能让代码更干净一些。
但这种结构体如果嵌套在别的结构体声明里，会因为不定长的成员导致无法决定其后边的成员的地址，有的编译器可能会报错。

zhugean

学习了！！！！！

peixiuhui

dr2001 发表于 2014-5-21 12:13
C99/N1256 6.7.2.1-16 flexible array member.

https://gcc.gnu.org/onlinedocs/gcc-4.9.0/gcc/Zero-Lengt ...

谢 了。

Zero-length arrays are allowed in GNU C. They are very useful as the last element of a structure that is really a header for a variable-length object:

     struct line {
       int length;
       char contents[0];
     };
     
     struct line *thisline = (struct line *)
       malloc (sizeof (struct line) + this_length);
     thisline->length = this_length;

In ISO C90, you would have to give contents a length of 1, which means either you waste space or complicate the argument to malloc.

In ISO C99, you would use a flexible array member, which is slightly different in syntax and semantics: