将整数赋给C位域[重复]

如果一个整数被分配给一个带位域的结构,结果是否定义得很好?

#include <stdio.h>
#include <stdint.h>

struct my_struct
{
  uint8_t a_byte;
  float a_float;
  uint8_t baz:1;
  uint8_t boo:1;
} __attribute__((__packed__));

int main ()
{
  struct my_struct foo;
  foo.a_byte = 5;
  foo.a_float = 3.14159;

  foo.boo = 0;
  foo.baz = 3;   /* this one */

  printf ("a_byte       = %i\n", foo.a_byte);
  printf ("a_float      = %f\n", foo.a_float);
  printf ("baz          = %i\n", foo.baz);
  printf ("boo          = %i\n", foo.boo);

  return 0;
}

用编译 gcc -Wall -Wextra main.c ,gcc警告

main.c:19:13: warning: large integer implicitly truncated to unsigned type 
[-Woverflow]
   foo.baz = 3;

a_byte       = 5
a_float      = 3.141590
baz          = 1
boo          = 0

所以在我的测试中,只有位0被分配给实际的位域(分配4将导致0)。