问题
Why output of the below code is -1 and -2, It should be 1 and 2, Right?
Also on a 64 bit server size of the below structure is 4 Bytes, It should be 8 Bytes right?
#include<stdio.h>
struct st
{
int a:1;
int b:2;
};
main()
{
struct st obj={1,2};
printf("a = %d\nb = %d\n",obj.a,obj.b);
printf("Size of struct = %d\n",sizeof(obj));
}
回答1:
Compile with all the warnings enabled, and read what your compiler says:
Georgioss-MacBook-Pro:~ gsamaras$ gcc -Wall main.c
main.c:7:1: warning: type specifier missing, defaults to 'int' [-Wimplicit-int]
main()
^
main.c:9:26: warning: implicit truncation from 'int' to bitfield changes value
from 2 to -2 [-Wbitfield-constant-conversion]
struct st obj={1,2};
^
main.c:11:40: warning: format specifies type 'int' but the argument has type
'unsigned long' [-Wformat]
printf("Size of struct = %d\n",sizeof(obj));
~~ ^~~~~~~~~~~
%lu
3 warnings generated.
Recall, that
a signed 1 bit variable can hold only two values, -1 and 0
as you can see in this answer.
So if you use this struct instead:
struct st
{
int a:2;
int b:3;
};
you will get the desired output.
This answer gives a nice explanation as well.
来源：https://stackoverflow.com/questions/43889846/why-output-of-the-below-code-is-1-and-2