container_of宏使用分析

#include <stdio.h>

#include <stdlib.h>

#include <stddef.h>

typedef struct student{

int x;

double y;

char z;

}mystudent;

//返回结构体的起始地址

#define container_of(ptr, type, member) ({ /

const typeof( ((type *)0)->member ) *__mptr = (ptr); /

(type *)( (char *)__mptr - offsetof(type,member) );})

 

int main(void)

{

mystudent *p = NULL;

mystudent x;

x.x = 10;

p = container_of(&x.z,mystudent,z);

printf("%d/n", p->x);

return 0;

}

 

 

 

 

 

typeof的用法

Another way to refer to the type of an expression is with typeof. The syntax of using of
this keyword looks like sizeof, but the construct acts semantically like a type name defined
with typedef.
There are two ways of writing the argument to typeof: with an expression or with a
type. Here is an example with an expression:
typeof (x[0](1))
This assumes that x is an array of pointers to functions; the type described is that of the
values of the functions.
Here is an example with a typename as the argument::

typeof (int *)
Here the type described is that of pointers to int.
If you are writing a header file that must work when included in ISO C programs, write
__typeof__ instead of typeof. See Section 5.39 [Alternate Keywords], page 239.
A typeof-construct can be used anywhere a typedef name could be used. For example,
you can use it in a declaration, in a cast, or inside of sizeof or typeof.
typeof is often useful in conjunction with the statements-within-expressions feature.
Here is how the two together can be used to define a safe “maximum” macro that operates
on any arithmetic type and evaluates each of its arguments exactly once:
#define max(a,b) /
({ typeof (a) _a = (a); /
typeof (b) _b = (b); /
_a > _b ? _a : _b; })
The reason for using names that start with underscores for the local variables is to avoid
conflicts with variable names that occur within the expressions that are substituted for a
and b. Eventually we hope to design a new form of declaration syntax that allows you to
declare variables whose scopes start only after their initializers; this will be a more reliable
way to prevent such conflicts.
Some more examples of the use of typeof:

• This declares y with the type of what x points to.
typeof (*x) y;
• This declares y as an array of such values.
typeof (*x) y[4];
• This declares y as an array of pointers to characters:
typeof (typeof (char *)[4]) y;
It is equivalent to the following traditional C declaration:
char *y[4];
To see the meaning of the declaration using typeof, and why it might be a useful way
to write, let’s rewrite it with these macros:
#define pointer(T) typeof(T *)
#define array(T, N) typeof(T [N])
Now the declaration can be rewritten this way:
array (pointer (char), 4) y;
Thus, array (pointer (char), 4) is the type of arrays of 4 pointers to char.
Compatibility Note: In addition to typeof, GCC 2 supported a more limited extension
which permitted one to write
typedef T = expr;
with the effect of declaring T to have the type of the expression expr. This extension does
not work with GCC 3 (versions between 3.0 and 3.2 will crash; 3.2.1 and later give an error).

Code which relies on it should be rewritten to use typeof:
typedef typeof(expr) T;
This will work with all versions of GCC.