Container_of in Linux

In Linux, The container_of (PTR, type, member) macro is used to pass in the Member Address PTR domain of the struct type. The first address of the struct variable is returned, which is defined as follows:

# Define container_of (PTR, type, member )({\
Const typeof (type *) 0)-> member) * _ mptr = (PTR );\
(Type *) (char *) _ mptr-offsetof (type, member ));})

 

There are two unique features of gnu c in this macro:

1: typeof (x) indicates the type of X, which is supported by gnu c and not supported by ansi c.

2: gnu c regards compound statements contained in parentheses as an expression called a statement expression, which can appear in any allowed expression. We can use loop variables and local variables that can only be used in composite statements in statement expressions.

For example, int AA = ({3; 43-5;}); if you use a compiler that does not support gnu c, an error is reported. If you use GCC, the value of AA is 38.
Using this feature of gnu c, we can avoid the side effects of some macro definitions, such as using

# Define min_t (type, x, y )\
({Type _ x = (x); Type _ y = (y) ;__ x <__ y? _ X :__ y })
Replace

# Define min (x, y) (x) <(y )? (X) :( y ))
Avoid the side effects of passing in min (A ++, B ++ ).

 

The following describes the implementation of container_of:

1: const typeof (type *) 0)-> member) * _ mptr = (PTR); defines a variable of the same type as member _ mptr, and assign the Member Address to it.

 

2: # define offsetof (type, member) (size_t) & (type *) 0)-> Member)
The macro is used to input the struct type and the domain member, and returns the offset address of the field in the struct.

 

3: (type *) (char *) _ mptr-offsetof (type, member); obtain the first address of the struct and convert it to type *.

 

 

The test procedure is as follows:
# Include <stdio. h>

# Define offsetof (type, member) (size_t) & (type *) 0)-> Member)

# Define container_of (PTR, type, member )({\
Const typeof (type *) 0)-> member) * _ mptr = (PTR );\
(Type *) (char *) _ mptr-offsetof (type, member ));})

Typedef struct contain_s
{
Char;
Int B;
Char C [10];
} Contain_t;

Int main (void)
{
Contain_t * pcon;
Contain_t con = {'3', 4, "ABC "};
Pcon = container_of (& con. B, contain_t, B );

Printf ("ADDR & con % P, ADDR pcon % P \ n", & con, pcon );

Return 0;
}

Output: ADDR & con 0xbfd4bad8, ADDR pcon 0xbfd4bad8

Usage of the typeof keyword

Http://www.cublog.cn/u3/101356/showart_2081601.html

Reprinted from:

Http://rookieubuntu.blog.sohu.com/179419212.html