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  • typeof, offsetof 和container_of

    要理解Linux中实现的双向循环链表("侵入式"链表),首先得弄明白宏container_of。 本文尝试从gcc的关键字typeof和宏offsetof入手,循序渐进地剖析宏container_of之实现原理。

    1. typeof (from: https://en.wikipedia.org/wiki/Typeof)

    typeof is an operator provided by several programming languages to
    determine the data type of a variable. This is useful when
    constructing programs that must accept multiple types of data
    without explicitly specifying the type.
    
    The GNU compiler (GCC) extensions for the C programming language provide
    typeof:
    
    #define max(a, b) 
           ({ typeof (a) _a = (a); 
              typeof (b) _b = (b); 
             _a > _b ? _a : _b; })

    typeof和sizeof一样,都是关键字。只不过typeof不是标准的c语言关键字,而是gcc支持的(扩展)关键字typeof的作用是取得某个变量的数据类型。例如:

    unsigned int a = 1; // typeof (a) is unsigned int 
    short b = 2;        // typeof (b) is short

    2. offsetof (from: include/linux/stddef.h)

    #define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)

    宏offsetof的作用是获取某个成员变量(MEMBER)在其所在的结构体(TYPE)里的偏移。 上面的宏实现得非常巧妙(如果你正好也熟悉汇编,一定会英雄所见略同),剖析如下:

    (1) P = (TYPE *)0  // 将地址0x0强制转化为类型为TYPE的结构体X的首地址
    (2) M = P->MEMBER  // 访问X的成员变量MEMBER
    (3) A = &M         // 取得X的成员变量MEMBER的内存地址
    (4) O = (size_t)A  // 将成员变量MEMBER的内存地址强制转换成偏移量(Offset),
          = (size_t)&M // 由于内存地址也是无符号整数,所以MEMBER相对于结构体X首地址的偏移Offset等于&M
          = (size_t)&(P->MEMBER) = (size_t)&P->MEMBER  // 注意: -> 比 & 优先级高
          = (size_t)&(P)->MEMBER = (size_t)&((TYPE *)0)->MEMBER

    为了理解更容易一些,不妨(多加几重括号)将宏offsetof定义为:

    #define offsetof(TYPE, MEMBER) ((size_t)(&(((TYPE *)0)->MEMBER)))

    进而,用图解析如下: (此图为本人原创,如需转载请注明出处

    3. container_of (from: include/linux/kernel.h)

     1 /**
     2  * container_of - cast a member of a structure out to the containing structure
     3  * @ptr:        the pointer to the member.
     4  * @type:       the type of the container struct this is embedded in.
     5  * @member:     the name of the member within the struct.
     6  *
     7  */
     8 #define container_of(ptr, type, member) ({                      
     9         const typeof( ((type *)0)->member ) *__mptr = (ptr);    
    10         (type *)( (char *)__mptr - offsetof(type,member) );})
    • L9:  用一个临时变量__mptr保存成员变量的指针ptr
    • L10: offsetof(type, member): 计算出成员变量相对于其所在的结构体的偏移,不妨记为OFFSET
    • L10: (char *)__mptr - OFFSET, 就是成员变量所在的结构体的首地址 (注意: 对(char *)巧妙的应用)

    因此, 宏container_of的作用就是根据某个成员变量的内存地址,反推出其所在的结构体变量的首地址用图解析如下: (此图为本人原创,如需转载请注明出处

    示例代码: foo.c

     1 #include <stdio.h>
     2 
     3 #define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)
     4 
     5 #define container_of(ptr, type, member) ({                              
     6                 const typeof( ((type *)0)->member ) *__mptr = (ptr);    
     7                 (type *)( (char *)__mptr - offsetof(type,member) );})
     8 
     9 typedef struct foo_s {
    10         int             m_int;
    11         short           m_short;
    12         char            m_char;
    13         long long       m_longlong;
    14 } foo_t;
    15 
    16 int
    17 main(int argc, char *argv[])
    18 {
    19         foo_t ox = {0x12345678, 0x1234, 'A', 0xfedcba9876543210};
    20         char *p3 = &ox.m_char;
    21         foo_t *p = container_of(p3, foo_t, m_char);
    22 
    23         printf("foo_t ox (%p) sizeof(foo_t) = %d
    ", &ox, sizeof (foo_t));
    24         printf("foo_t *p (%p) p3(%p)
    ", p, p3);
    25         printf("foo_t->m_int      = %#x		(%d)(%p)
    ",
    26             p->m_int, offsetof(foo_t, m_int), &ox.m_int);
    27         printf("foo_t->m_short    = %#x		(%d)(%p)
    ",
    28             p->m_short, offsetof(foo_t, m_short), &ox.m_short);
    29         printf("foo_t->m_char     = %c			(%d)(%p)
    ",
    30             p->m_char, offsetof(foo_t, m_char), &ox.m_char);
    31         printf("foo_t->m_longlong = %#llx	(%d)(%p)
    ",
    32             p->m_longlong, offsetof(foo_t, m_longlong), &ox.m_longlong);
    33 
    34         return 0;
    35 }

    编译并运行

    $ gcc -g -Wall -m32 -o foo foo.c
    $ ./foo
    foo_t ox (0xbfdfe990) sizeof(foo_t) = 16
    foo_t *p (0xbfdfe990) p3(0xbfdfe996)
    foo_t->m_int      = 0x12345678          (0)(0xbfdfe990)
    foo_t->m_short    = 0x1234              (4)(0xbfdfe994)
    foo_t->m_char     = A                   (6)(0xbfdfe996)
    foo_t->m_longlong = 0xfedcba9876543210  (8)(0xbfdfe998)

    反汇编并结合gcc -E foo.c

    (gdb) set disassembly-flavor intel
    (gdb) disas /m main
    Dump of assembler code for function main:
    18      {
       0x0804841d <+0>:     push   ebp
       0x0804841e <+1>:     mov    ebp,esp
       0x08048420 <+3>:     and    esp,0xfffffff0
       0x08048423 <+6>:     sub    esp,0x40
    
    19              foo_t ox = {0x12345678, 0x1234, 'A', 0xfedcba9876543210};
       0x08048426 <+9>:     mov    DWORD PTR [esp+0x30],0x12345678
       0x0804842e <+17>:    mov    WORD PTR [esp+0x34],0x1234
       0x08048435 <+24>:    mov    BYTE PTR [esp+0x36],0x41
       0x0804843a <+29>:    mov    DWORD PTR [esp+0x38],0x76543210
       0x08048442 <+37>:    mov    DWORD PTR [esp+0x3c],0xfedcba98
    
    20              char *p3 = &ox.m_char;
       0x0804844a <+45>:    lea    eax,[esp+0x30]
       0x0804844e <+49>:    add    eax,0x6
       0x08048451 <+52>:    mov    DWORD PTR [esp+0x24],eax
    
    21              foo_t *p = container_of(p3, foo_t, m_char);
       0x08048455 <+56>:    mov    eax,DWORD PTR [esp+0x24]
       0x08048459 <+60>:    mov    DWORD PTR [esp+0x28],eax
       0x0804845d <+64>:    mov    eax,DWORD PTR [esp+0x28]
       0x08048461 <+68>:    sub    eax,0x6
       0x08048464 <+71>:    mov    DWORD PTR [esp+0x2c],eax
    #
    # --- L21's output from "gcc -E foo.c" ---
    # 001               foo_t *p = ({
    # 002    const typeof( ((foo_t *)0)->m_char ) *__mptr = (p3);
    # 003    (foo_t *)( (char *)__mptr - ((size_t)&((foo_t *)0)->m_char) );
    # 004               });
    #

    结合汇编代码阅读下面两行,会更好懂:-)

    002 const typeof( ((foo_t *)0)->m_char ) *__mptr = (p3);
    003 (foo_t *)( (char *)__mptr - ((size_t)&((foo_t *)0)->m_char) );

    小结

    • typeof (VAR): 获取变量VAR的数据类型, (注意typeof是gcc支持的扩展关键字)
    • offsetof(TYPE, MEMBER): 获取成员变量MEMBER在其所在的结构体(类型为TYPE)里的偏移
    • container_of(PTR, TYPE, MEMBER): 根据成员变量MEMBER的内存首地址PTR, 反推出其所在的结构体(类型为TYPE)变量的内存首地址

    参考资料

    1. $ man -s3 offsetof

    2.  C Operators (which are copied from book C Programming: A Modern Approach, Second Edition) (快速查询C操作符的脚本戳这里)

    APPENDIX  A
    C Operators
    
    --------------------------------------------------------------------------------
    Precedence              Name                    Symbol(s)       Associativity
    --------------------------------------------------------------------------------
        1           Array subscripting              []              Left
        1           Function call                   ()              Left
        1           Structure and union member      .  ->           Left
        1           Increment (postfix)             ++              Left
        1           Decrement (postfix)             --              Left
    --------------------------------------------------------------------------------
        2           Increment (prefix)              ++              Right
        2           Decrement (prefix)              --              Right
        2           Address                         &               Right
        2           Indirection                     *               Right
        2           Unary plus                      +               Right
        2           Unary minus                     -               Right
        2           Bitwise complement              ~               Right
        2           Logical negation                !               Right
        2           Size                            sizeof          Right
    --------------------------------------------------------------------------------
        3           Cast                            ()              Right
    --------------------------------------------------------------------------------
        4           Multiplicative                  *  /  %         Left
    --------------------------------------------------------------------------------
        5           Additive                        +  -            Left
    --------------------------------------------------------------------------------
        6           Bitwise shift                   <<  >>          Left
    --------------------------------------------------------------------------------
        7           Relational                      <  >  <=  >=    Left
    --------------------------------------------------------------------------------
        8           Equality                        ==  !=          Left
    --------------------------------------------------------------------------------
        9           Bitwise and                     &               Left
    --------------------------------------------------------------------------------
       10           Bitwise exclusive or            ^               Left
    --------------------------------------------------------------------------------
       11           Bitwise inclusive or            |               Left
    --------------------------------------------------------------------------------
       12           Logical and                     &&              Left
    --------------------------------------------------------------------------------
       13           Logical or                      ||              Left
    --------------------------------------------------------------------------------
       14           Conditional                     ?:              Right
    --------------------------------------------------------------------------------
       15           Assignment                      =  *=  /=  %=   Right
                                                    += -=  <<= >>=
                                                    &= ^=  |=
    --------------------------------------------------------------------------------
       16           Comma                           ,               Left
    --------------------------------------------------------------------------------
    
                                                                                 735
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  • 原文地址:https://www.cnblogs.com/idorax/p/6796897.html
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