Linux kernel中有趣的語法[zz]

室友xin總結的Linux Kenel中關於C的一些有趣的語法，大部分是gcc的擴展，狂贊。特轉貼出來。 原文出處http://bbs.chinaunix.net/viewthread.php?tid=925106

原文如下：

經常從kernel裏面抄代碼，見到一些非常有趣的語法。我已經忘了它們位於哪些文件，但我記得它們的用法，就寫了一些小例子，放出來和大家分享一下，很好玩的。
我沒有去深究這些語法的出處，或許是gcc特有的，或許是C標準有的, 請知道的朋友點評。

1.算偏移 

CODE:

// 1. 先來個最常見的，算偏移
struct ex
{
    char a[100];
    int b;
    long c;
    char d;
};

#define OFFSET(x) ((unsigned long)(&((struct ex *)0)->x))

/*
   通過這種方法來計算每個元素在結構體裏的偏移量是很方便的。特別是在寫彙編程序的時候，這種自動得到偏移量的方法省去了程序員許多功夫。 通常的做法是把你想要知道的所有偏移量通過宏寫到一個.c文件中，在編譯時自動生成.h文件供彙編文件引用。
*/

 Comments:參考Linux kernel 裏面list_head的源程序

 2.結構體有名初始化

CODE:

// 也是很常見的，結構體有名初始化。
// 這我知道是c99標準裏面有的，因爲linux社區稱它爲“醜陋的”:lol:

struct   
{
    int i;
    int j;
} ex = { .i = 10,
         .j = 100};

 3. 數組有名初始化

CODE:

// 數組有名初始化
// 當時在寫這個數組的時候，我一直爲哪個字符在數組裏排第幾號煩惱
// 後來改成這種有名數組賦值就方便了

static char *ivt_vector[] =
{
    [0] = "vmx_vhpt_miss", [1] = "vmx_itlb_miss",
    [2] = "vmx_dtlb_miss", [3] = "vmx_alt_itlb_miss",
    [4] = "vmx_alt_dtlb_miss", [5] = "vmx_nested_dtlb_miss",
    [6] = "vmx_ikey_miss", [7] = "vmx_dkey_miss",
    [8] = "vmx_dirty_bit", [9] = "vmx_iaccess_bit",
    [10] = "vmx_daccess_bit", [11] = "vmx_break_fault",
    [12] = "vmx_interrupt", [13] = "vmx_virtual_exirq",
    [14] = "ia64_hypercall_setup", [15] = "reserved",
    [16] = "reserved", [17] = "reserved",
    [18] = "reserved", [19] = "reserved",
    [20] = "vmx_page_not_present", [21] = "vmx_key_permission",
    [22] = "vmx_iaccess_rights", [23] = "vmx_daccess_rights",
    [24] = "vmx_general_exception", [25] = "vmx_disabled_fp_reg",
    [26] = "vmx_nat_consumption", [27] = "vmx_speculation_vector",
    [28] = "reserved", [29] = "vmx_debug_vector",
    [30] = "vmx_unaligned_access", [31] = "vmx_unsupported_data_reference",
    [32] = "vmx_floating_point_fault", [33] = "vmx_floating_point_trap",
    [34] = "vmx_lower_privilege_trap", [35] = "vmx_taken_branch_trap",
    [36] = "vmx_single_step_trap", [37] = "vmx_virtualization_fault",
    [38] = "reserved",[39] = "reserved", [40] = "reserved",
    "reserved","reserved","reserved","reserved" "reserved",
    "reserved","reserved","reserved","reserved","reserved",
    "reserved","reserved","reserved","reserved","reserved",
    "reserved","reserved","reserved","reserved","reserved",
    "reserved","reserved","reserved","reserved","reserved",
    "reserved","reserved","reserved","reserved",
};

4.一次給數組中所有元素賦初值

CODE:

// 一次給數組中所有元素賦初值

int a[100] = {[0 ... 99] = 10};

 5. 語句塊返回值

CODE:

// 語句塊返回值
//這只能在IA64平臺（或許其他平臺也行）才能得到正確的值。它是通過mix指令實現的。

struct st { int  c; int d; };
struct st test()
{
    int a = 1;
    int b = 1;

    return ((struct st){a,b});
}

6. 把宏寫的像函數一樣

CODE:

//把宏寫的像函數一樣


struct ex
{
    int i;
    int j;
} ex1 = { .i = 10,
         .j = 100};

#define v(a,b) ({struct ex t; /
                if ( a > b )    /
                    t.i = a;    /
                else        /
                    t.i = b;    /
                    t;})

int main()
{
    printf("%d/n", v(1,2).i);
    printf("%d/n", v(3,2).i);
}

呵呵，是不是都很很有趣
其他的諸如typeof、attributes就很普通了，大家經常能在各種linux書籍中看到。