内核代码中一些c语言用法

container_of，（C 语言 up cast）

内核中的定义：

Cscope tag: container_of
   #   line filename / context / line
   1     27 drivers/gpu/drm/radeon/mkregtable.c <<container_of>>
             #define container_of(ptr, type, member) ({
   2     63 drivers/staging/lustre/include/linux/libcfs/libcfs.h <<container_of>>
             #define container_of(ptr, type, member)
   3     78 drivers/staging/rtl8192e/rtllib.h <<container_of>>
             #define container_of(ptr, type, member) ({
   4     61 drivers/staging/rtl8192u/ieee80211/ieee80211.h <<container_of>>
             #define container_of(ptr, type, member) ({
   5    791 include/linux/kernel.h <<container_of>>
             #define container_of(ptr, type, member) ({
   6     18 scripts/kconfig/list.h <<container_of>>
             #define container_of(ptr, type, member) ({
   7     26 tools/perf/util/include/linux/kernel.h <<container_of>>
             #define container_of(ptr, type, member) ({
   8     84 tools/virtio/linux/kernel.h <<container_of>>
             #define container_of(ptr, type, member) ({

scripts/kconfig/list.h

 1 #define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)
 2 
 3 /**
 4  * container_of - cast a member of a structure out to the containing structure
 5  * @ptr:        the pointer to the member.
 6  * @type:       the type of the container struct this is embedded in.
 7  * @member:     the name of the member within the struct.
 8  *
 9  */
10 #define container_of(ptr, type, member) ({                      
11         const typeof( ((type *)0)->member ) *__mptr = (ptr);    
12         (type *)( (char *)__mptr - offsetof(type,member) );})

分析详见：

http://www.cnblogs.com/cute/archive/2011/04/01/2002474.html

结构体和数据

谈结构体struct 初始化多出的点号“.”，数组[]初始化多出的逗号“,”

cat test.c

内容如下：

 1 #include <stdio.h>
 2 
 3 #define MAXTITL 30
 4 #define MAXAUTL 20
 5 
 6 struct book {
 7   float off;
 8   char title[MAXTITL];
 9   char author[MAXAUTL];
10   float value;
11 
12 };
13 
14 int main()
15 {
16     puts("C lange test!");
17     struct book b1 = { .value =  19.9, .author = "author", .title="C test tutorial" };
18     printf("Book value: %f
", b1.value);
19     printf("Book author: %s
", b1.author);
20     printf("Book title: %s
", b1.title);
21     printf("Book off: %f
", b1.off);
22     int m,n;
23     int a2[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
24     for(n=0;n<11;n++)
25         printf("%d
",a2[n]);
26     int a1[10] = {1, 2, 3, 4, 5, 6, 7, 8, 9, 0, };
27     for(m=0;m<11;m++)
28         printf("%d
",a1[m]);
29     return 0;
30 }

执行：

gcc test.c -o test
./test

结构体的空数组

example 1，求大小

 1 #include <string.h>
 2 #include <stdio.h>
 3 
 4 struct tag1
 5 {
 6       int a;
 7       int b;
 8 }__attribute ((packed));   //size = 8
 9 
10 struct tag2
11 {
12       int a;
13       int b;
14       char *c;
15 }__attribute ((packed));  //size = 12 on 32bit platform, 16 size on 64bit
16 
17 struct tag3
18 {
19     int a;
20     int b;
21     char c[0];
22 }__attribute ((packed));  //size = 8
23 
24 struct tag4
25 {
26     int a;
27     int b;
28     char c[1];
29 }__attribute ((packed));  //size = 9
30 
31 int main()
32 {
33       struct tag2 l_tag2;
34       struct tag3 l_tag3;
35       struct tag4 l_tag4;
36 
37       memset(&l_tag2,0,sizeof(struct tag2));
38       memset(&l_tag3,0,sizeof(struct tag3));
39       memset(&l_tag4,0,sizeof(struct tag4));
40 
41       printf("size of tag1 = %ld
",sizeof(struct tag1));
42       printf("size of tag2 = %ld
",sizeof(struct tag2));
43       printf("size of tag3 = %ld
",sizeof(struct tag3));
44       printf("size of tag4 = %ld
",sizeof(struct tag4));
45 
46       printf("l_tag2 = %p,&l_tag2.c = %p,l_tag2.c = %p
",&l_tag2,&l_tag2.c,l_tag2.c);
47       printf("l_tag3 = %p,l_tag3.c = %p
",&l_tag3,l_tag3.c);
48       printf("l_tag4 = %p,l_tag4.c = %p
",&l_tag4,l_tag4.c);
49       return 0;
50 }

example 2，柔性数组，动态内存

C语言0长度数组使用技巧，又解决了小内存碎片问题提高了性能。

#include <stdlib.h>
#include <string.h>
#include <stdio.h>

typedef struct{
    int stuID;
    int age;
    char address[];  // same with address[0]
}ST_STU_INFO,*pStuInfo; // size is 8

pStuInfo ComposeStuInfo( int stuID,int age, const char *paddress)
{
    pStuInfo ptmpInfo = malloc(sizeof(*ptmpInfo) + sizeof(char) * strlen(paddress) + 1);
    if(ptmpInfo != NULL){
        ptmpInfo->stuID = stuID;
        ptmpInfo->age = age;
        strcpy(ptmpInfo->address, paddress);
    }
    return ptmpInfo;
}

void printStuInfo(pStuInfo ptmpInfo)
{
    printf("stuID : %d, age : %d, 
Address: %s
"
           "Size of Struct：%ld

",
           ptmpInfo->stuID, ptmpInfo->age,
           ptmpInfo->address, sizeof(*ptmpInfo));
}

int main()
{
    pStuInfo CodeLab = ComposeStuInfo(
        100013, 20, "Tencent Building, Central District, High-tech Park, "
        "Nanshan District, Shenzhen");
    if(CodeLab != NULL){
        printStuInfo(CodeLab);
        free(CodeLab);
    }
    pStuInfo subCodeLab = ComposeStuInfo(200013, 23, "Tencent Building");
    if(subCodeLab != NULL){
        printStuInfo(subCodeLab);
        free(subCodeLab);
    }
    return 0;
}

c语言中__attribute__的意义

REF: