container_of 的用法及注意事项

container_of 的定义在 linux 内核 include/linux/kernel.h 中

/** * container_of - cast a member of a structure out to the containing structure * @ptr:        the pointer to the member. * @type:       the type of the container struct this is embedded in. * @member:     the name of the member within the struct. * */#define container_of(ptr, type, member) ({                      \        const typeof( ((type *)0)->member ) *__mptr = (ptr);    \        (type *)( (char *)__mptr - offsetof(type,member) );})

其中的offsetof 的定义在 include/linux/stddef.h

#ifdef __compiler_offsetof#define offsetof(TYPE,MEMBER) __compiler_offsetof(TYPE,MEMBER)#else#define offsetof(TYPE, MEMBER) ((size_t) &((TYPE *)0)->MEMBER)#endif

在使用过程中一般不会有什么 问题，如：

typedef struct frame {        int a;        char b;        char name[10];} frame_t;int main(int argc, char **argv){        frame_t fra, *pf;        fra.a = 1;        fra.b = 2;        snprintf(fra.name, 5, "cjz%d", 1);        pf = container_of(&fra.a, frame_t, a);        printf("fra.a = %d, fra.b = %d, fra.name = %s\n", fra.a, fra.b, fra.name);        return 0;}

但是下边的编译的时候就会有warning

typedef struct frame {        int a;        char b;        char name[10];} frame_t;int main(int argc, char **argv){        frame_t fra, *pf;        fra.a = 1;        fra.b = 2;        snprintf(fra.name, 5, "cjz%d", 1);        pf = container_of(fra.name, frame_t, name);        printf("fra.a = %d, fra.b = %d, fra.name = %s\n", fra.a, fra.b, fra.name);        return 0;}

编译过程：

gcc    -c -o main.o main.cmain.c: In function ‘main’:main.c:29:57: warning: initialization from incompatible pointer type [enabled by default]gcc -Wall  -I./ -o test main.o

注意红色部分，这是怎么回事呢？下面我们就来分析一下

container_of的部分展开

container_of(fra.name, frame_t, name) 展开如下 ({const typeof( ((frame_t *)0)->name ) *__mptr = (fra.name);    \   (frame_t *)( (char *)__mptr - offsetof(frame_t, name) );})这样看来好像也没有什么 问题啊，但为什么 会有警告呢？因为typeof(数组名) ，这个我个先把它称为数组名类型，而typeof( ) 中要的是一个C中定义的类型，应该为数组元素类型，所以我们这里应该传数组首元素。pf = container_of(fra.name, frame_t, name[0]);注意红色部分与之前的不同，之前是name, 而这里是name[0]改后再编译gcc    -c -o main.o main.cgcc -Wall  -I./ -o test main.o警告就没有了