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.c main.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.c gcc -Wall -I./ -o test main.o 警告就没有了