C++ 最新 container_of 宏中 __mptr 的用途是什么

由于“kernel.h: handle pointers to arrays better in container_of()”，__mptr不再用于类型判断.它用于消除scripts/gcc-plugins/randomize_layout_plugin.c的信息性消息，如inform(gimple_location(stmt), "found mismatched ssa struct pointer types: %qT and %qT\n", ptr_lhs_type, ptr_rhs_type);.此文件的工作之一是:

/*
 * iterate over all statements to find "bad" casts:
 * those where the address of the start of a structure is cast
 * to a pointer of a structure of a different type, or a
 * structure pointer type is cast to a different structure pointer type
 */

如果缺少__mptr，宏将包括您所说的代码:

(type *)((void*)ptr - offsetof(type, member)))

(PS:char *在这里更好，因为iso c标准保证它是byte foroffsetof, void*`仅由gnu c保证)

如果offsetof为零，ptr将包含 struct 类型member的起始地址，那么它将被转换为一个完全不同的类型:struct type *.此表单根据scripts/gcc-plugins/randomize_layout_plugin.c发生故障，并将由其检测.

随着void *__mptr = (void *)(ptr);的引入，编译器不知道__mptr和更多的类型，因此在将void * __mptr转换为(type *)时，scripts/gcc-plugins/randomize_layout_plugin.c将不再抱怨

下面是4.13之前的内核的原始答案，当时__mptr用于类型判断:

让我们简化container_of，看看这个例子:

#include <stdio.h>
#include <stddef.h>
#define container_of(ptr, type, member) ({          \
    const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
    (type *)( (char *)__mptr - offsetof(type,member) );})
#define container_of_without_typechecking(ptr, type, member) ({         \
    (type *)( (char *)ptr - offsetof(type,member) );})
struct Foo
{
    int a;
};
int main(int argc, char *argv[]) {
    struct Foo foo;
    int *a;

    printf("foo addr: %p\n", &foo);
    a = &foo.a;
    printf("container of a: %p\n", container_of_without_typechecking((unsigned long long*)a, struct Foo, a));
    printf("typecheck: container of a: %p\n", container_of((unsigned long long*)a, struct Foo, a));
    return 0;
}

container_of_without_typechecking没有__mptr，但container_of有.

编译时:

a.c: In function ‘main’:
a.c:13:55: warning: initialization of ‘const int *’ from incompatible pointer type ‘long long unsigned int *’ [-Wincompatible-pointer-types]
   13 |         const typeof( ((type *)0)->member ) *__mptr = (ptr);    \
      |                                                       ^
a.c:28:47: note: in expansion of macro ‘container_of’
   28 |     printf("typecheck: container of a: %p\n", container_of((unsigned long long*)a, struct Foo, a));
      |                                               ^~~~~~~~~~~~

如您所见，container_of会抛出incompatible pointer type警告，而container_of_without_typechecking不会，所以它只是用于类型判断.

另外，请注意，Linux内核将此警告视为错误:

KBUILD_CFLAGS   += $(call cc-option,-Werror=incompatible-pointer-types)

因此，如果将错误的类型传递到container_of，则会得到error而不是警告.