__mptr在最新的container_of宏中的用途是什么？
我正在阅读Linux5.17.5内核，现在查看container_of()宏。
/**
* 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) ({ \
void *__mptr = (void *)(ptr); \
static_assert(__same_type(*(ptr), ((type *)0)->member) || \
__same_type(*(ptr), void), \
"pointer type mismatch in container_of()"); \
((type *)(__mptr - offsetof(type, member))); })我的问题很简单：__mptr的目的是什么，我可以这样用(void *)ptr代替__mptr吗？
#define container_of(ptr, type, member) ({ \
static_assert(__same_type(*(ptr), ((type *)0)->member) || \
__same_type(*(ptr), void), \
"pointer type mismatch in container_of()"); \
((type *)((void*)ptr - offsetof(type, member))); })回答 1
Stack Overflow用户
发布于 2022-05-01 06:15:41
由于“kernel.h:在container_of()中更好地处理指向数组的指针”，__mptr不再用于类型检查。它用于消除scripts/gcc-plugins/randomize_layout_plugin.c的信息信息，如inform(gimple_location(stmt), "found mismatched ssa struct pointer types: %qT and %qT\n", ptr_lhs_type, ptr_rhs_type);。该文件的工作之一是：
/*
* 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offsetof为1字节，void *仅由gnu c保证)
如果offsetof获得零，并且ptr将包含struct类型member的开始地址，那么这将转换为完全不同的类型：struct type *。此表单每个scripts/gcc-plugins/randomize_layout_plugin.c都会出现故障，并会被它检测到。
随着void *__mptr = (void *)(ptr);的引入，编译器不再知道__mptr的类型了，所以当将void * __mptr转换为(type *)时，scripts/gcc-plugins/randomize_layout_plugin.c不会抱怨，您可以从https://lkml.org/lkml/2017/6/20/873看到实际的情况和相关的修复补丁
下面是用于类型检查的4.13之前的内核的原始答案：
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。
汇编时：
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抛出一个incompatible pointer type警告，而container_of_without_typechecking不抛出，因此它只是用于类型检查。
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，如果您将错误的类型传递给，您将得到一个错误，而不是一个警告。
https://stackoverflow.com/questions/72074089
复制相似问题
腾讯云开发者
