Question

在尝试从源代码Linux内核Definition of container_of理解 container_of（）的功能时，我发现宏定义的以下行似乎无所事事。

const typeof（（（（type *）0）-> member）* __ mptr =（ptr）; \

因此，我编写了一个不带上述行的简单C程序，并尝试比较所获得的结果。基本上，我试图理解新的实现 my_contianer_of（）与 container_of（） 有何不同？

public class MainActivity extends AppCompatActivity {

    LinearLayout verticallayout,horizontalLayout; //got

    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        verticallayout = findViewById(R.id.verticalLayout); //got

    }

    public void createRow(View view){

        createRow(); //got

    }



    private void createRow() { //got

        horizontalLayout = new LinearLayout(this);

        LinearLayout.LayoutParams horizontalParams = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        horizontalLayout.setOrientation(LinearLayout.HORIZONTAL);
        horizontalParams.setMargins(15, 5, 15, 5); // LEFT, TOP, RIGHT, BOTTOM
        horizontalLayout.setLayoutParams(horizontalParams);


        createSpinner();
        createCheckbox();
        createEditText();

        verticallayout.addView(horizontalLayout, horizontalParams);
    }








    private void createEditText() {
        EditText editText = new EditText(this);

        LinearLayout.LayoutParams editParams = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.MATCH_PARENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        editParams.setMargins(10, 10, 10, 10);

        editText.setLayoutParams(editParams);
        horizontalLayout.addView(editText);
    }


    private void createCheckbox() { //done
        CheckBox checkBox = new CheckBox(this);

        LinearLayout.LayoutParams checkParams = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        checkParams.setMargins(10, 10, 10, 10);
        checkParams.gravity = Gravity.LEFT;

        checkBox.setLayoutParams(checkParams);
        horizontalLayout.addView(checkBox);

    }

    private void createSpinner() {

        Spinner spinner = new Spinner(this);
        //ArrayAdapter<String> spinnerArrayAdapter = new ArrayAdapter<String>(this,android.R.layout.simple_spinner_dropdown_item, spinnerArray);
        //spinner.setAdapter(spinnerArrayAdapter);
        LinearLayout.LayoutParams spinnerParams = new LinearLayout.LayoutParams(
                LinearLayout.LayoutParams.WRAP_CONTENT, LinearLayout.LayoutParams.WRAP_CONTENT);
        spinnerParams.setMargins(10, 10, 10, 10);
        spinnerParams.gravity = Gravity.LEFT;

        spinner.setLayoutParams(spinnerParams);
        horizontalLayout.addView(spinner);
    }
}

以下是上述c程序的示例输出。

#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 my_container_of(ptr, type, member) ({ \
        (char *)ptr - offsetof(type, member);})

struct container {
    int *i;
    char c;
    float f;
};

void foo(char *cptr)
{
    printf("foo  : %p\n", my_container_of(cptr, struct container, c));
}

int main(int argc, char *argv[])
{
    struct container tc;

    printf("&tc  : %p\n", &tc);
    printf("cof  : %p\n", container_of(&tc.c, struct container, c));
    printf("mycof: %p\n", my_container_of(&tc.c, struct container, c));

    foo(&tc.c);

    return 0;
}

请不要将此问题标记为任何其他问题或此Understanding container_of macro in the linux kernel的重复，因为这些问题没有为我的查询提供所需的答案。

Answer 1

您的实现类型不安全。它不能确保您传递的指针与该成员具有相同的类型，这可能会导致错误。因此，例如：

my_container_of(&tc.f, struct container, c)

编译就可以了。内核版本至少会发出有关不兼容的指针类型的警告。还是完全清除，导致编译停止，具体取决于您的GCC标志。

自然地，内核版本不能防止所有错误（例如，如果f和c属于同一类型）。但这是可以捕获的，应该捕获。

替换Linux内核中使用的container_of宏

1 个答案: