代码之家 › 专栏 › 技术社区 › Dennis Miller

关于联合和堆分配内存的问题

heap-memory unions memory-management c++

Dennis Miller · 技术社区 · 14 年前

问题: 1) 有没有一种方法可以静态地在页面边界上声明fieldB alligned。基本上和valloc做同样的事情,但是在堆栈上?

2) 当在堆上分配字段B或任何字段时,是否可以进行并集?。不知道这是否合法。

#include <iostream>
#include <stdlib.h>
#include <string.h>
#include <stdio.h>

class Test
{
   public:
      Test(void)
      {
         // field B must be alligned to page boundary
         // Is there a way to do this on the stack???
         this->field.fieldB = (unsigned char*) valloc(10);
      };

      //I know this is bad, this class is being treated like 
      //a global structure. Its self contained in another class.
      unsigned char* PointerToFieldA(void)
      {
         return &this->field.fieldA[0];
      }

      unsigned char* PointerToFieldB(void)
      {
         return this->field.fieldB;
      }

      unsigned char* PointerToFieldC(void)
      {
         return &this->field.fieldC[0];
      }

      unsigned char* PointerToAllFields(void)
      {
         return &this->allFields[0];
      }

   private:
      // Is this union possible with field B being 
      // allocated on the heap?
      union
      {
         struct
         {
            unsigned char  fieldA[10];

            //This field has to be alligned to page boundary
            //Is there way to be declared on the stack
            unsigned char* fieldB;
            unsigned char  fieldC[10];
         } field;

         unsigned char allFields[30];
      };
};


int main()
{
   Test test;

   strncpy((char*) test.PointerToFieldA(), "0123456789", 10);
   strncpy((char*) test.PointerToFieldB(), "1234567890", 10);
   strncpy((char*) test.PointerToFieldC(), "2345678901", 10);

   char dummy[11];
   dummy[10] = '\0';

   strncpy(dummy, (char*) test.PointerToFieldA(), 10);
   printf("%s\n", dummy);

   strncpy(dummy, (char*) test.PointerToFieldB(), 10);
   printf("%s\n", dummy);

   strncpy(dummy, (char*) test.PointerToFieldC(), 10);
   printf("%s\n", dummy);

   char allFields[31];
   allFields[30] = '\0';
   strncpy(allFields, (char*) test.PointerToAllFields(), 30);
   printf("%s\n", allFields);

   return 0;
}

2 回复 | 直到 12 年前

Mark Wilkins 14 年前

我想你不能申报 fieldB 作为指针并获得所需的行为(假设我正确理解问题)。为了使联合在使用时有意义,您需要将它声明为联合中的数组。

我有点好奇,是否有可能重载新操作符,使类强制特定成员位于页面边界上。我很快就拼凑了超负荷的操作符来做这件事。它会导致每次分配一个完整的额外页面。它找到该字段所在位置的偏移量,然后按该量调整地址。因为额外的内存是分配的(假设我的计算正确),所以它是安全的。不过,很难看。

它将分配偏移量填充到类中的成员中,以便知道释放指针所需的“取消偏移”量。这真是可怕的代码。作为一个实验,它看起来还不错,但在生产代码中就不那么好了。

#define PAGE_SIZE 0x1000

class test
{
public:
   int allocoffset;
   void* operator new( size_t );
   void operator delete( void* );
    union
      {
         __declspec( align(4096)) struct
         {
            unsigned char  fieldA[10];

            //This field has to be alligned to page boundary
            //Is there way to be declared on the stack
            unsigned char  fieldB[10];
            unsigned char  fieldC[10];
         } field;

         unsigned char allFields[30];
      };
};

void* test::operator new(size_t size)
{
   // Allocate an entire extra page so we can offset it by any amount
   // less than the page size to ensure alignment of fieldB
   unsigned char *p = (unsigned char*)malloc( sizeof( test ) + PAGE_SIZE );
   uintptr_t addr;
   uintptr_t diff;

   std::cout << "new " << (void*)p << std::endl;

   // now offset the returned memory by the amount needed to align
   // fieldB on a page boundary.
   addr = (uintptr_t)p + (uintptr_t)( offsetof( test, field.fieldB ));

   diff = PAGE_SIZE - ( addr & (PAGE_SIZE - 1 ));

   p += diff;

   ((test*)p)->allocoffset = diff;

   return p;
}

void test::operator delete( void *p )
{
   // offset by appropriate amount that we allocated it by
   p = (void*)( (unsigned char*)p - ((test*)p)->allocoffset );
   std::cout << "delete " << p << std::endl;
   free(p);
}

int main()
{
   test *t;

   t = new test;

   std::cout << "allocation offset " << t->allocoffset << std::endl;
   std::cout << "address of fieldB " << (void*)&t->field.fieldB << std::endl;

   delete t;
}

new 00353FA0
allocation offset 86
address of fieldB 00355000
delete 00353FA0

gbjbaanb 14 年前

但是,有些编译器有pragma来对齐结构,MSVC有' __declspec align '其中可以指定数据成员的对齐方式,编译器将插入适当的字节数。

可以在堆上分配1个成员的情况下进行联合-联合将像往常一样包含所有字段,但堆分配的字段将只是一个指针。