I read this article, and I can not understand some of the techniques. If anyone can, please explain what happens, for example, here:
void FormatNewPage() { void* tmp = GetPage(); // получаем адрес начала блоков памяти head = tmp; // голову на начало блока памяти for(size_t i = 0; i < count-1; i++) { void* next = (char*)tmp + BlockSize; // получаем адрес следующего блока. *(void**)tmp = next; // ?? эта манипуляция мне не понятна tmp = next; // в "текущем" блоке указываем адрес следующего свободного блока } *(void**)tmp = NULL; // последний блок ни на что не указывает - терминируем. } The general idea is, of course, understandable - the “markup” of the memory goes, and using the properties of the pointers they make connections — You can say I get a singly linked list of pointers to an array.
But it is not at all clear why such a cast is in the code * (void **), because the type is so void *.
As I understand it, as a result of such a cast * (void **) tmp - the compiler looks at tmp as a dereferenced pointer of type void * —in fact, with my current knowledge — it seems to me that this should not change anything at all. But if you make such a simple experiment (g ++, linux 64bit)
char arr[]={'a','b','c','d','\0'}; void * vptr = new char[1024*2]; std::cout << vptr << '\n'; // 0x9c9010 std::cout << (void**)vptr << '\n'; // 0x9c9010 std::cout << (void*****)vptr << '\n'; // 0x9c9010 std::cout << *(void**)&vptr << '\n'; // 0x9c9010 std::cout << *(void**)vptr << '\n'; // 0 -- почему так ? memcpy((char*)vptr,arr,sizeof(arr)); // std::cout << *(void**)vptr << '\n'; // 0x64636261 -- почему так ? It becomes evident that * (void **) tmp somehow changes the address. As, unfortunately, I can not find information. If anyone knows, explain, or at least give the links something to read.
Studying a code from article, on f-ii
void* AllocBlock() { if (!head) FormatNewPage(); void* tmp = head; head = *(void**)head; return tmp; } It can be concluded that * (void **) head; makes some pointer address increment, but in my experiment this increment turns out to be very huge - and it seems to be significantly beyond the 1024 * 2 area by address arithmetic.
And for some reason I cannot find in any article, and in those books that I read (although there were not so many of them of course) there was no such method of passage to addresses as * (void **) (if this is of course used at all for some passage explicitly or indirectly)
In general, I do not understand. Thank you for your time.
