int *p = new int[5];

(char *)p - sizeof(size_t)

12.4
...

3 If a class has no user declared
destructor, a destructor is declared implicitly. An implicitly declared
destructor is an inline public member of its class. A destructor is 
trivial if it is an implicitlydeclared destructor and if:
— all of the direct base classes of its class have trivial destructors and
— for all of the nonstatic data members of its class that are of class type 
(or array thereof), each such class has a trivial destructor.

4 Otherwise, the destructor is nontrivial.

...

Н>>(char *)p - sizeof(size_t)
Н>>

int *p = new int[5];

int FDescr=_open(LocalFileName.operator LPCTSTR(), _O_BINARY|_O_RDONLY);
//               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

p = new char[lFileLength];

p = new char[lFileLength];

df>>p = new char[lFileLength];
df>>

FDescr=_open(LocalFileName, _O_BINARY|_O_RDONLY);
...
FLength=_filelength(FDescr);
...
pFileBuff=new char[FLength];
_read(FDescr, pFileBuff, FLength);
...

df>> FDescr = _open(LocalFileName,      // имя файла (CString)
                   _O_BINARY|_O_RDONLY);
df>>...
df>> FLength = _filelength(FDescr);     // размер в байтах
df>>...
df>> pFileBuff=new char[FLength];       //    
df>> _read(FDescr, pFileBuff, FLength); // в созданный буфер прочитал файл
df>>...

df>FDescr=_open(LocalFileName, _O_BINARY|_O_RDONLY);
df>...
df>FLength=_filelength(FDescr);
df>...
df>pFileBuff=new char[FLength];
df>_read(FDescr, pFileBuff, FLength);
df>...
df>

FDescr=_open(LocalFileName, _O_BINARY|_O_RDONLY);
...
FLength=_filelength(FDescr);
...
std::vector<char> FileBuff(FLength); 
FileBuff.resize( FLength ); //застолбили память
_read(FDescr, &FileBuff[0], FLength); //так как элементы вектора расположены неприрывно, то можно получить адрес начала блока памяти
...

vector<char> filebuf;
...
filebuf.resize(FLength); // или прямо в конструкторе указать
_read(FDescr, &filebuf[0], FLength);

G>FDescr=_open(LocalFileName, _O_BINARY|_O_RDONLY);
G>...
G>FLength=_filelength(FDescr);
G>...
G>std::vector<char> FileBuff(FLength); 
G>FileBuff.resize( FLength ); //застолбили память
G>_read(FDescr, &FileBuff[0], FLength); //так как элементы вектора расположены неприрывно, то можно получить адрес начала блока памяти
G>...

G>_read(FDescr, &FileBuff[0], FLength); //_если это сработает - будет гуд!_

df>p = new char[lFileLength];
df>

size_t lFileLength = ...;
p = new char[lFileLength + sizeof(size_t)];
*(reinterpret_cast< size_t* >(p)) = lFileLength;
Foo(p);

void Foo(char* _p) {
 size_t Size = *(reinterpret_cast< size_t* >(p));
 char* p = _p + sizeof(size_t);
 // Далее пользуем p как и раньше без изменений
}

_msize
Returns the size of a memory block allocated in the heap.

size_t _msize( void *memblock );

Parameter
— memblock
Pointer to memory block
Libraries
All versions of the C run-time libraries.

Return Value
_msize returns the size (in bytes) as an unsigned integer.

Remarks
The _msize function returns the size, in bytes, of the memory block allocated by a call to malloc, or realloc.

When the application is linked with a debug version of the C run-time libraries, _msize resolves to _msize_dbg. For more information about how the heap is managed during the debugging process, see Using C Run-Time Library Debugging Support.

Allocates memory blocks.

void *malloc(
   size_t size 
);

Parameter
size
Bytes to allocate.

Return Value
malloc returns a void pointer to the allocated space, or NULL if there is insufficient memory available. To return a pointer to a type other than void, use a type cast on the return value. The storage space pointed to by the return value is guaranteed to be suitably aligned for storage of any type of object. If size is 0, malloc allocates a zero-length item in the heap and returns a valid pointer to that item. Always check the return from malloc, even if the amount of memory requested is small.

Remarks
The malloc function allocates a memory block of at least size bytes. The block may be larger than size bytes because of space required for alignment and maintenance information.