#pragma data_seg在DLL中共享数据的应用
在Win32环境下要想在多个进程中共享数据，就必须进行必要的设置。在访问同一个Dll的各进程之间共享存储器是通过存储器映射文件技术实现的。也可以把这些需要共享的数据分离出来，放置在一个独立的数据段里，并把该段的属性设置为共享。必须给这些变量赋初值，否则编译器会把没有赋初始值的变量放在一个叫未被初始化的数据段中。
这是因为(MSDN):
The default segment in the .obj file for initialized variables is .data. Variables initialized to zero are considered uninitialized and are stored in .bss.
Example
// pragma_directive_data_seg.cpp
int h = 1; // stored in .data
int i = 0; // stored in .bss
#pragma data_seg(".my_data1")
int j = 1; // stored in "my_data1"
data_seg with no parameters resets the segment to .data
#pragma data_seg预处理指令用于设置共享数据段。例如：
#pragma data_seg("SharedDataName")
HHOOK hHook=NULL;
#pragma data_seg()
在#pragma data_seg("SharedDataName")和#pragma data_seg()之间的所有变量将被访问该Dll的所有进程看到和共享。再加上一条指令#pragma comment(linker,"/section:.SharedDataName,rws"),字母RWS表示段具有读、写和共享属性.那么这个数据节中的数据可以在所有DLL的实例之间共享。所有对这些数据的操作都针对同一个实例的，而不是在每个进程的地址空间中都有一份。
There are restrictions to consider before using a shared data segment:
· Any variables in a shared data segment must be statically initialized. In the above example,
i is initialized to 0(这里和先前MSDN里的描述有矛盾) and a is 32 characters initialized to hello world.
· All shared variables are placed in the compiled DLL in the specified data segment. Very large arrays can result in very large DLLs. This is true of all initialized global variables.
· Never store process-specific information in a shared data segment. Most Win32 data structures or values (such as HANDLEs) are really valid only within the context of a single process.
· Each process gets its own address space. It is very important that pointers are never stored in a variable contained in a shared data segment. A pointer might be perfectly valid in one application but not in another.
· It is possible that the DLL itself could get loaded at a different address in the virtual address spaces of each process. It is not safe to have pointers to functions in the DLL or to other shared variables.