为什么/ when是__declspec（dllimport）不需要？ [英] Why/when is __declspec( dllimport ) not needed?

问题描述

在使用server.dll和client.exe的项目中，我有 dllexport 从服务器dll中删除服务器符号，而不是 dllimport 将其写入客户端exe。

In a project using a server.dll and a client.exe, I have dllexported a server symbol from the server dll, and not dllimported it into the client exe.

然而，应用程序链接并启动，没有任何问题。 dllimport 不需要，然后

Still, the application links, and starts, without any problem. Is dllimport not needed, then???

详细信息：

我有这个'server'dll：

I have this 'server' dll:

// server.h
#ifdef SERVER_EXPORTS
  #define SERVER_API __declspec(dllexport)
#else
  #define SERVER_API // =====> not using dllimport!
#endif
class  SERVER_API CServer {
   static long s;
   public:
   CServer();
};

// server.cpp
CServer::CServer(){}

long CServer::s;

和此客户端可执行文件：

and this client executable:

#include <server.h>
int main() {
   CServer s;
}

服务器命令行：

cl.exe /Od  /D "WIN32" /D "_DEBUG" /D "_WINDOWS" /D "_USRDLL" 
 /D "SERVER_EXPORTS" /D "_UNICODE" /D "UNICODE" /D "_WINDLL" 
 /Gm /EHsc /RTC1 /MDd /Yu"stdafx.h" 
 /Fp"Debug\server.pch" /Fo"Debug\\" /Fd"Debug\vc80.pdb" 
 /W3 /nologo /c /Wp64 /ZI /TP /errorReport:prompt

cl.exe /OUT:"U:\libs\Debug\server.dll" /INCREMENTAL:NO /NOLOGO /DLL 
/MANIFEST /MANIFESTFILE:"Debug\server.dll.intermediate.manifest" 
/DEBUG /PDB:"u:\libs\Debug\server.pdb" 
/SUBSYSTEM:WINDOWS /MACHINE:X86 /ERRORREPORT:PROMPT 
kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib advapi32.lib 
shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib

客户端命令行：

cl.exe /Od /I "..\server" 
 /D "WIN32" /D "_DEBUG" /D "_CONSOLE" /D "_UNICODE" /D "UNICODE" 
 /Gm /EHsc /RTC1 /MDd /Fo"Debug\\" /Fd"Debug\vc80.pdb" /W3 /c /Wp64 /ZI /TP 
 .\client.cpp

cl.exe /OUT:"U:\libs\Debug\Debug\client.exe" /INCREMENTAL 
/LIBPATH:"U:\libs\Debug" 
/MANIFEST /MANIFESTFILE:"Debug\client.exe.intermediate.manifest" 
/DEBUG /PDB:"u:\libs\debug\debug\client.pdb" 
/SUBSYSTEM:CONSOLE /MACHINE:X86 
server.lib kernel32.lib user32.lib gdi32.lib winspool.lib comdlg32.lib 
advapi32.lib shell32.lib ole32.lib oleaut32.lib uuid.lib odbc32.lib odbccp32.lib

推荐答案

这不是必需的。这是一个优化，提示编译器DLL将直接导出函数指针，而不仅仅是DLL的IAT中的条目。名为foo（）的函数的导出函数指针将为__imp_foo。它允许它生成更好的代码，从IAT保存函数指针加载和间接跳转。

It isn't required. It is an optimization, a hint to the compiler that the DLL is going to export the function pointer directly rather than just an entry in the IAT of the DLL. The exported function pointer for a function named foo() will be __imp_foo. Which allows it to generate better code, saving a function pointer load from the IAT and an indirect jump. It is a time optimization, not space.

这博客帖子有详细信息。

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