通常，我们在Windows上加载一个dll并调用标记为的函数__declspec(dllexport)，但是我可以从dll调用已加载程序中实现的函数吗？

首先，这可以在Linux上完成，请检查我之前提出的问题。

我编写了一个测试程序对此进行测试：CMakeList.txt（对不起，我是Windows编程的新手，并且不知道如何使用Visual Studio）：

cmake_minimum_required(VERSION 3.16)
project(untitled4)

set(CMAKE_CXX_STANDARD 17)

add_library(lib1 SHARED lib1.cpp)

add_executable(untitled4 main.cpp)

main.cpp

#include "iostream"
#include "windows.h"
extern "C" {
  // this is the function I want to get called from dll
__declspec(dllexport)
float get_e() {
  return 2.71;
}
}
int main() {
  auto *handler = LoadLibrary("lib1.dll");
  if (!handler) {
    std::cerr << ERROR_DELAY_LOAD_FAILED << std::endl;
    exit(1);
  }
  auto p = (float (*)()) GetProcAddress(handler, "get_pi");
  std::cout << p() << std::endl;
}

lib1.cpp：

#include "iostream"
extern "C" {
  // implemented in main.cpp
__declspec(dllimport)
float get_e();

__declspec(dllexport)
float get_pi() {
  std::cout << get_e() << std::endl; // comment this line will compile, just like the normal case
  return 3.14;
}
}

构建lib1时，编译将失败：

NMAKE : fatal error U1077: '"C:\Program Files\JetBrains\CLion 2020.1.1\bin\cmake\win\bin\cmake.exe"' : return code '0xffffffff'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\bin\HostX86\x64\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\bin\HostX86\x64\nmake.exe"' : return code '0x2'
Stop.
NMAKE : fatal error U1077: '"C:\Program Files (x86)\Microsoft Visual Studio\2019\BuildTools\VC\Tools\MSVC\14.27.29110\bin\HostX86\x64\nmake.exe"' : return code '0x2'
LINK Pass 1: command "C:\PROGRA~2\MICROS~2\2019\BUILDT~1\VC\Tools\MSVC\1427~1.291\bin\Hostx86\x64\link.exe /nologo @CMakeFiles\lib1.dir\objects1.rsp /out:lib1.dll /implib:lib1.lib /pdb:C:\Users\derwe\CLionProjects\untitled\cmake-build-debug\lib1.pdb /dll /version:0.0 /machine:x64 /debug /INCREMENTAL kernel32.lib user32.lib gdi32.lib winspool.lib shell32.lib ole32.lib oleaut32.lib uuid.lib comdlg32.lib advapi32.lib /MANIFEST /MANIFESTFILE:CMakeFiles\lib1.dir/intermediate.manifest CMakeFiles\lib1.dir/manifest.res" failed (exit code 1120) with the following output:
   Creating library lib1.lib and object lib1.exp
lib1.cpp.obj : error LNK2019: unresolved external symbol __imp_get_e referenced in function get_pi
lib1.dll : fatal error LNK1120: 1 unresolved externals
Stop.

那么我可以在Windows上执行此操作吗？那就是从dll调用main中的函数？

PS在dll中添加注册表功能并 get_e通过它的想法非常感谢，但在我的实际情况中不能考虑。