build - CMake link shared library on Windows

Question

There are three files, (m.c,m.h, and **main.c*).

File m.h

// m.h
int m();

File m.c

// m.c
#include <stdio.h>
#include "m.h"


int m(){
    printf("Hello,m!
");
    return 0;
}

File main.c

// main.c
#include "m.h"
int main(){
    return m();
}

While I prefer a shared library (m.dll), I've made the CMakeLists.txt file:

    PROJECT("app1")
    ADD_LIBRARY(m SHARED m.c)
    ADD_EXECUTABLE(myexe main.c)
    TARGET_LINK_LIBRARIES(myexe m)

The CMake configuration is done and generated done. Opening app1.sln and building with Visual Studio, it crashes as

LNK1104:Can't open file "Debugm.lib"

It only works as STATIC at ADD_LIBRARY(). Why doesn't it work on Windows?

If I got another shared library (mylib.dll), how could I invoke its functions in my main.c and CMakeLists.txt files?

Answer 1

There are differences between dynamic library linking on different platforms which also needs some additional code. The good news is, that CMake can help you with this. I found the following blog post by Gernot Klingler very useful:

• Creating and using shared libraries with different compilers on different operating systems

In short you need some "export prefix" defined for whatever is declared in m.h. Otherwise the build process will not generate an "import library" for statically linking named m.lib (see also CMAKE_IMPORT_LIBRARY_SUFFIX).

Here is your code with the modifications needed:

m.h

#include "m_exports.h"

int M_EXPORTS m();

m.c

#include "m.h"
#include <stdio.h>

int m(){
    printf("Hello,m!
");
    return 0;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.0)

include(GenerateExportHeader)

PROJECT("app1")

INCLUDE_DIRECTORIES("${CMAKE_CURRENT_BINARY_DIR}")
ADD_LIBRARY(m SHARED m.c m.h m_exports.h)
GENERATE_EXPORT_HEADER(m           
    BASE_NAME m
    EXPORT_MACRO_NAME M_EXPORTS
    EXPORT_FILE_NAME m_exports.h
    STATIC_DEFINE SHARED_EXPORTS_BUILT_AS_STATIC)

ADD_EXECUTABLE(myexe main.c)
TARGET_LINK_LIBRARIES(myexe m)

Additional References

• GenerateExportHeader macro
• cmake and GenerateExportHeader
• How do I get CMake to create a dll and its matching lib file?
• MSDN: Walkthrough: Creating and Using a Dynamic Link Library (C++)