在Windows上使用Mingw的gcc构建共享库时出现“未定义的引用”问题

Question

I am trying to build a shared library (.dll) for windows using Mingw. As a dependencies, the shared library has a set of c files, header files and couple of static libraries (*.a built with mingw). When I tried to build the shared library with source files and static libraries, the following error occurs: "undefined reference to". The error comes from one of the static library which has dependency on other static files.

The following example demonstrates the issue I am facing

There is a file ops.c, built as a static library

#include<stdio.h>

extern void print_num();

void print_num()
{
    printf("HELLO WORLD IN OPS\n");
}

Compiled with the command:

gcc -c ops.c -o ops.o
ar cru libops.a ops.o

The second files mp4.c, which is also built as static library

#include<stdio.h>

void print_ops_num()
{
    printf("IN mp4c, will call ops\n");

    print_num();
}

Compiled with the command:

gcc -c mp4.c -o mp4.o
ar cru libmp4.a mp4.o

The list files mss.c, this will be built as shared library with above 2 static libraries

#include<stdio.h>

void demo_func()
{
     printf("Demo dynamic build and linking \n");

     print_ops_num();

     print_num();

}

Commands:

 gcc -o libmss.dll mss.c -o mss.o -L libops.a -L libmp4.a -Wl,--out-implib=libmss.dll.a

and also used below

 gcc -o libmss.dll mss.c -o mss.o -L -lops -L -lmp4

The following error is seen while build the shared library

C:\Users\AppData\Local\Temp\cc6tISrV.o:mss.c:(.text+0x15): undefined reference to `print_ops_num'
C:\Users\AppData\Local\Temp\cc6tISrV.o:mss.c:(.text+0x1a): undefined reference to `print_num'
g:/work/temp/mingw64/bin/../lib/gcc/x86_64-w64-mingw32/4.8.1/../../../../x86_64-w64-mingw32/bin/ld.exe: C:\Users\AppData\Local\Temp\cc6tISrV.o: ba
eloc address 0x0 in section `.pdata'
collect2.exe: error: ld returned 1 exit statusc  ops.a  ops.c  ops.o

The above problem does not occur when the same procedure is folowed on a linux platform. I am new Windows platform, can someone help me to solve the above issue?

Answer 1

The line

gcc -o libmss.dll mss.c -o mss.o -L libops.a -L libmp4.a -Wl,--out-implib=libmss.dll.a

is incorrect. You are using the -L flags, which point the linker to directories where to look for libraries. These directories do not exist, obviously, so nothing happens and your DLL is not linked to the static libraries. You need to use the -l flags:

gcc -shared -o libmss.dll mss.c -o mss.o -L . -l ops -l mp4 -Wl,--out-implib=libmss.dll.a

The reason why this worked in Linux and not in Windows is that DLL needs to have all external dependencies resolved during linking, while in Linux SO can have undefined symbols, which are resolved only when the library is used.

By the way, with modern MinGW, you don't need to create import libraries ( libmss.dll.a ). Is is possible to link directly to the DLL.