What are the exact steps for creating and then linking against a Win32 DLL on the command line?
See this related question on how to build the DLL.
Your library code as it stands does not export any symbols and your executable does not import the symbols from your library. Two typical patterns for doing that are shown below but you might want to read up on that first.
The first method uses __declspec()
to declare in the code what functions (or other items) are exported from your DLL and imported by other executables. You use a header file to declare the exported items and have a preprocessor flag used to control whether the symbols are exports or imports:
mylib.h:
#ifndef MYLIB_H
#define MYLIB_H
#if defined(BUILDING_MYLIB)
#define MYLIB_API __declspec(dllexport) __stdcall
#else
#define MYLIB_API __declspec(dllimport) __stdcall
#endif
#ifdef __cplusplus
extern "C" {
#endif
int MYLIB_API helloworld(void);
#ifdef __cplusplus
}
#endif
#endif
I have also specifically set the calling convention to __stdcall
as are most DLL functions (I could have used WINAPI
instead of __stdcall
if I had included windows.h) and have declared the functions as extern "C"
so their names do not get mangled when compiled as C++. Not such a problem here as it's all C, but if you were to build the DLL from C source and then try to use it from a C++ executable then the imported names would be incorrect.
The code could then look like this:
mylib.c
#include "mylib.h"
#include <stdio.h>
int MYLIB_API helloworld(void)
{
printf("Hello World DLL");
return 42;
}
You'd build your DLL using the following command line. As well as creating the DLL it will create the import library (.lib) required to use your DLL from another executable (as well as the export file, but that is only required in certain circumstances):
cl /DBUILDING_MYLIB mylib.c /LD
The /DBUILDING_MYLIB
argument defines the preprocessor symbol used to control whether the functions in the DLL are exports (if it is defined) or imports (not defined). So you'd define it when building the DLL but not when building your application.
The /LD
parameter tells cl to produce a DLL.
The second method is to use module definition files as mentioned in the comments. You can use the code you already have but you also need to create the module definition file. At it's simplest it looks like this:
LIBRARY mylib
EXPORTS
helloworld
In this case to build the DLL you require the following command line:
cl /LD mylib.c /link /DEF:mylib.def
You could then code your application so that it used your library header with the imported version of your DLL function:
main.c
/* No need to include this if you went the module definition
* route, but you will need to add the function prototype.
*/
#include "mylib.h"
int main(void)
{
helloworld();
return (0);
}
Which you could then compile with the following command line (assuming the import library from the DLL creation is in the same directory as your main.c). This step is the same whether you used declspec or module definition files:
cl main.c /link mylib.lib
Arguments passed after the /link
argument are passed onto the linker command line as they appear, so as just a filename it is used as extra input to link into the executable. In this case we specify the import library generated when we built the DLL.
The command lines I've shown here are pretty much the absolute minimum you'd need but it'll allow you to create a DLL and link an application to it.
I have assumed the calling convention is correct in all of the above and I have not experimented much to see whether I got it wrong at any point.
merlin2011
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Updated on June 05, 2022Comments
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merlin2011 almost 2 years
Here's my library
Lib.c
file:#include <stdio.h> int helloworld(){ printf("Hello World DLL"); }
Here's my exe
Main.c
file:int helloworld(); int main(int argc, char** argv){ helloworld(); }
I would like to create
Lib.dll
, andMain.exe
, whereLib.dll
comes fromLib.c
andMain.exe
links againstLib.dll
.What are the specific steps to achieve this?
-
Mark Ransom over 12 yearsYou should be more explicit about the fact that creating the .dll also creates a .lib at the same time.
-
Adrian McCarthy over 12 yearsThis answer is good. If you don't like using the
__declspec
extension, you can create a [module definition file][msdn.microsoft.com/en-us/library/28d6s79h(v=vs.100).aspx] that specifies the exported interface. It can give more control over what (and how) you expose externally. -
Ben Voigt over 12 yearsYou probably want to use
extern "C"
for exported symbols, and maybe also__stdcall
(or theWINAPI
macro). -
tomlogic over 12 yearsI assume the definition of
MYLIB_API
should be different whenBUILDING_MYLIB
isn't defined. Do you just remove the__declspec(dllexport)
and keep the__stdcall
for C++ code linking against the DLL? -
tinman over 12 years@tomlogic: it was different but I broke it in an edit, thanks. I'll edit to change but linking with C++ works without changes (once I've fixed it :)
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merlin2011 over 12 yearsI had already seen the related question cited here, but the answers were nowhere near as complete as this one, most likely because the asker already had more knowledge so didn't require as much completeness.