Building and using DLLs in C

At some point you might find that while higher level languages like C# are awesome and get the job done faster than regular C, the overhead of using C# libraries might cause issues when computing heavy loads. For example, if you wanted to write a bruteforcer or crypto algorithm, you want the speed of C.

Creating the DLL

In this tutorial I'll create a file named test_dll.c and test_dll.h.

test_dll.c

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

EXPORT void message(){
    printf("Hello World");
}

The code is pretty straight foward. I created a simple function that prints 'Hello World'. The main thing to note is the use of the EXPORT keyword. It's a keyword I defined in test_dll.h

test_dll.h

#define EXPORT __declspec(dllexport)

EXPORT void message (void);

The EXPORT keyword is defined as __declspec(dllexport) which is a DLL export directive. Using this directive basically tells the linker to expose the function for import to other DLLs or programs.

This can be compiled like so:

gcc -c test_dll.c
gcc -shared -o TestDll.dll -Wl,--out-implib,libtstdll.a test_dll.o

I'm compiling it such that the result is TestDll.dll.

Creating the program

For this part, I just created a file main.c.
main.c

#include "test_dll.h"

int main(){
    message();
    return 0;
}

I included the dll using the same name as its header file and then was able to simply call the function defined in it.

It can be compiled like so:

gcc -o testProg main.c -L. -lTestDll

I just named my program 'testProg' because of lack of creativity. The -L. part tells the linker to look in the current directory for libs. The -lTestDll tells it to look for a lib named 'TestDll', which is our DLL.

Conclusion
Thats basically DLLs in a nutshell. They are very useful for making small libraries and APIs that you can use with C/C++ or C#.