Hi, I am a computer science resercher. I am looking for a piece of
free software that enables me to do wireless communication. More
specifically, I need the code( C++, C#, visual basic is fine too but
not as desirable) of an application that can be deployed on hand-held
devices (e.g., PDA's) or laptops and these kind of devices, such that
I can send and receive packets WIRELESSLY between these devices. So I
just have to write the code of my application to generate packets,
then call a function in this software to send my packet wirelessly to
another like device.
The provided application should be accessible from C/C++ or c#, as
this is how I will be using it.
Thanks!

It would matter if you need radio or can do line of site, and what
speed you're expecting.

Wireless serial communication via infrared has been available for a
while. In fact, it's not even a major issue to program for serial
communications (read: COM1/COM2) ... certainly not wireless specific.
If you have any serial communications software, it's up to the
hardware to behave properly, but you don't have to program
specifically for the hardware. You just have to program specifically
for the port. Other wireless communication, such as WIFI, might be a
bit on the more technical side, but Infrared is wireless, and the code
will be essentially Open port, send data, and open rcvr, receive data.

It takes more than just software to do this.  First, you should select
the hardware technology you want to use, then find the software to
drive it.