If you use fopen("test1.dat","r") which is the typical usage (no path information given), fopen will look in the current working directory. This the directory where your project file (.dsw in case of VC++ 6.0 and .sln in case of VC++ 7.1) is present. If u run your programe from explorer, say by clicking on it, current woring directry is one in which .exe of your application is present.

In addition, if the file is NOT one the current working directory the program may have to change the current working directory to where the file is. There are a few operating systems where there is no concept of "current working directory", in which case the program has no choice but to specify the full path.

thanks dragon