Your assumption is wrong about extensions. If fopen is returning NULL, you should output the result of strerror(errno) or use the perror() function to see why it failed.
Edit: The problem is probably because you have "c:\tc\bin\hi.ttcn". in C, "\t" is interpreted as tab, for example.
You could do
"c:\\tc\\bin\\hi.ttcn"
But this is extremely ugly, and your system should accept:
"c:/tc/bin/hi.ttcn"
The most likely culprit is your Turbo C, an ancient compiler. It's techincally a DOS compiler, not Windows. That would limit it's RunTme Library to 8.3 filenames. Upgrade to something newer - Turbo C++ seems like a logical successor, but Microsoft's VC++ Express would work as well.
Now that you've posted the code, another problem comes to light.
The following line:
fp=fopen("c:\tc\bin\hi.ttcn","r");
Should instead read:
fp=fopen("c:\\tc\\bin\\hi.ttcn","r");
In C strings, the backslash (\) is an escape character that is used to encode special characters (e.g. \n represents a newline character, \t a tab character). To actually use a literal backslash, you need to double it. As it stands, the compiler is actually trying to open a file named "C:<tab>c<backspace>in\hi.ttcn" -- needless to say, no such file exists!
MS-DOS does not know about long file names, thos including files with extensions longer than 3 characters. Therefore, the CRT provided by Turbo C most probably does not look for the name you are providing, but a truncated one - or something else.
Windows conveniently provides a short (i.e. matching the 8.3 format, most of the time ending in ~1 unless you play with files having the same 8-character prefix) file name for those; one way to discover it is to open a console window and to run "dir /x" in the folder your file is stored.
Find the short name associated to your file and patch it into your C source file.
Edit: Darn, I'll read the comments next time. All credits to j_random_hacker.