clear/truncate file in C when already open in "r+" mode

With standard C, the only way is to reopen the file in "w+" mode every time you need to truncate. You can use freopen() for this. "w+" will continue to allow reading from it, so there's no need to close and reopen yet again in "r+" mode. The semantics of "w+" are:

Open for reading and writing. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file.

(Taken from the fopen(3) man page.)

You can pass a NULL pointer as the filename parameter when using freopen():

my_file = freopen(NULL, "w+", my_file);

If you don't need to read from the file anymore at all, when "w" mode will also do just fine.

You can write a function something like this:(pseudo code)

if(this is linux box) 
use truncate()
else if (this is windows box)
use _chsize_s()

This is the most straightforward solution for your requirement.

Refer: man truncate and _chsize_s at msdn.microsoft.com

and include necessary header files too.