Determine the size of a pipe without calling read()

I need a function called SizeOfPipe() which should return the size of a pipe - I only want to know how much data is in the pipe and not actually read data off the pipe itself.

I thought the following code would work:

fseek (pPipe, 0 , SEEK_END);
*pBytes = ftell (pPipe);
rewind (pPipe);

but fseek() doesn't work on file descriptors. Another option would be to read the pipe then write the data back but would like to avoid this if possible. Any suggestions?