Query regarding buffering in streams

Hai,
I could able to do the follwoing thing in linux, aix but not in IRIX.

cat testfile.dat | testprog

testfile.dat is the input to testprog.

testprog could able to get all the input properly in Linux without using setbuf in "testprog"but failed in IRIX. So when I used

setbuf(stdin, NULL) in "testprog" where I am opening stdin explicitly it worked well. Can anyone explain me what is going on explaining me the usage of setbuf, setvbuf with clear understanding using streams. I jst hve a vague idea and made it work.

with regards,
Karthik.

I recommend that you check the manpage for setbuf; it explains it all in detail.
By the way, why "cat testfile.dat | testprog"? Why not just "testprog <testfile.dat"?