mmap() on Qnx Rtp Question

I am new to QNX, and I have download the QNX RTP iso and installed it on
my box.
When I use mmap() call in my programm, it seems it can not map a disk file
the memory, I use perror to examing why and was told “not support.” My
question is
is the really true? And if true, is here any alternative for me?
Thank you.