[Proj] Proj binary input and output

[pw]

Hello,

The documentation for proj states binary input is
expected to be system byte order and double.

I am getting invalid results with binary
input, yet if I manually enter lonlats asking
for binary output I get the expected output.

Right my data is double precision floating point (64bit)
organized as a series of lonlats (x then y).

Byte order appears to be OK since the program chokes if I
reverse the byte order.

What format does my binary input file (binary_file.bin)
need to be in for the following  conversion?

proj +proj=utm +zone=18 +ellps=WGS84 -i binary_file.bin


Thanks for any insights you may have,

Peter Willis