socat experiments

socat.bash

# Create input FIFO
rm -f /tmp/input.fifo
mkfifo /tmp/input.fifo
# Bind to FD in the shell to keep the FIFO alive
exec 10<>/tmp/input.fifo

# Create output FIFO
rm -f /tmp/output.fifo
mkfifo /tmp/output.fifo
# Bind to FD in the shell to keep the FIFO alive
exec 11<>/tmp/output.fifo

# Configure the FIFOs to have a 4096 byte buffer (the minimum)
#   1031 is F_SETPIPE_SZ, see http://man7.org/linux/man-pages/man2/fcntl.2.html
python -c 'import fcntl; fifo = open("/tmp/input.fifo", "wb"); print(fcntl.fcntl(fifo.fileno(), 1031, 4096))'
python -c 'import fcntl; fifo = open("/tmp/output.fifo", "rb"); print(fcntl.fcntl(fifo.fileno(), 1031, 4096))'

# Generate a 4k test file
python -c "import sys; sys.stdout.write(bytes.join(b'', [chr(X) for X in range(256)] * 16))" >4k.bin

# Launch server
socat -d -d -d tcp-listen:2101,reuseaddr,fork - >/tmp/output.fifo </tmp/input.fifo &

# Write more data to the server than the pipes can hold
dd if=4k.bin bs=4096 count=4 | socat - tcp:localhost:2101

# Check how much data is in the pipe:
cat /tmp/output.fifo | xxd

# Last line of output should look like this (showing that there's only 4k of data):
# > 00000ff0: f0f1 f2f3 f4f5 f6f7 f8f9 fafb fcfd feff  ................