Thanks to ffiore I discovered a nice way for communicating data between processes and thread: Message Queues. MQs are a Linux feature that implement a kernel-based push-pop stack. Linux has two types of MQs: Posix and SystemV. This post is about SystemV.

Instead of reading/writing data to a standard pipe/socket where you have to manage the split of the messages and other issues, you can simply use MQs with the following pseudocode procedure:

# pusher thread

struct mq_msg msg;

msg.mtext = “my message”

mq = mq_create()

msgsnd(mq, msg, ..)

while the pop-er thread gets messages

# popper thread

msg = msgrcv(mq,..)

print msg.mtext

Linux provides some command line tools to manage the queue, while queue size is managed by sysctl and ulimit.

ananke # sysctl -a | grep msg

kernel.msgmnb = 65536 # max queue size = (msgsize*#msgs)

kernel.msgmni = 16

kernel.msgmax = 65536 # maximum  number  of  messages  in  a  queue.

fs.mqueue.msgsize_max = 8192 # maximum message size.

fs.mqueue.msg_max = 10

If we want to queue 10k messages of 12kbytes each, we should set:

# sysclt -w fs.mqueue.msgsize_max=12000

# sysclt -w kernel.msgmnb=120000000

You can view message queues with the #ipcs command:

ananke# sudo apt-get install util-linux

an MQ follows, containing 12 messages summing up 390 bytes

ananke #  ipcs -q

—— Message Queues ——–

key        msqid      owner      perms      used-bytes   messages

0x00000000 0          rpolli     666        390          13

For further info on System V and Posix MQs

#man svipc mq_overview