Pipes are Fun!

2024-04-11 574 words 3 minutes

Contents

Pipes are fun. Pipes are cool. All the cool kids use pipes for communication.

You are probably thinking - wait a minute! I thought signals were used for IPC and for triggering specific actions in processes. And you’d be right. Signals are used to communicate b/w 2 processes. Signals like SIGINT, SIGTERM, SIGKILL can be used to communicate and alter state of processes. However, Pipes can be also be used for IPC.

Note: ALPC are used for high performance, high throughput IPC in Windows. Checkout how ALPC and CLFS play together. CLFS is highly optimised for performance.

You have probably already used `pipes` before

Where have I used them? - you ask. Well, let’s take a look at this program. Here’s a simple example:

#include <stdio.h>

int main() {
	printf("hi");
}

Above mentioned program writes output to stdout

Same program can be re-written as:

int main() {
	fprintf(stdout, "hi");
}

And you can have separate pipes for stdout and stderr and you can just decide to redirect any of them to some file.

Use > to redirect stdout
Use 2> to redirect stderr

Also, you can redirect these pipes to some other process, effectively connecting both the processes to each other.

./proc1 | grep hula

ps aux | grep edge | wc

But those pipes are ephemeral. They go away as soon as process is done. What to do if you want to have them stay persistent for some time? You give them names. That way, pipes will live in the files system.

mkfifo somepipe

While it looks as a file, it is not a regular file. FS knows that it is a pipe. This pipe can be used for IPC. When a process writes to it, it blocks the process that wrote to it. And when some other process reads from it, writing process can finally proceed ahead.

Just use a regular file bro

Files are slow. If you just want to do IPC then using pipes would be faster. This is because files are going to commit data to the disk. Pipes are not going to do that. Pipes will hold on to data only until either

Writing process lives
Reading process consumes it
Whichever happens first from above two

Named pipes are also faster than sockets. And, yes, sockets can also be used for IPC.

Windows has Pipes too

Some properties of pipes in Windows

Used for IPC
Can be one-way or duplex for comm b/w pipe server and one or more clients
Have their own file system called NPFS
Any process can access them depending upon security clearance
All instances of a named pipe share the same pipe name but each instance has it’s own buffers and handles

Named pipes can be accessed remotely

IPC can be local or remote
IPC can be done over RPC
Useful in distributed computing envs
IONinja tool can sniff pipes

Properties of Windows named pipes

Number of min and max connections
ACLs - DACLs and SACLs
Enumerating and scanning for named pipes

# Get the list of named pipes using .NET classes
$namedPipes = [System.IO.Directory]::GetFiles("\\.\\pipe\\") | ForEach-Object { $_ -replace '\\\\.\\pipe\\', '' }

# Output the list of named pipes
$namedPipes

What other methods for IPC in Windows?

Shared memory/mapping
RPC
LPC/ALPC
COM - Uses RPC or LPC
Windows Messages
Sockets
Named Pipes
Named - Mutex, Semaphore, Events
Debugging APIs
Regular ol’ Files
Custom Kernel Driver