Advanced · Module 6 · Lesson 8 of 10

Unix Domain Sockets and File Descriptor Passing

Establish bidirectional inter-process streams with AF_UNIX, pass live open file descriptors across process boundaries using sendmsg and SCM_RIGHTS, and authenticate peer credentials.

ProAbout 16 minutes

By the end you can

Construct bidirectional stream and datagram channels using AF_UNIX and socketpair
Transmit open kernel file descriptors across process boundaries via sendmsg and SCM_RIGHTS
Parse ancillary control messages using CMSG_FIRSTHDR, CMSG_DATA, and CMSG_SPACE macros
Verify peer process identity (PID, UID, GID) using SO_PEERCRED

Builds on Process Synchronization with POSIX Semaphores.

01 · A question first

Why can an unprivileged worker process with no filesystem permissions read a protected file after receiving an 8-byte message from a privileged daemon? And why is sending an integer fd over a pipe useless, while passing it over a Unix domain socket gives the recipient a live kernel handle?

Make a guess before you read on.

02 · The idea

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What's inside

Start with the problem
How the idea works
Production Implementation: Passing Descriptors
Why it is correct
What it costs
Pitfalls and variations
Where it is used
Core Track: Modules 1–3 Complete