Basics

Why Threads?

fork() is so beautiful

• New process

• New address space

• ⟶ no race conditions

• ⟶ simple is beautiful!

But …

• Process creation is expensive

• Separate address space ⟶ communication is cumbersome

• Portability: Windows has no idea

Typical Uses

• Use of multiple processors for compute-intensive calculation

• One is force to use a library that blocks

  • A no-go in a GUI application for example

  • Push it in a thread, call it there, and communicate with the thread however you feel best

  • Communication ⟶ later

• Blocking I/O

  • Like the blocking library: push it in a dedicated thread

  • But there are better anti-naive solutions (Unix is not Windows)

Overview

• Creating threads

• Synchronisation: Mutex

• Communication: Condition variable

• Thread specific data (a.k.a. thread local storage)

• One-time initialization

Legal (1)

Threads of one process share the following resources:

• Process memory

• PID and PPID

• Credentials

• Open files

• Signal handler

• Umask, Current Working Directory, etc.

• …

Legal (2)

Threads have the following attributes of their own:

• Thread ID (TID)

  • Scheduler only cares about threads

  • A process is just a container (which happens to have the ID of the main thread)

• Stack

• errno

• Signal mask

• Thread specific data (TSD)

• …

POSIX Thread API

• POSIX thread API is not implemented in the kernel

  • User space library

  • man 3 ...

  • strace is of limited use

• errno is thread spezific ⟶ “semi-global”

• No PThread function sets errno

  • They generally return what otherwise would be -errno

  • Thank you!

• gcc -pthread

  • Defines macro _REENTRANT

  • Links -lpthread

  • C++: thread safe initialization of local static