Inter Process Communication

tags : Systems, Operating Systems, Processes

We can make two processes communicate with each other using IPC.

Shared memory (Indirect) §

• One process creates an area in memory for others to access
• Fast, no syscalls but does need some kind of synchronization.

Message Passing (Direct) §

• Shared memory created in Kernel
• syscalls such as send, recieve etc are involved.
• Pipes make use of message passing. (unidirectional)
• See Message Passing

Signals §

• Asynchronous, unidirectional communication between processes.
• Process receives a signal
  • Processes the signal immediately
  • Without finishing the currently running instruction
• We can list all signals in the system with kill -l and man 7 signal

Relationship w Interrupts §

• The kernel may pass an interrupt as a signal to processes

Signal Handlers §

• Signal handler executes in userspace.

Traps §

• So signals are things that one process can send to another
• Steps
  • A sends signal to B via a syscall
  • Kernel intercepts the signal, checks if A can send the signal to B or no and goes on or drops based on that.
  • If signal is SIGSTOP/SIGKILL it’ll send the signal regardless if B is blocking or not.
  • Kernel sends the signal to B if B is not blocking the signal.
    • Kernel sets a trap on the thread(B); adds it to the “pending signal set”
    • Next time the thread(B) is scheduled to run, B handles the signal instead of processing itself.
    • This means, if the context switch of B never happens, signal never will get processed.
  • When handing the signal, B has few options
    • Ignore the signal
    • Let the defult thing happen
    • Use a signal handler

Resources/Links §

• How fast are Linux pipes anyway?