Signals and Interprocess Communication

• SIGTERM: A Unix signal providing a mechanism for notifying of system events.
  • Two different types of SIGTERMs
    1. Asynchronous: Generated by events outside the control of the process that receives them, and arrive at unpredictable times such as user input Ctrl+C
    2. Synchronous: Generated by exceptions, or errors, which are caused by invalid operations in a program such as dividing by zero.
• POSIX standard specifies 20 signals 2 of which are user definable

Reciveing Sigterms

• When receiving a asynchronous SIGTERM the processor can 1. ignore or 2. executed specified procedure
  • If unspecified there are 5 default procedures which may happen
    1. Terminate process
    2. Write execution context and contents of the address space to a file (called core dump) and terminate
    3. Ignore
    4. Suspend process
    5. Resume the process’s execution if it was previously stopped

IPC Resources: Semaphores, message queues, and shared memory

• Kernel implements these as IPC resources and a process can declare these data structures with function calls which
  • Return System V data structure identifiers specific to the function eg. semget() returns a semaphore set identifier

    shmget(), semget(), msgget()

• These data structures stick around like file descriptors and must be deallocated.  For example semaphores will be deallocated with

  sem_destroy()

  which returns 0 on success and -1 otherwise.

• Message Queue: Allow processes to exchange messages with the following functions

  msgsnd() and msgrcv()

  • Note POSIX has its own standard which is simpler file based interface vs Sys V
• Shared Memory:  Allocated memory from

  shmget()

  which returns a memory segment identifier if suceess.

  • For any process to access shared memory it must invoke

    shmat()

    which attaches the System V shared memory segment identified by shmid to the address space of the calling process.  And once finished detaches by using

    shmdt()