Task Control: Signals and Alarms Chapter 7 and 8

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Task ControlSignals and AlarmsChapter 7 and 8

• B. Ramamurthy

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Multi-tasking

• How to create multiple tasks? Ex Xinu create()
• How to control them?
• ready()
• resched()
• How to synchronize them? How to communicate among
  them?
• XINU semaphores, send and receive messages
• How to (software) interrupt a process? signals

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Examples

• Consider g myProg.c
• You want to kill this process after you started
  the compilation..hit cntrl-C
• Consider execution of a program called badprog
• gtbadprog
• It core dumps .. What happened? The error in the
  program results in a signal to kernel to stop and
  dump the offending code
• Consider kill p ltpidgt
• Kill issues a termination signal to the process
  identified by the pid

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Linux Processes

• Similar to XINU Procs.
• Lets understand how to create a linux process and
  control it.
• Chapter 7 and 8 of text book.
• Chapter 7 multi-tasking
• Chapter 8 Task communication and synchronization

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Process creation

• Four common events that lead to a process
  creation are
• 1) When a new batch-job is presented for
  execution.
• 2) When an interactive user logs in / system
  initialization.
• 3) When OS needs to perform an operation (usually
  IO) on behalf of a user process, concurrently
  with that process.
• 4) To exploit parallelism an user process can
  spawn a number of processes.

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Termination of a process

• Normal completion, time limit exceeded, memory
  unavailable
• Bounds violation, protection error, arithmetic
  error, invalid instruction
• IO failure, Operator intervention, parent
  termination, parent request, killed by another
  process
• A number of other conditions are possible.
• Segmentation fault usually happens when you try
  write/read into/from a non-existent
  array/structure/object component. Or access a
  pointer to a dynamic data before creating it.
  (new etc.)
• Bus error Related to function call and return.
  You have messed up the stack where the return
  address or parameters are stored.

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Process control

• Process creation in unix is by means of the
  system call fork().
• OS in response to a fork() call
• Allocate slot in the process table for new
  process.
• Assigns unique pid to the new process..
• Makes a copy of the process image, except for the
  shared memory.
• both child and parent are executing the same code
  following fork()
• Move child process to Ready queue.
• it returns pid of the child to the parent, and a
  zero value to the child.

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Process control (contd.)

• All the above are done in the kernel mode in the
  process context. When the kernel completes these
  it does one of the following as a part of the
  dispatcher
• Stay in the parent process. Control returns to
  the user mode at the point of the fork call of
  the parent.
• Transfer control to the child process. The child
  process begins executing at the same point in the
  code as the parent, at the return from the fork
  call.
• Transfer control another process leaving both
  parent and child in the Ready state.

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Process Creation (contd.)

• Parent process create children processes, which,
  in turn create other processes, forming a tree of
  processes
• Generally, process identified and managed via a
  process identifier (pid)
• Resource sharing
• Parent and children share all resources
• Children share subset of parents resources
• Parent and child share no resources
• Execution
• Parent and children execute concurrently
• Parent waits until children terminate

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Process Termination

• Process executes last statement and asks the
  operating system to delete it (exit)
• Output data from child to parent (via wait)
• Process resources are deallocated by operating
  system
• Parent may terminate execution of children
  processes (abort)
• Child has exceeded allocated resources
• Task assigned to child is no longer required
• If parent is exiting
• Some operating system do not allow child to
  continue if its parent terminates
• All children terminated - cascading termination

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Example Code

1. int retVal
2. printf(" Just one process so far\n")
3. printf(" Invoking/Calling fork() system
   call\n")
4. retVal fork() / create new process/
5. if (retVal 0)
6. printf(" I am the child d \n",getpid())
7. else if (retVal gt 0)
8. printf(" I am the parent, child has pid d
   \n", retVal)
9. else
10. printf(" Fork returned an error d \n",
    retVal)

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Signals

• Signals provide a simple method for transmitting
  software interrupts to UNIX process
• Signals cannot carry information directly, which
  limits their usefulness as an general
  inter-process communication mechanism
• However each type of signal is given a mnemonic
  name Ex SIGINT
• See signal.h for others
• SIGHUP, SIGINT, SIGILL, SIGTRAP, SIGFPE, SIGKILL
• SIGALRM (sent by kernel to a process after an
  alarm timer has expired)
• SIGTERM
• signal (signal id, function) simply arms the
  signal

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Intercept Signals
Task1
Task2
Two essential parameters are destination process
identifier and the signal code number kill
(pid, signal) Signals are a useful way of
handling intermittent data arrivals or rare
error conditions.
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Handling Signals

• Look at the examples
• Catching SIGALRM
• Ignoring SIGALRM
• sigtest.c
• sigHandler.c
• pingpong.c
• See /usr/include/sys/iso/signal_iso.h for signal
  numbers

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Signals and Alarms

• include ltsignal.hgt
• unsigned int alarm( unsigned int seconds )
• alarm(a) will start a timer for a secsonds and
  will interrupt the calling process after a secs.
• time(t) will get you current time in the
  variable t declared as time_t t
• ctime(t) will convert time to ascii format

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Sample programs

• Starting new tasks in linux page 165
• Programs in pages 174-180 on signals and alarms
• See demos directory for first
• See page 175 for the second program
• See page 178 for the second program

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Pingpong
Parent
PSIG 43
Child
CSIG 42
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Volatile (from code reading last lecture)

• A variable should be declared volatile whenever
  its value could change unexpectedly. In practice,
  only three types of variables could change
• Memory-mapped peripheral registers
• Global variables modified by an interrupt service
  routine
• Global variables within a multi-threaded
  application