Task Control: Signals and Alarms Havilland and Salama

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Task ControlSignals and AlarmsHavilland and
Salamas Unix Systems Programming

• 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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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

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Handling Signals

• Look at the examples
• Catching SIGALRM
• Ignoring SIGALRM
• sigtest.c

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Signals and Alarms

• include ltunistd.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 program

• Lets look at a sample program

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Volatile

• 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