File: libc.info,  Node: Setting an Alarm,  Next: Sleeping,  Prev: Precision Tim\e,  Up: Date and Time

Setting an Alarm
================

   The `alarm' and `setitimer' functions provide a mechanism for a
process to interrupt itself at some future time.  They do this by
setting a timer; when the timer expires, the process receives a signal.

   Each process has three independent interval timers available:

   * A real-time timer that counts clock time.  This timer sends a
     `SIGALRM' signal to the process when it expires.

   * A virtual timer that counts CPU time used by the process.  This
     timer sends a `SIGVTALRM' signal to the process when it expires.

   * A profiling timer that counts both CPU time used by the process,
     and CPU time spent in system calls on behalf of the process.  This
     timer sends a `SIGPROF' signal to the process when it expires.

     This timer is useful for profiling in interpreters.  The interval
     timer mechanism does not have the fine granularity necessary for
     profiling native code.

   You can only have one timer of each kind set at any given time.  If
you set a timer that has not yet expired, that timer is simply reset to
the new value.

   You should establish a handler for the appropriate alarm signal using
`signal' or `sigaction' before issuing a call to `setitimer' or
`alarm'.  Otherwise, an unusual chain of events could cause the timer
to expire before your program establishes the handler, and in that case
it would be terminated, since that is the default action for the alarm
signals.  *Note Signal Handling::.

   The `setitimer' function is the primary means for setting an alarm.
This facility is declared in the header file `sys/time.h'.  The `alarm'
function, declared in `unistd.h', provides a somewhat simpler interface
for setting the real-time timer.

 - Data Type: struct itimerval
     This structure is used to specify when a timer should expire.  It
     contains the following members:
    `struct timeval it_interval'
          This is the interval between successive timer interrupts.  If
          zero, the alarm will only be sent once.

    `struct timeval it_value'
          This is the interval to the first timer interrupt.  If zero,
          the alarm is disabled.

     The `struct timeval' data type is described in *Note
     High-Resolution Calendar::.

 - Function: int setitimer (int WHICH, struct itimerval *NEW, struct
          itimerval *OLD)
     The `setitimer' function sets the timer specified by WHICH
     according to NEW.  The WHICH argument can have a value of
     `ITIMER_REAL', `ITIMER_VIRTUAL', or `ITIMER_PROF'.

     If OLD is not a null pointer, `setitimer' returns information
     about any previous unexpired timer of the same kind in the
     structure it points to.

     The return value is `0' on success and `-1' on failure.  The
     following `errno' error conditions are defined for this function:

    `EINVAL'
          The timer interval was too large.

 - Function: int getitimer (int WHICH, struct itimerval *OLD)
     The `getitimer' function stores information about the timer
     specified by WHICH in the structure pointed at by OLD.

     The return value and error conditions are the same as for
     `setitimer'.

`ITIMER_REAL'
     This constant can be used as the WHICH argument to the `setitimer'
     and `getitimer' functions to specify the real-time timer.

`ITIMER_VIRTUAL'
     This constant can be used as the WHICH argument to the `setitimer'
     and `getitimer' functions to specify the virtual timer.

`ITIMER_PROF'
     This constant can be used as the WHICH argument to the `setitimer'
     and `getitimer' functions to specify the profiling timer.

 - Function: unsigned int alarm (unsigned int SECONDS)
     The `alarm' function sets the real-time timer to expire in SECONDS
     seconds.  If you want to cancel any existing alarm, you can do
     this by calling `alarm' with a SECONDS argument of zero.

     The return value indicates how many seconds remain before the
     previous alarm would have been sent.  If there is no previous
     alarm, `alarm' returns zero.

   The `alarm' function could be defined in terms of `setitimer' like
this:

     unsigned int
     alarm (unsigned int seconds)
     {
       struct itimerval old, new;
       new.it_interval.tv_usec = 0;
       new.it_interval.tv_sec = 0;
       new.it_value.tv_usec = 0;
       new.it_value.tv_sec = (long int) seconds;
       if (setitimer (ITIMER_REAL, &new, &old) < 0)
         return 0;
       else
         return old.it_value.tv_sec;
     }

   There is an example showing the use of the `alarm' function in *Note
Handler Returns::.

   If you simply want your process to wait for a given number of
seconds, you should use the `sleep' function.  *Note Sleeping::.

   You shouldn't count on the signal arriving precisely when the timer
expires.  In a multiprocessing environment there is typically some
amount of delay involved.

   *Portability Note:* The `setitimer' and `getitimer' functions are
derived from BSD Unix, while the `alarm' function is specified by the
POSIX.1 standard.  `setitimer' is more powerful than `alarm', but
`alarm' is more widely used.



