The getitimer() function shall store the current value of the timer
specified by which into the structure pointed to by value. The
setitimer() function shall set the timer specified by which to
the value specified in the structure pointed to by value, and if
ovalue is not a null pointer, store the previous value of the timer in
the structure pointed to by ovalue.
A timer value is defined by the itimerval structure, specified in
<sys/time.h>. If it_value is non-zero, it shall indicate
the time to the next timer expiration. If it_interval is non-zero, it
shall specify a value to be used in reloading it_value when the timer
expires. Setting it_value to 0 shall disable a timer, regardless of the
value of it_interval. Setting it_interval to 0 shall disable a
timer after its next expiration (assuming it_value is non-zero).
Implementations may place limitations on the granularity of timer values. For
each interval timer, if the requested timer value requires a finer granularity
than the implementation supports, the actual timer value shall be rounded up
to the next supported value.
An XSI-conforming implementation provides each process with at least three
interval timers, which are indicated by the which argument:
Decrements in real time. A SIGALRM signal is delivered when
this timer expires.
Decrements in process virtual time. It runs only when the
process is executing. A SIGVTALRM signal is delivered when it
Decrements both in process virtual time and when the system
is running on behalf of the process. It is designed to be used by
interpreters in statistically profiling the execution of interpreted
programs. Each time the ITIMER_PROF timer expires, the SIGPROF signal is
The interaction between setitimer() and any of alarm(),
sleep(), or usleep() is unspecified.
Portions of this text are reprinted and reproduced in electronic form from IEEE
Std 1003.1, 2003 Edition, Standard for Information Technology -- Portable
Operating System Interface (POSIX), The Open Group Base Specifications Issue
6, Copyright (C) 2001-2003 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard, the original
IEEE and The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.opengroup.org/unix/online.html