The system provides each process with four interval timers, defined in sys/time.h. The getitimer() function stores the current value of the timer specified by which into the structure pointed to by value. The setitimer() function call sets the value of the timer specified by which to the value specified in the structure pointed to by value, and if ovalue is not NULL, stores the previous value of the timer in the structure pointed to by ovalue.

A timer value is defined by the itimerval structure (see gettimeofday(3C)) for the definition of timeval), which includes the following members:

struct timeval it_interval; /* timer interval */ struct timeval it_value; /* current value */

The it_value member indicates the time to the next timer expiration. The it_interval member specifies a value to be used in reloading it_value when the timer expires. Setting it_value to 0 disables a timer, regardless of the value of it_interval. Setting it_interval to 0 disables a timer after its next expiration (assuming it_value is non-zero).

Time values smaller than the resolution of the system clock are rounded up to the resolution of the system clock, except for ITIMER_REALPROF, whose values are rounded up to the resolution of the profiling clock. The four timers are as follows:

ITIMER_REAL

Decrements in real time. A SIGALRM signal is delivered when this timer expires.

ITIMER_VIRTUAL

Decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is delivered when it expires.

ITIMER_PROF

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 delivered. Because this signal may interrupt in-progress functions, programs using this timer must be prepared to restart interrupted functions.

ITIMER_REALPROF

Decrements in real time. It is designed to be used for real-time profiling of multithreaded programs. Each time the ITIMER_REALPROF timer expires, one counter in a set of counters maintained by the system for each lightweight process (lwp) is incremented. The counter corresponds to the state of the lwp at the time of the timer tick. All lwps executing in user mode when the timer expires are interrupted into system mode. When each lwp resumes execution in user mode, if any of the elements in its set of counters are non-zero, the SIGPROF signal is delivered to the lwp. The SIGPROF signal is delivered before any other signal except SIGKILL. This signal does not interrupt any in-progress function. A siginfo structure, defined in <sys/siginfo.h>, is associated with the delivery of the SIGPROF signal, and includes the following members:

si_tstamp; /* high resolution timestamp */ si_syscall; /* current syscall */ si_nsysarg; /* number of syscall arguments */ si_sysarg[]; /* actual syscall arguments */ si_fault; /* last fault type */ si_faddr; /* last fault address */ si_mstate[]; /* ticks in each microstate */

The enumeration of microstates (indices into si_mstate) is defined in <sys/msacct.h>.

Upon successful completion, 0 is returned. Otherwise, -1 is returned and errno is set to indicate the error.

The getitimer() and setitimer() functions will fail if:

EINVAL

The specified number of seconds is greater than 100,000,000, the number of microseconds is greater than or equal to 1,000,000, or the which argument is unrecognized.

See attributes(5) for descriptions of the following attributes:

alarm(2), gettimeofday(3C), sleep(3C), sysconf(3C), attributes(5), standards(5)

The microseconds field should not be equal to or greater than one second.

The setitimer() function is independent of the alarm() function.

Do not use setitimer(ITIMER_REAL) with the sleep() routine. A sleep(3C) call wipes out knowledge of the user signal handler for SIGALRM.

The ITIMER_PROF and ITIMER_REALPROF timers deliver the same signal and have different semantics. They cannot be used together.

The granularity of the resolution of alarm time is platform-dependent.