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setitimer(2) [redhat man page]

GETITIMER(2)						     Linux Programmer's Manual						      GETITIMER(2)

getitimer, setitimer - get or set value of an interval timer SYNOPSIS
#include <sys/time.h> int getitimer(int which, struct itimerval *value); int setitimer(int which, const struct itimerval *value, struct itimerval *ovalue); DESCRIPTION
The system provides each process with three interval timers, each decrementing in a distinct time domain. When any timer expires, a signal is sent to the process, and the timer (potentially) restarts. ITIMER_REAL decrements in real time, and delivers SIGALRM upon expiration. ITIMER_VIRTUAL decrements only when the process is executing, and delivers SIGVTALRM upon expiration. ITIMER_PROF decrements both when the process executes and when the system is executing on behalf of the process. Coupled with ITIMER_VIRTUAL, this timer is usually used to profile the time spent by the application in user and kernel space. SIGPROF is delivered upon expiration. Timer values are defined by the following structures: struct itimerval { struct timeval it_interval; /* next value */ struct timeval it_value; /* current value */ }; struct timeval { long tv_sec; /* seconds */ long tv_usec; /* microseconds */ }; The function getitimer fills the structure indicated by value with the current setting for the timer indicated by which (one of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF). The element it_value is set to the amount of time remaining on the timer, or zero if the timer is disabled. Similarly, it_interval is set to the reset value. The function setitimer sets the indicated timer to the value in value. If ovalue is nonzero, the old value of the timer is stored there. Timers decrement from it_value to zero, generate a signal, and reset to it_interval. A timer which is set to zero (it_value is zero or the timer expires and it_interval is zero) stops. Both tv_sec and tv_usec are significant in determining the duration of a timer. Timers will never expire before the requested time, instead expiring some short, constant time afterwards, dependent on the system timer resolution (currently 10ms). Upon expiration, a signal will be generated and the timer reset. If the timer expires while the process is active (always true for ITIMER_VIRT) the signal will be delivered immediately when generated. Otherwise the delivery will be offset by a small time dependent on the system loading. RETURN VALUE
On success, zero is returned. On error, -1 is returned, and errno is set appropriately. ERRORS
EFAULT value or ovalue are not valid pointers. EINVAL which is not one of ITIMER_REAL, ITIMER_VIRT, or ITIMER_PROF. CONFORMING TO
SVr4, 4.4BSD (This call first appeared in 4.2BSD). SEE ALSO
gettimeofday(2), sigaction(2), signal(2) BUGS
Under Linux, the generation and delivery of a signal are distinct, and there each signal is permitted only one outstanding event. It's therefore conceivable that under pathologically heavy loading, ITIMER_REAL will expire before the signal from a previous expiration has been delivered. The second signal in such an event will be lost. Linux 0.99.11 1993-08-05 GETITIMER(2)

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GETITIMER(2)						      BSD System Calls Manual						      GETITIMER(2)

getitimer, setitimer -- get/set value of interval timer SYNOPSIS
#include <sys/time.h> #define ITIMER_REAL 0 #define ITIMER_VIRTUAL 1 #define ITIMER_PROF 2 int getitimer(int which, struct itimerval *value); int setitimer(int which, const struct itimerval *restrict value, struct itimerval *restrict ovalue); DESCRIPTION
The system provides each process with three interval timers, defined in <sys/time.h>. The getitimer() call returns the current value for the timer specified in which in the structure at value. The setitimer() call sets a timer to the specified value (returning the previous value of the timer if ovalue is non-nil). A timer value is defined by the itimerval structure: struct itimerval { struct timeval it_interval; /* timer interval */ struct timeval it_value; /* current value */ }; If it_value is non-zero, it indicates the time to the next timer expiration. If it_interval is non-zero, it specifies a value to be used in reloading it_value when the timer expires. Setting it_value to 0 disables a timer. Setting it_interval to 0 causes a timer to be disabled after its next expiration (assuming it_value is non-zero). Time values smaller than the resolution of the system clock are rounded up to this resolution (typically 10 milliseconds). The ITIMER_REAL timer decrements in real time. A SIGALRM signal is delivered when this timer expires. The ITIMER_VIRTUAL timer decrements in process virtual time. It runs only when the process is executing. A SIGVTALRM signal is delivered when it expires. The ITIMER_PROF timer 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 system calls, programs using this timer must be prepared to restart interrupted system calls. NOTES
Three macros for manipulating time values are defined in <sys/time.h>. Timerclear sets a time value to zero, timerisset tests if a time value is non-zero, and timercmp compares two time values (beware that >= and <= do not work with this macro). RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and the global integer variable errno is set to indicate the error. ERRORS
Getitimer() and setitimer() will fail if: [EFAULT] The value parameter specified a bad address. [EINVAL] The value parameter specified a time that was too large to be handled or not in the canonical form. [EINVAL] The which parameter was invalid. SEE ALSO
gettimeofday(2), select(2), sigaction(2) HISTORY
The getitimer() function call appeared in 4.2BSD. 4.2 Berkeley Distribution December 11, 1993 4.2 Berkeley Distribution

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