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BSD 2.11 - man page for setitimer (bsd section 2)

GETITIMER(2)					 System Calls Manual					 GETITIMER(2)

getitimer, setitimer - get/set value of interval timer
#include <sys/time.h> #define ITIMER_REAL 0 /* real time intervals */ #define ITIMER_VIRTUAL 1 /* virtual time intervals */ #define ITIMER_PROF 2 /* user and system virtual time */ getitimer(which, value) int which; struct itimerval *value; setitimer(which, value, ovalue) int which; struct itimerval *value, *ovalue;
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 nonzero). 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 (on the VAX, 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.
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). NOTES (PDP-11) On the PDP-11, setitimer rounds timer values up to seconds resolution. (This saves some space and computation in the overburdened PDP-11 kernel.)
If the calls succeed, a value of 0 is returned. If an error occurs, the value -1 is returned, and a more pre- cise error code is placed in the global variable errno.
The possible errors are: [EFAULT] The value parameter specified a bad address. [EINVAL] A value parameter specified a time was too large to be handled.
sigvec(2), gettimeofday(2) 4.2 Berkeley Distribution August 26, 1985 GETITIMER(2)

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