getitimer.2

早期freebsd实现

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.\"     @(#)getitimer.2	8.2 (Berkeley) 12/11/93
.\"
.Dd December 11, 1993
.Dt GETITIMER 2
.Os BSD 4.2
.Sh NAME
.Nm getitimer ,
.Nm setitimer
.Nd get/set value of interval timer
.Sh SYNOPSIS
.Fd #include <sys/time.h>
.Fd #define ITIMER_REAL		0
.Fd #define ITIMER_VIRTUAL	1
.Fd #define ITIMER_PROF		2
.Ft int
.Fn getitimer "int which" "struct itimerval *value"
.Ft int
.Fn setitimer "int which" "struct itimerval *value" "struct itimerval *ovalue"
.Sh DESCRIPTION
The system provides each process with three interval timers,
defined in
.Ao Pa sys/time.h Ac .
The
.Fn getitimer
call returns the current value for the timer specified in
.Fa which
in the structure at
.Fa value .
The
.Fn setitimer
call sets a timer to the specified
.Fa value
(returning the previous value of the timer if
.Fa ovalue
is non-nil).
.Pp
A timer value is defined by the 
.Fa itimerval
structure:
.Bd -literal -offset indent
struct itimerval {
	struct	timeval it_interval;	/* timer interval */
	struct	timeval it_value;	/* current value */
};
.Ed
.Pp
If
.Fa it_value
is non-zero, it indicates the time to the next timer expiration. 
If
.Fa it_interval
is non-zero, it specifies a value to be used in reloading 
.Fa it_value
when the timer expires.
Setting 
.Fa it_value
to 0 disables a timer.  Setting 
.Fa it_interval
to 0 causes a timer to be disabled after its next expiration (assuming
.Fa it_value
is non-zero).
.Pp
Time values smaller than the resolution of the
system clock are rounded up to this resolution
(typically 10 milliseconds).
.Pp
The
.Dv ITIMER_REAL
timer decrements in real time.  A
.Dv SIGALRM
signal is
delivered when this timer expires.
.Pp
The
.Dv ITIMER_VIRTUAL
timer decrements in process virtual time.
It runs only when the process is executing.  A
.Dv SIGVTALRM
signal
is delivered when it expires.
.Pp
The
.Dv 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
.Dv ITIMER_PROF
timer expires, the
.Dv 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.
.Sh NOTES
Three macros for manipulating time values are defined in
.Ao Pa sys/time.h Ac .
.Fa Timerclear
sets a time value to zero,
.Fa timerisset
tests if a time value is non-zero, and
.Fa timercmp
compares two time values (beware that >= and <= do not
work with this macro).
.Sh RETURN VALUES
If the calls succeed, a value of 0 is returned.  If an error occurs,
the value -1 is returned, and a more precise error code is placed
in the global variable
.Va errno .
.Sh ERRORS
.Fn Getitimer
and
.Fn setitimer
will fail if:
.Bl -tag -width Er
.It Bq Er EFAULT
The
.Fa value
parameter specified a bad address.
.It Bq Er EINVAL
A
.Fa value
parameter specified a time that was too large
to be handled.
.El
.Sh SEE ALSO
.Xr select 2 ,
.Xr sigvec 2 ,
.Xr gettimeofday 2
.Sh HISTORY
The
.Nm
function call appeared in
.Bx 4.2 .