libbsd/man/timeval.3bsd

.\" $NetBSD: timeval.3,v 1.12 2011/04/12 08:39:26 jruoho Exp $
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.\" by Jukka Ruohonen.
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.Dd April 12, 2011
.Dt TIMEVAL 3bsd
.Os
.Sh NAME
.Nm timeval ,
.Nm timespec
.Nd time structures
.Sh LIBRARY
.ds str-Lb-libbsd Utility functions from BSD systems (libbsd, \-lbsd)
.ds doc-str-Lb-libbsd \*[str-Lb-libbsd]
.Lb libbsd
.Sh SYNOPSIS
.In sys/time.h
(See
.Xr libbsd 7
for include usage.)
.Ft void
.Fn TIMEVAL_TO_TIMESPEC "struct timeval *tv" "struct timespec *ts"
.Ft void
.Fn TIMESPEC_TO_TIMEVAL "struct timeval *tv" "struct timespec *ts"
.Sh DESCRIPTION
The
.In sys/time.h
header, included by
.In time.h ,
defines various structures related to time and timers.
.Bl -enum -offset 1n
.It
The following structure is used by
.Xr gettimeofday 2 ,
among others:
.Bd -literal -offset indent
struct timeval {
	time_t		tv_sec;
	suseconds_t	tv_usec;
};
.Ed
.Pp
The
.Va tv_sec
member represents the elapsed time, in whole seconds.
The
.Va tv_usec
member captures rest of the elapsed time,
represented as the number of microseconds.
.It
The following structure is used by
.Xr nanosleep 2 ,
among others:
.Bd -literal -offset indent
struct timespec {
	time_t		tv_sec;
	long		tv_nsec;
};
.Ed
.Pp
The
.Va tv_sec
member is again the elapsed time in whole seconds.
The
.Va tv_nsec
member represents the rest of the elapsed time in nanoseconds.
.Pp
A microsecond is equal to one millionth of a second,
1000 nanoseconds, or 1/1000 milliseconds.
To ease the conversions, the macros
.Fn TIMEVAL_TO_TIMESPEC
and
.Fn TIMESPEC_TO_TIMEVAL
can be used to convert between
.Em struct timeval
and
.Em struct timespec .
.El
.Sh EXAMPLES
It can be stressed that the traditional
.Tn UNIX
.Va timeval
and
.Va timespec
structures represent elapsed time, measured by the system clock.
The following sketch implements a function suitable
for use in a context where the
.Va timespec
structure is required for a conditional timeout:
.Bd -literal -offset indent
static void
example(struct timespec *spec, time_t minutes)
{
	struct timeval elapsed;

	(void)gettimeofday(&elapsed, NULL);

	_DIAGASSERT(spec != NULL);
	TIMEVAL_TO_TIMESPEC(&elapsed, spec);

	/* Add the offset for timeout in minutes. */
	spec->tv_sec = spec->tv_sec + minutes * 60;
}
.Ed
.Pp
A better alternative would use the more precise
.Xr clock_gettime 2 .
.Sh SEE ALSO
.Xr timeradd 3bsd