Unix Programming - <sys/time.h>: timercmp

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<sys/time.h>: timercmp

Adrian 'Dagurashibanipal' von Bidder

2004-05-15, 3:31 pm

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Yo all!

In the glibc 2.3.2 headers, I find the following:

/* NOTE: `timercmp' does not work for >= or <=. */
....
# define timercmp(a, b, CMP) \
(((a)->tv_sec == (b)->tv_sec) ? \
((a)->tv_usec CMP (b)->tv_usec) : \
((a)->tv_sec CMP (b)->tv_sec))

I'm probably just tired, but why would >=/<= not work?

thanks & bye
- -- vbi

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Fletcher Glenn

2004-05-17, 12:35 pm

Adrian 'Dagurashibanipal' von Bidder wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Yo all!
>
> In the glibc 2.3.2 headers, I find the following:
>
> /* NOTE: `timercmp' does not work for >= or <=. */
> ...
> # define timercmp(a, b, CMP) \
> (((a)->tv_sec == (b)->tv_sec) ? \
> ((a)->tv_usec CMP (b)->tv_usec) : \
> ((a)->tv_sec CMP (b)->tv_sec))
>
> I'm probably just tired, but why would >=/<= not work?
>

A struct timeval looks like this:

struct timeval
{
time_t tv_sec; // seconds
time_t tv_usec; // microseconds
};

You cannot directly compare one struct with another. You must instead
compare the elements. Now think about what you know if
a->tv_sec != b->tv_sec. Right, you cannot conclude anything execpt
> or <. The <= and >= can only work if a->tv_sec == b->tv_sec.

--

Fletcher Glenn

Adrian 'Dagurashibanipal' von Bidder

2004-05-19, 3:31 am

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Clinging to sanity, Fletcher Glenn mumbled in his beard:
(well, in this case it's probably me who clings to sanity, or tries to)=

=20[color=darkred]
\[color=darkred]
[color=darkred]
[color=darkred]
> struct timeval
> {
> time_t tv_sec; // seconds
> time_t tv_usec; // microseconds
> };
>=20
> You cannot directly compare one struct with another. You must instea=
d
> compare the elements. Now think about what you know if
> a->tv_sec !=3D b->tv_sec. Right, you cannot conclude anything execpt=
[color=darkred]
ec.

Hmm.

Let's say a and b are struct timevals.

If a < b and sec are !=3D, timercmp(a, b, <=3D) should return true, whi=
ch it
does because the third line of the define is used (a->sec <=3D b->sec)

If a < b and sec are =3D=3D, result is the same, from the second line o=
f the
define (a->usec <=3D b->usec)

If a =3D=3D b, again, the comparison will be done on usec and the resul=
t will be
as expected.

If a > b within the same sec, the usec will be compared and result is f=
alse
as a->usec is not <=3D b->usec.

If a > b and not within the same sec, same but comparison is a->sec <=3D=

b->sec which is not true.

Still don't see the light, sorry.

Of course, the probability of two gettimeofday() (or two intervals
cal=C3=B6culated with timersub) being =3D=3D is near zero, but in my ca=
se I have a
sorted list where some of the timevals are actually really equal. Using=

timercmp(.. <=3D) ensures that new records with equal time are sorted i=
n the
right position in that list automatically and makes the if (...)
conditional shorter.

greetings
- -- vbi

=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D
#include <stdio.h>
#include <sys/time.h>

struct timeval a, b;

void print_case(struct timeval * a, struct timeval * b) {
if (timercmp(a, b, <=3D))
printf("a <=3D b\n");
else
printf("a > b\n");
}

int main() {
// case 1
a.tv_sec =3D 500;
a.tv_usec =3D 265346;
b.tv_sec =3D 501;
b.tv_usec =3D 23454;
print_case(&a, &b);
// case 2
a.tv_sec =3D 500;
a.tv_usec =3D 26546;
b.tv_sec =3D 500;
b.tv_usec =3D 2345456;
print_case(&a, &b);
// case 3
a.tv_sec =3D 500;
a.tv_usec =3D 265346;
b.tv_sec =3D 500;
b.tv_usec =3D 265346;
print_case(&a, &b);
// case 1
a.tv_sec =3D 500;
a.tv_usec =3D 265346;
b.tv_sec =3D 500;
b.tv_usec =3D 23454;
print_case(&a, &b);
// case 1
a.tv_sec =3D 502;
a.tv_usec =3D 265346;
b.tv_sec =3D 501;
b.tv_usec =3D 23454;
print_case(&a, &b);

return 0;
}
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D

- --=20
Today is Prickle-Prickle, the 66th day of Discord in the YOLD 3170

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