srand(time(NULL))

[Ioannis Vranos]

Regarding C95:


Is srand(time(NULL)); an effective solution for seeding rand(), or is
there any better approach?

 

[Mark Bluemel]

Regarding C95:


Is srand(time(NULL)); an effective solution for seeding rand(), or is
there any better approach?

There's some discussion in this area in section 13 of the C FAQ
(http://www.c-faq.com). Try Question 13.17

 

[Mark Bluemel]

Regarding C95:


Is srand(time(NULL)); an effective solution for seeding rand(), or is
there any better approach?

In addition to my previous reply, Google gives loads of hits for
"seeding a random number generator", many of which look helpful.

 

[Richard Bos]

Is srand(time(NULL)); an effective solution for seeding rand(),

Unless your time_t is a floating point type and the current time is
larger than UINT_MAX (not likely, in particular the first part);
as long as you don't call it more than once every time_t tick;
and as long as your rand() is at least of semi-reasonable quality (i.e.,
successive seeds don't lead to similar series), it's fine.

Richard

 

[Ben Pfaff]

Is srand(time(NULL)); an effective solution for seeding rand(), or is
there any better approach?

By default, the C random number generator produces the same
sequence every time the program is run. In order to generate
different sequences, it has to be "seeded" using srand() with a
unique value. The function below to do this is carefully
designed. It uses time() to obtain the current time; the
alternative clock() is a poor choice because it measures CPU time
used, which is often more or less constant among runs. The
actual value of a time_t is not portable, so it computes a "hash"
of the bytes in it using a multiply-and-add technique. The
factor used for multiplication normally comes out as 257, a prime
and therefore a good candidate.

References: Knuth, _The Art of Computer Programming, Vol. 2:
Seminumerical Algorithms_, section 3.2.1; Aho, Sethi, and Ullman,
_Compilers: Principles, Techniques, and Tools_, section 7.6.

#include <limits.h>
#include <stdlib.h>
#include <time.h>

/* Choose and return an initial random seed based on the current time.
Based on code by Lawrence Kirby <[email protected]>.
Usage: srand (time_seed ()); */
unsigned
time_seed (void)
{
time_t timeval; /* Current time. */
unsigned char *ptr; /* Type punned pointed into timeval. */
unsigned seed; /* Generated seed. */
size_t i;

timeval = time (NULL);
ptr = (unsigned char *) &timeval;

seed = 0;
for (i = 0; i < sizeof timeval; i++)
seed = seed * (UCHAR_MAX + 2u) + ptr;

return seed;
}

 

[Ioannis Vranos]

Unless your time_t is a floating point type

The function prototype of srand() is

void srand(unsigned int seed);


Whatever time_t is, isn't there an implicit conversion to unsigned?

and the current time is
larger than UINT_MAX (not likely, in particular the first part);

If the value is larger than UINT_MAX, doesn't the argument seed just
wrap around?

 

[Ioannis Vranos]

The FAQ mentions:

#include <stdlib.h>
#include <time.h>

srand((unsigned int)time((time_t *)NULL));


I think the casting of NULL to "time_t *" is unnecessary in C, since
NULL can be assigned to any pointer type without casting (also the
conversion of void * to whatever_type * is implicit).


Also since the srand() function protype is

void srand(unsigned int seed);


I think the casting of the return value of time() to unsigned int is
unnecessary, since the return value is implicitly converted to the
unsigned int argument.

 

[Richard Bos]

The function prototype of srand() is

void srand(unsigned int seed);

Whatever time_t is, isn't there an implicit conversion to unsigned?

Yes. But the conversion of a floating point value to an integral type
causes UB if that value, rounded down, would not fit into the integral
type - even if it's an unsigned type.

If the value is larger than UINT_MAX, doesn't the argument seed just
wrap around?

You'd think so, but no, not on conversion from a floating point value.

At least, not in C90. I haven't looked up whether this remains true in
C99.

Richard

 

[Ben Pfaff]

Yes. But the conversion of a floating point value to an integral type
causes UB if that value, rounded down, would not fit into the integral
type - even if it's an unsigned type.

Additionally, a floating point time value may not be useful as a
random seed, for example if the time value is always between 0
and 1. (I am not aware of any systems that do this.)

 

[Eric Sosman]

The FAQ mentions:

#include <stdlib.h>
#include <time.h>

srand((unsigned int)time((time_t *)NULL));


I think the casting of NULL to "time_t *" is unnecessary in C, since
NULL can be assigned to any pointer type without casting (also the
conversion of void * to whatever_type * is implicit).


Also since the srand() function protype is

void srand(unsigned int seed);


I think the casting of the return value of time() to unsigned int is
unnecessary, since the return value is implicitly converted to the
unsigned int argument.

"Unnecessary" is not always the same as "undesirable" --
do you use indentation to indicate block nesting? -- and the
question of whether to use or omit these casts is in some
degree a matter of taste. Myself, I'd omit them. Yet the
Ten Commandments For C Programmers takes the opposite view
(Third Commandment), so I am probably in a state of sin.

 

[Ioannis Vranos]

Additionally, a floating point time value may not be useful as a
random seed, for example if the time value is always between 0
and 1. (I am not aware of any systems that do this.)

Also I assume in the floating point case, casting to unsigned is
needless, right?

 

[Keith Thompson]

"Unnecessary" is not always the same as "undesirable" --
do you use indentation to indicate block nesting? -- and the
question of whether to use or omit these casts is in some
degree a matter of taste. Myself, I'd omit them. Yet the
Ten Commandments For C Programmers takes the opposite view
(Third Commandment), so I am probably in a state of sin.

Reference: <http://www.lysator.liu.se/c/ten-commandments.html>,
written by Henry Spencer.

The original version of this fine document was written, I believe,
before the existence of ANSI C. The Third Commandment is:

Thou shalt cast all function arguments to the expected type if
they are not of that type already, even when thou art convinced
that this is unnecessary, lest they take cruel vengeance upon thee
when thou least expect it.

The newer annotation reads, in part:

It may be thought that the radical new blessing of ``prototypes''
might eliminate the need for caution about argument types. Not so,
brethren. Firstly, when confronted with the twisted strangeness of
variable numbers of arguments, the problem returns... and he who
has not kept his faith strong by repeated practice shall surely
fall to this subtle trap. Secondly, the wise men have observed
that reliance on prototypes doth open many doors to strange
errors, and some indeed had hoped that prototypes would be decreed
for purposes of error checking but would not cause implicit
conversions. Lastly, reliance on prototypes causeth great
difficulty in the Real World today, when many cling to the old
ways and the old compilers out of desire or necessity, and no man
knoweth what machine his code may be asked to run on tomorrow.

This was good advice *at the time it was written*. Today, C compilers
that don't support prototypes are vanishingly rare, and this advice is
IMHO obsolete. You still need to be careful when calling variadic
functions; for example, when passing a pointer to printf with the "%p"
format, you should cast it to void*. (Quibbles: If it's already
void*, you don't need to cast it; if it's char*, you can probably get
away without casting it, but you should anyway; if it's a function
pointer, you can't portably print it with "%p" with or without a
cast.)

I'd say the 4th and 9th commandments are similarly obsolete. The 8th
(which mandates K&R brace style) is something that I *wish* everyone
obeyed, but if you're working under a coding standard that requires
some other style, it's more important to be consistent with that, and
with the existing code, than to use the (IMHO better) K&R style. The
only thing worse than code that uses an ugly brace style is code that
uses a mixture of different brace styles.

The remaining Commandments are still good advice today.

In particular, the Eight Commandment includes one of the best pieces
of programming advice I've ever seen, even if you disagree with the
premise that K&R defined the One True Brace Style:

... for thy creativity is better used in solving problems than in
creating beautiful new impediments to understanding.

 

[Richard Heathfield]

Keith Thompson said:

I'd say [Henry Spencer's] 4th and 9th commandments are similarly
obsolete. The 8th
(which mandates K&R brace style) is something that I *wish* everyone
obeyed,

Fortunately, your wishing well is broken. K&R style is *dire*.

Cue the Brace Wars.

<snip>

 

[Keith Thompson]

Also I assume in the floating point case, casting to unsigned is
needless, right?

Correct; the conversion is implicit for any arithmetic type (which
time_t is guaranteed to be). If time_t is a floating-point type, then
other problems appear: if the result of time() doesn't vary outside a
small range, then you won't get much variation in seed values, and if
it can go outside the range 0 .. UINT_MAX, the behavior of the
conversion can be undefined.

In theory, you could even have problems with an integer time_t.
Imagine that unsigned int is 32 bits, and time_t is a 64-bit integer
type whose high-order 32 bits represent time in seconds and whose
low-order 32 bits represent fractions of a second. Imagine further
that the underlying system clock can only represent whole seconds, so
the low-order 32 bits are always 0. Then converting *any* time_t
value returned by time() to unsigned int will yield 0.

Ben Pfaff's solution (treating the time_t result as an array of
unsigned char and hashing it) avoids all these problems, none of which
are likely but all of which are allowed by the standard.

In practice, using srand(time(NULL)) is likely to be good enough for a
quick-and-dirty program, and is unlikely to break in any of the above
described ways on any system you're likely to run into in real life.
On the other hand, the implementation-provided rand() function is
typically not very good; if you want *good* random numbers, you should
use some other technique. <HANDWAVE>See the literature for
details.</HANDWAVE>

Incidentally, I've e-mailed Steve Summit (the FAQ maintainer) about this;
he says:

Say, you're right. That's a last vestige of my PDP-11/V7/K&R C
days -- I thought I'd stamped those all out.

 

[Keith Thompson]

Keith Thompson said:

I'd say [Henry Spencer's] 4th and 9th commandments are similarly
obsolete. The 8th
(which mandates K&R brace style) is something that I *wish* everyone
obeyed,

Fortunately, your wishing well is broken. K&R style is *dire*.

Cue the Brace Wars.

<snip>

NOOOOOOOOOOO!!!!!!!

 

[Richard Heathfield]

Keith Thompson said:

Ben Pfaff's solution (treating the time_t result as an array of
unsigned char and hashing it) avoids all these problems,

As Ben would be the first to acknowledge, that was actually Lawrence
Kirby's solution.

<snip>

 

[Ben Pfaff]

Also I assume in the floating point case, casting to unsigned is
needless, right?

Casting a floating-point value to unsigned int will have the same
effect as allowing it to be implicitly converted to unsigned int
by omitting the cast when a prototype of srand() is visible, so a
cast to unsigned int would not be useful.

 

[Ben Pfaff]

Incidentally, I've e-mailed Steve Summit (the FAQ maintainer) about this;
he says:

Say, you're right. That's a last vestige of my PDP-11/V7/K&R C
days -- I thought I'd stamped those all out.

It's very odd to see the cast in the online FAQ, because the text
version that I keep on my local file system, which says it was
last modified February 7, 1999, omits the cast. Odd that it was
apparently (re?)introduced after that.

I should really update my local copy.

 

[Morris Dovey]

Keith Thompson said:

I'd say [Henry Spencer's] 4th and 9th commandments are similarly
obsolete. The 8th
(which mandates K&R brace style) is something that I *wish* everyone
obeyed,

Fortunately, your wishing well is broken. K&R style is *dire*.

Cue the Brace Wars.

<snip>

K&R style is easily the most maintainable and easiest on the eye I have
ever used. It just "makes sense. header, body, footer. e.g

if(expr){
expr;
}

Now loading trebuchet with barn waste...

 

[Antoninus Twink]

Keith Thompson said:

I'd say [Henry Spencer's] 4th and 9th commandments are similarly
obsolete. The 8th
(which mandates K&R brace style) is something that I *wish* everyone
obeyed,

Fortunately, your wishing well is broken. K&R style is *dire*.

Cue the Brace Wars.

The familiar Heathfield combination of minimal taste and maximal
polemic...