# FreeBSD Manual Pages

```RAND48(3)		 BSD Library Functions Manual		     RAND48(3)

NAME
drand48, erand48, lrand48,	nrand48, mrand48, jrand48, srand48, seed48,
lcong48 --	pseudo random number generators	and initialization routines

LIBRARY
Standard C	Library	(libc, -lc)

SYNOPSIS
#include <stdlib.h>

double
drand48(void);

double
erand48(unsigned short xseed);

long
lrand48(void);

long
nrand48(unsigned short xseed);

long
mrand48(void);

long
jrand48(unsigned short xseed);

void
srand48(long seed);

unsigned short *
seed48(unsigned short xseed);

void
lcong48(unsigned short p);

DESCRIPTION
The functions described in	this manual page are not cryptographically se-
cure.  Cryptographic applications should use arc4random(3)	instead.

The rand48() family of functions generates	pseudo-random numbers using a
linear congruential algorithm working on integers 48 bits in size.	 The
particular	formula	employed is r(n+1) = (a	* r(n) + c) mod	m where	the
default values are	for the	multiplicand a = 0x5deece66d = 25214903917 and
the addend	c = 0xb	= 11.  The modulo is always fixed at m = 2 ** 48.
r(n) is called the	seed of	the random number generator.

For all the six generator routines	described next,	the first computa-
tional step is to perform a single	iteration of the algorithm.

The drand48() and erand48() functions return values of type double.  The
full 48 bits of r(n+1) are	loaded into the	mantissa of the	returned
value, with the exponent set such that the	values produced	lie in the in-
terval [0.0, 1.0).

The lrand48() and nrand48() functions return values of type long in the
range [0, 2**31-1].  The high-order (31) bits of r(n+1) are loaded	into
the lower bits of the returned value, with	the topmost (sign) bit set to
zero.

The mrand48() and jrand48() functions return values of type long in the
range [-2**31, 2**31-1].  The high-order (32) bits	of r(n+1) are loaded
into the returned value.

The drand48(), lrand48(), and mrand48() functions use an internal buffer
to	store r(n).  For these functions the initial value of r(0) =
0x1234abcd330e = 20017429951246.

On	the other hand,	erand48(), nrand48(), and jrand48() use	a user-sup-
plied buffer to store the seed r(n), which	consists of an array of	3
shorts, where the zeroth member holds the least significant bits.

All functions share the same multiplicand and addend.

The srand48() function is used to initialize the internal buffer r(n) of
drand48(),	lrand48(), and mrand48() such that the 32 bits of the seed
value are copied into the upper 32	bits of	r(n), with the lower 16	bits
of	r(n) arbitrarily being set to 0x330e.  Additionally, the constant mul-
tiplicand and addend of the algorithm are reset to	the default values
given above.

The seed48() function also	initializes the	internal buffer	r(n) of
drand48(),	lrand48(), and mrand48(), but here all 48 bits of the seed can
be	specified in an	array of 3 shorts, where the zeroth member specifies
the lowest	bits.  Again, the constant multiplicand	and addend of the al-
gorithm are reset to the default values given above.  The seed48()	func-
tion returns a pointer to an array	of 3 shorts which contains the old
seed.  This array is statically allocated,	thus its contents are lost af-
ter each new call to seed48().

Finally, lcong48()	allows full control over the multiplicand and addend
used in drand48(),	erand48(), lrand48(), nrand48(), mrand48(), and
jrand48(),	and the	seed used in drand48(),	lrand48(), and mrand48().  An
array of 7	shorts is passed as argument; the first	three shorts are used
to	initialize the seed; the second	three are used to initialize the mul-
tiplicand;	and the	last short is used to initialize the addend.  It is
thus not possible to use values greater than 0xffff as the	addend.

Note that all three methods of seeding the	random number generator	always
also set the multiplicand and addend for any of the six generator calls.