| RAND48(3) | Library Functions Manual | RAND48(3) |
drand48, erand48,
jrand48, lcong48,
lrand48, mrand48,
nrand48, seed48,
srand48 — pseudo random
number generators and initialization routines
Standard C Library (libc, -lc)
#include
<stdlib.h>
double
drand48(void);
double
erand48(unsigned short
xsubi[3]);
long
jrand48(unsigned short
xsubi[3]);
void
lcong48(unsigned short
param[7]);
long
lrand48(void);
long
mrand48(void);
long
nrand48(unsigned short
xsubi[3]);
unsigned short *
seed48(unsigned short
seed16v[3]);
void
srand48(long seedval);
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. The default value for the
multiplicand `a' is 0x5deece66d (25214903917). The default value for the the
addend `c' is 0xb (11). The modulo is always fixed at m = 2 ** 48. r(n) is
called the seed of the random number generator.
For the six generator routines described next, the first computational 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 interval [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-supplied 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 multiplicand 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 algorithm
are reset to the default values given above. The
seed48() function returns a pointer to an array of 3
shorts which contains the old seed. This array is statically allocated;
thus, its contents are lost after 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
multiplicand; 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.
For a more powerful random number generator, see random(3).
Martin Birgmeier
| October 8, 1993 | Mac OS X 12 |