| RAND48(3) | Library Functions Manual | RAND48(3) | 
drand48, erand48,
  lrand48, nrand48,
  mrand48, jrand48,
  srand48, seed48,
  lcong48 —
#include <stdlib.h>
double
  
  drand48(void);
double
  
  erand48(unsigned
    short xseed[3]);
long
  
  lrand48(void);
long
  
  nrand48(unsigned
    short xseed[3]);
long
  
  mrand48(void);
long
  
  jrand48(unsigned
    short xseed[3]);
void
  
  srand48(long
    seed);
unsigned short *
  
  seed48(unsigned
    short xseed[3]);
void
  
  lcong48(unsigned
    short p[7]);
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 modulus 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 computational step is to perform a single iteration of the algorithm.
drand48() and
    erand48() return values of type double. The full 48
    bits of r(n+1) are loaded into the significand of the returned value, with
    the exponent set such that the values produced lie in the interval [0.0,
    1.0).
lrand48() and
    nrand48() 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.
mrand48() and
    jrand48() 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.
drand48(),
    lrand48(), and mrand48() 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 unsigned shorts, where the zeroth member holds the least significant
  bits.
All functions share the same multiplicand and addend.
srand48() 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.
seed48() 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 unsigned
    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. seed48() returns a pointer to an
    array of 3 unsigned 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 unsigned shorts is passed
    as parameter; 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).
| February 22, 2020 | NetBSD 10.0 |