RAND48(3) BSD Library Functions Manual RAND48(3)
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, lcong48 -- pseudo-
random number generators and initialization routines
Standard C Library (libc, -lc)
erand48(unsigned short xseed);
nrand48(unsigned short xseed);
jrand48(unsigned short xseed);
unsigned short *
seed48(unsigned short xseed);
lcong48(unsigned short p);
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 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 per-
form 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 mantissa 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 func-
tions 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.
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 shorts, where the zeroth
member specifies the lowest bits. Again, the constant multiplicand and addend of the algo-
rithm are reset to the default values given above. seed48() returns a pointer to an array
of 3 shorts which contains the old seed. This array is statically allocated, thus its con-
tents 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 parameter; the first three
shorts are used to initialize the seed; the second three are used to initialize the multi-
plicand; 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 mul-
tiplicand and addend for any of the six generator calls.
For a more powerful random number generator, see random(3).
BSD October 8, 1993 BSD