rand48, drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, lcong48—pseudo-random number generators and initialization routinesSynopsis
#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]);
Description
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 = 0xfdeece66d = 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 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 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 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.
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 algorithm 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 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 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.
Portability
SUS requires these functions.
No supporting OS subroutines are required.