Originální popis anglicky:
drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, lcong48 -
generate uniformly distributed pseudo-random numbers
Návod, kniha: Linux Programmer's Manual
#include <stdlib.h>
double drand48(void);
double erand48(unsigned short xsubi[3]);
long int lrand48(void);
long int nrand48(unsigned short xsubi[3]);
long int mrand48(void);
long int jrand48(unsigned short xsubi[3]);
void srand48(long int seedval);
unsigned short *seed48(unsigned short seed16v[3]);
void lcong48(unsigned short param[7]);
These functions generate pseudo-random numbers using the linear congruential
algorithm and 48-bit integer arithmetic.
The
drand48() and
erand48() functions return non-negative
double-precision floating-point values uniformly distributed between [0.0,
1.0).
The
lrand48() and
nrand48() functions return non-negative long
integers uniformly distributed between 0 and 2^31.
The
mrand48() and
jrand48() functions return signed long integers
uniformly distributed between -2^31 and 2^31.
The
srand48(),
seed48() and
lcong48() functions are
initialization functions, one of which should be called before using
drand48(),
lrand48() or
mrand48(). The functions
erand48(),
nrand48() and
jrand48() do not require an
initialization function to be called first.
All the functions work by generating a sequence of 48-bit integers,
Xi,
according to the linear congruential formula:
Xn+1 = (aXn + c) mod m, where n >= 0
The parameter
m = 2^48, hence 48-bit integer arithmetic is performed.
Unless
lcong48() is called,
a and
c are given by:
The value returned by any of the functions
drand48(),
erand48(),
lrand48(),
nrand48(),
mrand48() or
jrand48() is
computed by first generating the next 48-bit
Xi in the sequence. Then
the appropriate number of bits, according to the type of data item to be
returned, is copied from the high-order bits of
Xi and transformed into
the returned value.
The functions
drand48(),
lrand48() and
mrand48() store the
last 48-bit
Xi generated in an internal buffer. The functions
erand48(),
nrand48() and
jrand48() require the calling
program to provide storage for the successive
Xi values in the array
argument
xsubi. The functions are initialized by placing the initial
value of
Xi into the array before calling the function for the first
time.
The initializer function
srand48() sets the high order 32-bits of
Xi to the argument
seedval. The low order 16-bits are set to the
arbitrary value 0x330E.
The initializer function
seed48() sets the value of
Xi to the
48-bit value specified in the array argument
seed16v. The previous
value of
Xi is copied into an internal buffer and a pointer to this
buffer is returned by
seed48().
The initialization function
lcong48() allows the user to specify initial
values for
Xi,
a and
c. Array argument elements
param[0-2] specify
Xi,
param[3-5] specify
a, and
param[6] specifies
c. After
lcong48() has been called, a
subsequent call to either
srand48() or
seed48() will restore the
standard values of
a and
c.
SVID 3
These functions are declared obsolete by SVID 3, which states that rand(3)
should be used instead.
rand(3),
random(3)
