случайные числа (random numbers)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
drand48, erand48, jrand48, lcong48, lrand48, mrand48, nrand48,
seed48, srand48 — generate uniformly distributed pseudo-random
numbers
Синопсис (Synopsis)
#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);
Описание (Description)
This family of functions shall generate pseudo-random numbers
using a linear congruential algorithm and 48-bit integer
arithmetic.
The drand48() and erand48() functions shall return non-negative,
double-precision, floating-point values, uniformly distributed
over the interval [0.0,1.0).
The lrand48() and nrand48() functions shall return non-negative,
long integers, uniformly distributed over the interval [0,231).
The mrand48() and jrand48() functions shall return signed long
integers uniformly distributed over the interval [-231,231).
The srand48(), seed48(), and lcong48() functions are
initialization entry points, one of which should be invoked
before either drand48(), lrand48(), or mrand48() is called.
(Although it is not recommended practice, constant default
initializer values shall be supplied automatically if drand48(),
lrand48(), or mrand48() is called without a prior call to an
initialization entry point.) The erand48(), nrand48(), and
jrand48() functions do not require an initialization entry point
to be called first.
All the routines work by generating a sequence of 48-bit integer
values, X_i , according to the linear congruential formula:
Xn+1 = (aX_n +c)mod m n≥ 0
The parameter m=2^48; hence 48-bit integer arithmetic is
performed. Unless lcong48() is invoked, the multiplier value a
and the addend value c are given by:
a = 5DEECE66D16 = 2736731631558
c = B16 = 138
The value returned by any of the drand48(), erand48(), jrand48(),
lrand48(), mrand48(), or nrand48() functions is computed by first
generating the next 48-bit X_i in the sequence. Then the
appropriate number of bits, according to the type of data item to
be returned, are copied from the high-order (leftmost) bits of
X_i and transformed into the returned value.
The drand48(), lrand48(), and mrand48() functions store the last
48-bit X_i generated in an internal buffer; that is why the
application shall ensure that these are initialized prior to
being invoked. The erand48(), nrand48(), and jrand48() functions
require the calling program to provide storage for the successive
X_i values in the array specified as an argument when the
functions are invoked. That is why these routines do not have to
be initialized; the calling program merely has to place the
desired initial value of X_i into the array and pass it as an
argument. By using different arguments, erand48(), nrand48(),
and jrand48() allow separate modules of a large program to
generate several independent streams of pseudo-random numbers;
that is, the sequence of numbers in each stream shall not depend
upon how many times the routines are called to generate numbers
for the other streams.
The initializer function srand48() sets the high-order 32 bits of
X_i to the low-order 32 bits contained in its argument. The low-
order 16 bits of X_i are set to the arbitrary value 330E_16 .
The initializer function seed48() sets the value of X_i to the
48-bit value specified in the argument array. The low-order 16
bits of X_i are set to the low-order 16 bits of seed16v[0]. The
mid-order 16 bits of X_i are set to the low-order 16 bits of
seed16v[1]. The high-order 16 bits of X_i are set to the low-
order 16 bits of seed16v[2]. In addition, the previous value of
X_i is copied into a 48-bit internal buffer, used only by
seed48(), and a pointer to this buffer is the value returned by
seed48(). This returned pointer, which can just be ignored if
not needed, is useful if a program is to be restarted from a
given point at some future time—use the pointer to get at and
store the last X_i value, and then use this value to reinitialize
via seed48() when the program is restarted.
The initializer function lcong48() allows the user to specify the
initial X_i , the multiplier value a, and the addend value c.
Argument array elements param[0-2] specify X_i , param[3-5]
specify the multiplier a, and param[6] specifies the 16-bit
addend c. After lcong48() is called, a subsequent call to either
srand48() or seed48() shall restore the standard multiplier and
addend values, a and c, specified above.
The drand48(), lrand48(), and mrand48() functions need not be
thread-safe.
Возвращаемое значение (Return value)
As described in the DESCRIPTION above.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
These functions should be avoided whenever non-trivial
requirements (including safety) have to be fulfilled.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
initstate(3p), rand(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)
