функции случайных чисел (random number functions)
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)
initstate, random, setstate, srandom — pseudo-random number
functions
Синопсис (Synopsis)
#include <stdlib.h>
char *initstate(unsigned seed, char *state, size_t size);
long random(void);
char *setstate(char *state);
void srandom(unsigned seed);
Описание (Description)
The random() function shall use a non-linear additive feedback
random-number generator employing a default state array size of
31 long integers to return successive pseudo-random numbers in
the range from 0 to 231-1. The period of this random-number
generator is approximately 16 x (231-1). The size of the state
array determines the period of the random-number generator.
Increasing the state array size shall increase the period.
With 256 bytes of state information, the period of the random-
number generator shall be greater than 269.
Like rand(), random() shall produce by default a sequence of
numbers that can be duplicated by calling srandom() with 1 as the
seed.
The srandom() function shall initialize the current state array
using the value of seed.
The initstate() and setstate() functions handle restarting and
changing random-number generators. The initstate() function
allows a state array, pointed to by the state argument, to be
initialized for future use. The size argument, which specifies
the size in bytes of the state array, shall be used by
initstate() to decide what type of random-number generator to
use; the larger the state array, the more random the numbers.
Values for the amount of state information are 8, 32, 64, 128,
and 256 bytes. Other values greater than 8 bytes are rounded down
to the nearest one of these values. If initstate() is called with
8≤size<32, then random() shall use a simple linear congruential
random number generator. The seed argument specifies a starting
point for the random-number sequence and provides for restarting
at the same point. The initstate() function shall return a
pointer to the previous state information array.
If initstate() has not been called, then random() shall behave as
though initstate() had been called with seed=1 and size=128.
Once a state has been initialized, setstate() allows switching
between state arrays. The array defined by the state argument
shall be used for further random-number generation until
initstate() is called or setstate() is called again. The
setstate() function shall return a pointer to the previous state
array.
Возвращаемое значение (Return value)
If initstate() is called with size less than 8, it shall return
NULL.
The random() function shall return the generated pseudo-random
number.
The srandom() function shall not return a value.
Upon successful completion, initstate() and setstate() shall
return a pointer to the previous state array; otherwise, a null
pointer shall be returned.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
After initialization, a state array can be restarted at a
different point in one of two ways:
1. The initstate() function can be used, with the desired seed,
state array, and size of the array.
2. The setstate() function, with the desired state, can be used,
followed by srandom() with the desired seed. The advantage of
using both of these functions is that the size of the state
array does not have to be saved once it is initialized.
Although some implementations of random() have written messages
to standard error, such implementations do not conform to
POSIX.1‐2008.
Issue 5 restored the historical behavior of this function.
Threaded applications should use erand48(), nrand48(), or
jrand48() instead of random() when an independent random number
sequence in multiple threads is required.
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)
drand48(3p), rand(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)
