генератор случайных чисел (random number generator)
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)
rand, rand_r, srand — pseudo-random number generator
Синопсис (Synopsis)
#include <stdlib.h>
int rand(void);
int rand_r(unsigned *seed);
void srand(unsigned seed);
Описание (Description)
For rand() and srand(): The functionality described on this
reference page is aligned with the ISO C standard. Any conflict
between the requirements described here and the ISO C standard is
unintentional. This volume of POSIX.1‐2017 defers to the ISO C
standard.
The rand() function shall compute a sequence of pseudo-random
integers in the range [0,{RAND_MAX}] with a period of at least
232.
The rand() function need not be thread-safe.
The rand_r() function shall compute a sequence of pseudo-random
integers in the range [0,{RAND_MAX}]. (The value of the
{RAND_MAX} macro shall be at least 32767.)
If rand_r() is called with the same initial value for the object
pointed to by seed and that object is not modified between
successive returns and calls to rand_r(), the same sequence shall
be generated.
The srand() function uses the argument as a seed for a new
sequence of pseudo-random numbers to be returned by subsequent
calls to rand(). If srand() is then called with the same seed
value, the sequence of pseudo-random numbers shall be repeated.
If rand() is called before any calls to srand() are made, the
same sequence shall be generated as when srand() is first called
with a seed value of 1.
The implementation shall behave as if no function defined in this
volume of POSIX.1‐2017 calls rand() or srand().
Возвращаемое значение (Return value)
The rand() function shall return the next pseudo-random number in
the sequence.
The rand_r() function shall return a pseudo-random integer.
The srand() function shall not return a value.
Ошибки (Error)
No errors are defined.
The following sections are informative.
Примеры (Examples)
Generating a Pseudo-Random Number Sequence
The following example demonstrates how to generate a sequence of
pseudo-random numbers.
#include <stdio.h>
#include <stdlib.h>
...
long count, i;
char *keystr;
int elementlen, len;
char c;
...
/* Initial random number generator. */
srand(1);
/* Create keys using only lowercase characters */
len = 0;
for (i=0; i<count; i++) {
while (len < elementlen) {
c = (char) (rand() % 128);
if (islower(c))
keystr[len++] = c;
}
keystr[len] = '\0';
printf("%s Element%0*ld\n", keystr, elementlen, i);
len = 0;
}
Generating the Same Sequence on Different Machines
The following code defines a pair of functions that could be
incorporated into applications wishing to ensure that the same
sequence of numbers is generated across different machines.
static unsigned long next = 1;
int myrand(void) /* RAND_MAX assumed to be 32767. */
{
next = next * 1103515245 + 12345;
return((unsigned)(next/65536) % 32768);
}
void mysrand(unsigned seed)
{
next = seed;
}
Использование в приложениях (Application usage)
The drand48() and random() functions provide much more elaborate
pseudo-random number generators.
The limitations on the amount of state that can be carried
between one function call and another mean the rand_r() function
can never be implemented in a way which satisfies all of the
requirements on a pseudo-random number generator.
These functions should be avoided whenever non-trivial
requirements (including safety) have to be fulfilled.
Обоснование (Rationale)
The ISO C standard rand() and srand() functions allow per-process
pseudo-random streams shared by all threads. Those two functions
need not change, but there has to be mutual-exclusion that
prevents interference between two threads concurrently accessing
the random number generator.
With regard to rand(), there are two different behaviors that may
be wanted in a multi-threaded program:
1. A single per-process sequence of pseudo-random numbers that
is shared by all threads that call rand()
2. A different sequence of pseudo-random numbers for each thread
that calls rand()
This is provided by the modified thread-safe function based on
whether the seed value is global to the entire process or local
to each thread.
This does not address the known deficiencies of the rand()
function implementations, which have been approached by
maintaining more state. In effect, this specifies new thread-safe
forms of a deficient function.
Будущие направления (Future directions)
The rand_r() function may be removed in a future version.
Смотри также (See also)
drand48(3p), initstate(3p)
The Base Definitions volume of POSIX.1‐2017, stdlib.h(0p)
