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.
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.
If initstate() is called with size less than 8, it shall return NULL.
The random() function shall return the generated pseudo-random
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
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.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) In the event of any discrepancy between this version and
the original IEEE and The Open Group Standard, the original IEEE and
The Open Group Standard is the referee document. The original
Standard can be obtained online at http://www.unix.org/online.html .
Any typographical or formatting errors that appear in this page are
most likely to have been introduced during the conversion of the
source files to man page format. To report such errors, see
IEEE/The Open Group 2013 INITSTATE(3P)