.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
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.\" References consulted:
.\" Linux libc source code
.\" Modified Sat Jul 24 18:13:39 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sun Aug 20 21:47:07 2000, aeb
.\"
-.TH RANDOM 3 2000-08-20 "GNU" "Linux Programmer's Manual"
+.TH RANDOM 3 2019-03-06 "GNU" "Linux Programmer's Manual"
.SH NAME
random, srandom, initstate, setstate \- random number generator
.SH SYNOPSIS
.nf
.B #include <stdlib.h>
-.sp
+.PP
.B long int random(void);
-.nl
+.PP
.BI "void srandom(unsigned int " seed );
-.nl
+.PP
.BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n );
-.nl
+.PP
.BI "char *setstate(char *" state );
.fi
+.PP
+.in -4n
+Feature Test Macro Requirements for glibc (see
+.BR feature_test_macros (7)):
+.in
+.PP
+.ad l
+.BR random (),
+.BR srandom (),
+.BR initstate (),
+.BR setstate ():
+.RS 4
+_XOPEN_SOURCE\ >=\ 500
+.\" || _XOPEN_SOURCE\ &&\ _XOPEN_SOURCE_EXTENDED
+ || /* Glibc since 2.19: */ _DEFAULT_SOURCE
+ || /* Glibc versions <= 2.19: */ _SVID_SOURCE || _BSD_SOURCE
+.RE
+.ad
.SH DESCRIPTION
-The \fBrandom()\fP function uses a non-linear additive feedback random
+The
+.BR random ()
+function uses a nonlinear additive feedback random
number generator employing a default table of size 31 long integers to
-return successive pseudo-random numbers in the range from 0 to \fBRAND_MAX\fR.
+return successive pseudo-random numbers in
+the range from 0 to \fBRAND_MAX\fR.
The period of this random number generator is very large, approximately
-16*((2**31)\-1).
+.IR "16\ *\ ((2^31)\ \-\ 1)" .
.PP
-The \fBsrandom()\fP function sets its argument as the seed for a new
-sequence of pseudo-random integers to be returned by \fBrandom()\fP.
-These sequences are repeatable by calling \fBsrandom()\fP with the same
-seed value. If no seed value is provided, the \fBrandom()\fP function
+The
+.BR srandom ()
+function sets its argument as the seed for a new
+sequence of pseudo-random integers to be returned by
+.BR random ().
+These sequences are repeatable by calling
+.BR srandom ()
+with the same
+seed value.
+If no seed value is provided, the
+.BR random ()
+function
is automatically seeded with a value of 1.
.PP
-The \fBinitstate()\fP function allows a state array \fIstate\fP to
-be initialized for use by \fBrandom()\fP. The size of the state array
-\fIn\fP is used by \fBinitstate()\fP to decide how sophisticated a
-random number generator it should use \(em the larger the state array,
-the better the random numbers will be. \fIseed\fP is the seed for the
+The
+.BR initstate ()
+function allows a state array \fIstate\fP to
+be initialized for use by
+.BR random ().
+The size of the state array
+\fIn\fP is used by
+.BR initstate ()
+to decide how sophisticated a
+random number generator it should use\(emthe larger the state array,
+the better the random numbers will be.
+Current "optimal" values for the size of the state array \fIn\fP are
+8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
+the nearest known amount.
+Using less than 8 bytes results in an error.
+\fIseed\fP is the seed for the
initialization, which specifies a starting point for the random number
sequence, and provides for restarting at the same point.
.PP
-The \fBsetstate()\fP function changes the state array used by the
-\fBrandom()\fP function. The state array \fIstate\fP is used for
-random number generation until the next call to \fBinitstate()\fP
-or \fBsetstate()\fP. \fIstate\fP must first have been initialized
-using \fBinitstate()\fP or be the result of a previous call of
-\fBsetstate()\fP.
-.SH "RETURN VALUE"
-The \fBrandom()\fP function returns a value between 0 and RAND_MAX.
-The \fBsrandom()\fP function returns no value. The \fBinitstate()\fP
-and \fBsetstate()\fP functions return a pointer to the previous state
-array, or NULL on error.
+The
+.BR setstate ()
+function changes the state array used by the
+.BR random ()
+function.
+The state array \fIstate\fP is used for
+random number generation until the next call to
+.BR initstate ()
+or
+.BR setstate ().
+\fIstate\fP must first have been initialized
+using
+.BR initstate ()
+or be the result of a previous call of
+.BR setstate ().
+.SH RETURN VALUE
+The
+.BR random ()
+function returns a value between 0 and
+.BR RAND_MAX .
+The
+.BR srandom ()
+function returns no value.
+.PP
+The
+.BR initstate ()
+function returns a pointer to the previous state array.
+On error,
+.I errno
+is set to indicate the cause.
+.PP
+On success,
+.BR setstate ()
+returns a pointer to the previous state array.
+On error, it returns NULL, with
+.I errno
+set to indicate the cause of the error.
.SH ERRORS
.TP
.B EINVAL
-A state array of less than 8 bytes was specified to \fBinitstate()\fP.
+The
+.I state
+argument given to
+.BR setstate ()
+was NULL.
+.TP
+.B EINVAL
+A state array of less than 8 bytes was specified to
+.BR initstate ().
+.SH ATTRIBUTES
+For an explanation of the terms used in this section, see
+.BR attributes (7).
+.TS
+allbox;
+lbw23 lb lb
+l l l.
+Interface Attribute Value
+T{
+.BR random (),
+.BR srandom (),
+.br
+.BR initstate (),
+.BR setstate ()
+T} Thread safety MT-Safe
+.TE
+.SH CONFORMING TO
+POSIX.1-2001, POSIX.1-2008, 4.3BSD.
.SH NOTES
-Current "optimal" values for the size of the state array \fIn\fP are
-8, 32, 64, 128, and 256 bytes; other amounts will be rounded down to
-the nearest known amount. Using less than 8 bytes will cause an
-error.
-.SH "CONFORMING TO"
-BSD 4.3
-.SH "SEE ALSO"
+The
+.BR random ()
+function should not be used in multithreaded programs
+where reproducible behavior is required.
+Use
+.BR random_r (3)
+for that purpose.
+.PP
+Random-number generation is a complex topic.
+.I Numerical Recipes in C: The Art of Scientific Computing
+(William H.\& Press, Brian P.\& Flannery, Saul A.\& Teukolsky, William
+T.\& Vetterling; New York: Cambridge University Press, 2007, 3rd ed.)
+provides an excellent discussion of practical random-number generation
+issues in Chapter 7 (Random Numbers).
+.PP
+For a more theoretical discussion which also covers many practical issues
+in depth, see Chapter 3 (Random Numbers) in Donald E.\& Knuth's
+.IR "The Art of Computer Programming" ,
+volume 2 (Seminumerical Algorithms), 2nd ed.; Reading, Massachusetts:
+Addison-Wesley Publishing Company, 1981.
+.SH BUGS
+According to POSIX,
+.BR initstate ()
+should return NULL on error.
+In the glibc implementation,
+.I errno
+is (as specified) set on error, but the function does not return NULL.
+.\" http://sourceware.org/bugzilla/show_bug.cgi?id=15380
+.SH SEE ALSO
+.BR getrandom (2),
+.BR drand48 (3),
.BR rand (3),
+.BR random_r (3),
.BR srand (3)
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