| .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) |
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| .\" References consulted: |
| .\" Linux libc source code |
| .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) |
| .\" 386BSD man pages |
| .\" Modified Sun Mar 28 00:25:51 1993, David Metcalfe |
| .\" 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 2016-03-15 "GNU" "Linux Programmer's Manual" |
| .SH NAME |
| random, srandom, initstate, setstate \- random number generator |
| .SH SYNOPSIS |
| .nf |
| .B #include <stdlib.h> |
| .sp |
| .B long int random(void); |
| |
| .BI "void srandom(unsigned int " seed ); |
| |
| .BI "char *initstate(unsigned int " seed ", char *" state ", size_t " n ); |
| .br |
| .BI "char *setstate(char *" state ); |
| .fi |
| .sp |
| .in -4n |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .in |
| .sp |
| .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 |
| .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. |
| The period of this random number generator is very large, approximately |
| .IR "16\ *\ ((2^31)\ \-\ 1)" . |
| .PP |
| 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 |
| .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. |
| \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 |
| .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. |
| |
| The |
| .BR initstate () |
| function returns a pointer to the previous state array. |
| On error, |
| .I errno |
| is set to indicate the cause. |
| |
| 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 |
| 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. |
| .PP |
| This function should not be used in cases where multiple threads use |
| .BR random () |
| and the behavior should be reproducible. |
| 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 drand48 (3), |
| .BR getrandom (2), |
| .BR rand (3), |
| .BR random_r (3), |
| .BR srand (3) |