| .\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk) |
| .\" |
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| .\" Since the Linux kernel and libraries are constantly changing, this |
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| .\" References consulted: |
| .\" Linux libc source code |
| .\" Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991) |
| .\" 386BSD man pages |
| .\" Modified Sat Jul 24 19:46:03 1993 by Rik Faith (faith@cs.unc.edu) |
| .TH DRAND48 3 2021-03-22 "" "Linux Programmer's Manual" |
| .SH NAME |
| drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, |
| lcong48 \- generate uniformly distributed pseudo-random numbers |
| .SH SYNOPSIS |
| .nf |
| .B #include <stdlib.h> |
| .PP |
| .B double drand48(void); |
| .BI "double erand48(unsigned short " xsubi [3]); |
| .PP |
| .B long lrand48(void); |
| .BI "long nrand48(unsigned short " xsubi [3]); |
| .PP |
| .B long mrand48(void); |
| .BI "long jrand48(unsigned short " xsubi [3]); |
| .PP |
| .BI "void srand48(long " seedval ); |
| .BI "unsigned short *seed48(unsigned short " seed16v [3]); |
| .BI "void lcong48(unsigned short " param [7]); |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| All functions shown above: |
| .\" .BR drand48 (), |
| .\" .BR erand48 (), |
| .\" .BR lrand48 (), |
| .\" .BR nrand48 (), |
| .\" .BR mrand48 (), |
| .\" .BR jrand48 (), |
| .\" .BR srand48 (), |
| .\" .BR seed48 (), |
| .\" .BR lcong48 (): |
| .nf |
| _XOPEN_SOURCE |
| || /* Glibc since 2.19: */ _DEFAULT_SOURCE |
| || /* Glibc <= 2.19: */ _SVID_SOURCE |
| .fi |
| .SH DESCRIPTION |
| These functions generate pseudo-random numbers using the linear congruential |
| algorithm and 48-bit integer arithmetic. |
| .PP |
| The |
| .BR drand48 () |
| and |
| .BR erand48 () |
| functions return nonnegative |
| double-precision floating-point values uniformly distributed over the interval |
| [0.0,\ 1.0). |
| .PP |
| The |
| .BR lrand48 () |
| and |
| .BR nrand48 () |
| functions return nonnegative |
| long integers uniformly distributed over the interval [0,\ 2^31). |
| .PP |
| The |
| .BR mrand48 () |
| and |
| .BR jrand48 () |
| functions return signed long |
| integers uniformly distributed over the interval [\-2^31,\ 2^31). |
| .PP |
| The |
| .BR srand48 (), |
| .BR seed48 (), |
| and |
| .BR lcong48 () |
| functions are |
| initialization functions, one of which should be called before using |
| .BR drand48 (), |
| .BR lrand48 (), |
| or |
| .BR mrand48 (). |
| The functions |
| .BR erand48 (), |
| .BR nrand48 (), |
| and |
| .BR jrand48 () |
| do not require |
| an initialization function to be called first. |
| .PP |
| All the functions work by generating a sequence of 48-bit integers, |
| .IR Xi , |
| according to the linear congruential formula: |
| .PP |
| .in +4n |
| .EX |
| .B Xn+1 = (aXn + c) mod m, where n >= 0 |
| .EE |
| .in |
| .PP |
| The parameter |
| .I m |
| = 2^48, hence 48-bit integer arithmetic is performed. |
| Unless |
| .BR lcong48 () |
| is called, |
| .IR a |
| and |
| .I c |
| are given by: |
| .PP |
| .in +4n |
| .EX |
| .B a = 0x5DEECE66D |
| .B c = 0xB |
| .EE |
| .in |
| .PP |
| The value returned by any of the functions |
| .BR drand48 (), |
| .BR erand48 (), |
| .BR lrand48 (), |
| .BR nrand48 (), |
| .BR mrand48 (), |
| or |
| .BR jrand48 () |
| is |
| computed by first generating the next 48-bit |
| .I Xi |
| in the sequence. |
| Then the appropriate number of bits, according to the type of data item to |
| be returned, is copied from the high-order bits of |
| .I Xi |
| and transformed |
| into the returned value. |
| .PP |
| The functions |
| .BR drand48 (), |
| .BR lrand48 (), |
| and |
| .BR mrand48 () |
| store |
| the last 48-bit |
| .I Xi |
| generated in an internal buffer. |
| The functions |
| .BR erand48 (), |
| .BR nrand48 (), |
| and |
| .BR jrand48 () |
| require the calling |
| program to provide storage for the successive |
| .I Xi |
| values in the array |
| argument |
| .IR xsubi . |
| The functions are initialized by placing the initial |
| value of |
| .I Xi |
| into the array before calling the function for the first |
| time. |
| .PP |
| The initializer function |
| .BR srand48 () |
| sets the high order 32-bits of |
| .I Xi |
| to the argument |
| .IR seedval . |
| The low order 16-bits are set |
| to the arbitrary value 0x330E. |
| .PP |
| The initializer function |
| .BR seed48 () |
| sets the value of |
| .I Xi |
| to |
| the 48-bit value specified in the array argument |
| .IR seed16v . |
| The |
| previous value of |
| .I Xi |
| is copied into an internal buffer and a |
| pointer to this buffer is returned by |
| .BR seed48 (). |
| .PP |
| The initialization function |
| .BR lcong48 () |
| allows the user to specify |
| initial values for |
| .IR Xi , |
| .IR a , |
| and |
| .IR c . |
| Array argument |
| elements |
| .I param[0\-2] |
| specify |
| .IR Xi , |
| .I param[3\-5] |
| specify |
| .IR a , |
| and |
| .I param[6] |
| specifies |
| .IR c . |
| After |
| .BR lcong48 () |
| has been called, a subsequent call to either |
| .BR srand48 () |
| or |
| .BR seed48 () |
| will restore the standard values of |
| .I a |
| and |
| .IR c . |
| .SH ATTRIBUTES |
| For an explanation of the terms used in this section, see |
| .BR attributes (7). |
| .ad l |
| .nh |
| .TS |
| allbox; |
| lbx lb lb |
| l l l. |
| Interface Attribute Value |
| T{ |
| .BR drand48 (), |
| .BR erand48 (), |
| .BR lrand48 (), |
| .BR nrand48 (), |
| .BR mrand48 (), |
| .BR jrand48 (), |
| .BR srand48 (), |
| .BR seed48 (), |
| .BR lcong48 () |
| T} Thread safety T{ |
| MT-Unsafe race:drand48 |
| T} |
| .TE |
| .hy |
| .ad |
| .sp 1 |
| .PP |
| The above |
| functions record global state information for the random number generator, |
| so they are not thread-safe. |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008, SVr4. |
| .SH SEE ALSO |
| .BR rand (3), |
| .BR random (3) |