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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.

SYNOPSIS top

#include <stdlib.h> double drand48(void); double erand48(unsigned shortxsubi[3]); long jrand48(unsigned shortxsubi[3]); void lcong48(unsigned shortparam[7]); long lrand48(void); long mrand48(void); long nrand48(unsigned shortxsubi[3]); unsigned short *seed48(unsigned shortseed16v[3]); void srand48(longseedval);

DESCRIPTION top

This family of functions shall generate pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic. Thedrand48() anderand48() functions shall return non-negative, double-precision, floating-point values, uniformly distributed over the interval [0.0,1.0). Thelrand48() andnrand48() functions shall return non-negative, long integers, uniformly distributed over the interval [0,231). Themrand48() andjrand48() functions shall return signed long integers uniformly distributed over the interval [-231,231). Thesrand48(),seed48(), andlcong48() functions are initialization entry points, one of which should be invoked before eitherdrand48(),lrand48(), ormrand48() is called. (Although it is not recommended practice, constant default initializer values shall be supplied automatically ifdrand48(),lrand48(), ormrand48() is called without a prior call to an initialization entry point.) Theerand48(),nrand48(), andjrand48() functions do not require an initialization entry point to be called first. All the routines work by generating a sequence of 48-bit integer values,X_i , according to the linear congruential formula:Xn+1 = (aX_n +c)modmn≥ 0 The parameterm=2^48; hence 48-bit integer arithmetic is performed. Unlesslcong48() is invoked, the multiplier valuea and the addend valuec are given by:a = 5DEECE66D16 = 2736731631558c = B16 = 138 The value returned by any of thedrand48(),erand48(),jrand48(),lrand48(),mrand48(), ornrand48() functions is computed by first generating the next 48-bitX_i in the sequence. Then the appropriate number of bits, according to the type of data item to be returned, are copied from the high-order (leftmost) bits ofX_i and transformed into the returned value. Thedrand48(),lrand48(), andmrand48() functions store the last 48-bitX_i generated in an internal buffer; that is why the application shall ensure that these are initialized prior to being invoked. Theerand48(),nrand48(), andjrand48() functions require the calling program to provide storage for the successiveX_i values in the array specified as an argument when the functions are invoked. That is why these routines do not have to be initialized; the calling program merely has to place the desired initial value ofX_i into the array and pass it as an argument. By using different arguments,erand48(),nrand48(), andjrand48() allow separate modules of a large program to generate severalindependent streams of pseudo-random numbers; that is, the sequence of numbers in each stream shallnot depend upon how many times the routines are called to generate numbers for the other streams. The initializer functionsrand48() sets the high-order 32 bits ofX_i to the low-order 32 bits contained in its argument. The low- order 16 bits ofX_i are set to the arbitrary value 330E_16 . The initializer functionseed48() sets the value ofX_i to the 48-bit value specified in the argument array. The low-order 16 bits ofX_i are set to the low-order 16 bits ofseed16v[0]. The mid-order 16 bits ofX_i are set to the low-order 16 bits ofseed16v[1]. The high-order 16 bits ofX_i are set to the low- order 16 bits ofseed16v[2]. In addition, the previous value ofX_i is copied into a 48-bit internal buffer, used only byseed48(), and a pointer to this buffer is the value returned byseed48(). This returned pointer, which can just be ignored if not needed, is useful if a program is to be restarted from a given point at some future time—use the pointer to get at and store the lastX_i value, and then use this value to reinitialize viaseed48() when the program is restarted. The initializer functionlcong48() allows the user to specify the initialX_i , the multiplier valuea, and the addend valuec. Argument array elementsparam[0-2] specifyX_i ,param[3-5] specify the multipliera, andparam[6] specifies the 16-bit addendc. Afterlcong48() is called, a subsequent call to eithersrand48() orseed48() shall restore the standard multiplier and addend values,a andc, specified above. Thedrand48(),lrand48(), andmrand48() functions need not be thread-safe.

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. 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 athttp://www.opengroup.org/unix/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, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017DRAND48(3P)