AT&T UNIX™PC Unix System V Users Manual - tenox
AT&T UNIX™PC Unix System V Users Manual - tenox AT&T UNIX™PC Unix System V Users Manual - tenox
DRAND48 ( 3C ) DRAND48 ( 3C ) a = 5DEECE66D 16 = 273673163155 8 c = B 16 = 13 8. The value returned by any of the functions drand48, erand48, lrand48, nrand48, mrand48 or jrand48 is computed by first generating the next 48-bit � 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 of � and transformed into the returned value. The functions drand48, lrand48 and mrand48 store the last 48-bit xi generated in an internal buffer; that is why they must be initialized prior to being invoked. The functions erand48, nrand48 and jrand48 require the calling program to provide storage for the successive � 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 of � into the array and pass it as an argument. By using different arguments, functions erand48, nrand48 and jrand48 allow separate modules of a large program to generate several independent streams of pseudo-random numbers, i.e., the sequence of numbers in each stream will not depend upon how many times the routines have been called to generate numbers for the other streams. The initializer function srand48 sets the high-order 32 bits of Xi to the 32 bits contained in its argument. The low-order 16 bits of � are set to the arbitrary value 330E16• 'l The initializer function seed48 sets the value of � to the 48-bit value specified in the argument array. In addition, the previous value of � is copied into a 48-bit internal buffer, used only by seed48, and a pointer to this buffer is the value returned by seed48. 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 last Xi value, and then use this value to reinitialize via seed48 when the program is restarted. The initialization function lcong48 allows the user to specify the initial xi ' the multiplier value a ' and the addend value c. Argument array elements param(0-2/ specify � , param(9-5J specify the multiplier a , and param(6j specifies the 16-bit addend c . After lcong48 has been called, a subsequent call to either srand48 or seed48 will restore the "standard" multiplier and addend values, a and c , specified on the previous page. NOTES The versions of these routines for the VAX- 11 and PDP-11 are coded in assembly language for maximum speed. It requires approximately 80 J.lSec on a VAX- 11/780 and 130 J.lsec on a PDP- 11/70 to generate one pseudo-random number. On other computers, the routines are coded in portable C. The source code for the portable version can even be used on computers which do not - 2-
DRAND48 ( 3C ) DRAND48 ( 3C ) have floating-point arithmetic. In such a situation, functions drand48 and erand48 do not exist; instead, they are replaced by the two new functions below. long irand48 (m) unsigned short m; long krand48 (xsubi, m) unsigned short xsubi[3], m; Functions irand48 and krand48 return non-negative long integers uniformly distributed over the interval [0, m -1]. SEE ALSO rand(3C). - 3-
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DRAND48 ( 3C ) DRAND48 ( 3C )<br />
have floating-point arithmetic. In such a situation, functions<br />
drand48 and erand48 do not exist; instead, they are replaced by<br />
the two new functions below.<br />
long irand48 (m)<br />
unsigned short m;<br />
long krand48 (xsubi, m)<br />
unsigned short xsubi[3], m;<br />
Functions irand48 and krand48 return non-negative long integers<br />
uniformly distributed over the interval [0, m -1].<br />
SEE ALSO<br />
rand(3C).<br />
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