drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, lcong48 - generate uniformly distributed pseudo-random numbers

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

This family of functions generates pseudo-random numbers using the well-known linear congruential algorithm and 48- bit integer arithmetic. Functions drand48() and erand48() return non-negative double-precision floating-point values uniformly distributed over the interval [0.0, 1.0). Functions lrand48() and nrand48() return non-negative long integers uniformly distributed over the interval [0, 2**31]. Functions mrand48() and jrand48() return signed long integers uniformly distributed over the interval [-2**31 , 2 **31 ]. Functions srand48(), seed48(), and lcong48() are initializa- tion entry points, one of which should be invoked before either drand48(), lrand48(), or mrand48() is called. (Although it is not recommended practice, constant default initializer values will be supplied automatically if drand48(), lrand48(), or mrand48() is called without a prior call to an initialization entry point.) Functions erand48(), nrand48(), and jrand48() do not require an initialization entry point to be called first. All the routines work by generating a sequence of 48-bit integer values, Xi , according to the linear congruential formula X n+1= (aX n+c) mod m n>=0. The parameterm= 2**48; hence 48-bit integer arithmetic is performed. Unless lcong48() has been invoked, the multiplier valueaand the addend valuecare given bya= 5DEECE66D16 = 2736731631558c= B16 = 138 . The value returned by any of the functions drand48(), erand48(), lrand48(), nrand48(), mrand48(), or jrand48() is computed by first generating the next 48-bit Xi 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 Xi and transformed into the returned value. The functions drand48(), lrand48(), and mrand48() store the last 48-bit Xi generated in an internal buffer. Xi must be initialized prior to being invoked. The functions erand48(), nrand48(), and jrand48() require the calling program to pro- vide storage for the successive Xi values in the array specified as an argument when the functions are invoked. These routines do not have to be initialized; the calling program must place the desired initial value of Xi into the array and pass it as an argument. By using different argu- ments, functions erand48(), nrand48(), and jrand48() allow separate modules of a large program to generate severalindependentstreams of pseudo-random numbers, that is, the sequence of numbers in each stream willnotdepend 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 Xi are set to the arbitrary value 330E16 . The initializer function seed48() sets the value of Xi to the 48-bit value specified in the argument array. In addi- tion, the previous value of Xi 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 seed48() when the program is restarted. The initialization function lcong48() allows the user to specify the initial Xi the multiplier valuea, and the addend valuec. Argument array elementsparam[0-2] specify Xi,param[3-5] specify the multipliera, andparam[6] speci- fies the 16-bit addendc. After lcong48() has been called, a subsequent call to either srand48() or seed48() will restore the ``standard'' multiplier and addend values,aandc, specified above.

Seeattributes(5)for descriptions of the following attri- butes: ____________________________________________________________ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | |_____________________________|_____________________________|| MT-Level | Safe | |_____________________________|_____________________________|

rand(3C),attributes(5)

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