drand48(S) 6 January 1993 drand48(S) Name drand48, erand48, jrand48, lcong48, lrand48, nrand48, mrand48, srand48, seed48 - generate uniformly distributed pseudo-random numbers Syntax cc . . . -lc double drand48 () double erand48 (xsubi) unsigned short xsubi[3]; long jrand48 (xsubi) unsigned short xsubi[3]; void lcong48 (param) unsigned short param[7]; long lrand48 () long nrand48 (xsubi) unsigned short xsubi[3]; long mrand48 () void srand48 (seedval) long seedval; unsigned short *seed48 (seed16v) unsigned short seed16v[3]; Description 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 dis- tributed over the interval [-2^31, 2^31). Functions srand48, seed48, and lcong48 are initialization 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 are 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 Xn+1 = (aXn+c)mod m n>=0. The parameter m=248; hence 48-bit integer arithmetic is performed. Unless lcong48 has been invoked, the multiplier value a and the addend value c are given by a = 5DEECE66D16 = 2736731631558 c = 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, accord- ing 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 gen- erated in an internal buffer, and must be initialized prior to being invoked. The functions erand48, nrand48, and jrand48 require the calling program to provide 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 arguments, functions erand48, nrand48, and jrand48 allow separate modules of a large program to generate several independent streams of pseudo-random numbers; that is, the sequence of numbers in each stream does 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 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 addition, 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 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 ini- tial Xi, the multiplier value a, and the addend value c. Argument array elements param[0-2] specify Xi, param[3-5] specify the multiplier a, and param[6] specifies the 16-bit addend c. After lcong48 has been called, a subsequent call to either srand48 or seed48 restores the ``standard'' multiplier and addend values, a and c, specified on the previous page. Notes The source code for the portable version can be used on computers which do not have floating-point arithmetic. In such a situation, functions drand48 and erand48 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(S) Standards conformance drand48, erand48, jrand48, lrand48, mrand48, nrand48, seed48 and srand48 are conformant with: AT&T SVID Issue 2; and X/Open Portability Guide, Issue 3, 1989. lcong48 is conformant with: AT&T SVID Issue 2.