#include <stdlib.h>double drand48(void);
double erand48(unsigned short xsubi[3]);
long lrand48(void);
long nrand48(unsigned short xsubi[3]);
long mrand48(void);
long jrand48(unsigned short xsubi[3]);
void srand48(long seedval);
unsigned short
seed48(unsigned short seed16v[3]);
void lcong48(unsigned short param[7]);
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 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 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, X[i], according to the linear congruential formula
X[n+1] = (aX[n] + c)[mod m] n
0.
The parameter m=2[48]; 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 = 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 X[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 of X[i] and transformed into the returned value.
The functions drand48, lrand48, and mrand48 store the last 48-bit X[i] generated in an internal buffer. X[i] must be initialized prior to being invoked. The functions erand48, nrand48, and jrand48 require the calling program to provide storage for the successive X[i] 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 X[i] 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 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 X[i] to the 32 bits contained in its argument. The low-order 16 bits of X[i] are set to the arbitrary value 330E[16].
The initializer function seed48 sets the value of X[i] to the 48-bit value specified in the argument array. In addition, the previous value of X[i] 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 X[i] 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 X[i], the multiplier value a, and the addend value c. Argument array elements param[0-2] specify X[i], 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 will restore the ``standard'' multiplier and addend values, a and c, specified on the previous page.