This family of functions generates pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic. drand48() and erand48() return non-negative, double-precision, floating-point values, uniformly distributed over the interval [0.0 , 1.0].
lrand48() and nrand48() return non-negative, long integers, uniformly distributed over the interval [0, 231].
mrand48() and jrand48() return signed long integers uniformly distributed over the interval [-231, 231].
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, drand48(), lrand48() or mrand48() can be invoked without a prior call to an initialization entry point, since default initializer values are supplied automatically in such cases.
erand48(), nrand48() and rand48() 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 = 13 8 The value returned by any of the drand48(), erand48(), jrand48(), lrand48(), mrand48() or nrand48() functions is computed by first generating the next 48-bit Xi in the sequence. Then the appropriate number of bits, according to the type of variable to be returned, are copied from the high-order (leftmost) bits of Xi and transformed into the returned value. The drand48(), lrand48() and mrand48() functions store the last 48-bit Xi generated in an internal buffer and must therefore be initialized prior to being invoked. The erand48(), nrand48() and jrand48() functions require the calling program to provide storage for the successive Xi values in the array specified as an argument when the functions are invoked.
Consequently, these functions do not have to be initialized; the calling program merely has to place the desired initial value of Xi into the array and pass it as an argument. By using different arguments, 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 on how many times the routines are called to generate numbers for the other streams. The initializer function srand48() sets the high-order 32 bits of Xi to the value of the {LONG_BIT} 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; the pointer can be used to get at and store the last Xi value, and this value can then be used to reinitialize via seed48() when the program is restarted. The initializer function lcong48() allows the user to specify default values for Xi, the multiplier value a, and the addend value c. Argument array elements param[0] to param[2] specify Xi, param[3] to param[5] specify the multiplier a, and param[6] specifies the 16-bit addend c. After lcong48() is called, a subsequent call to either srand48() or seed48() will restore the "standard" multiplier and addend values, a and c, specified above. | 