drand48 (original) (raw)

This family of functions shall generate pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic.

The drand48() and erand48() functions shall return non-negative, double-precision, floating-point values, uniformly distributed over the interval [0.0,1.0).

The lrand48() and nrand48() functions shall return non-negative, long integers, uniformly distributed over the interval [0,231).

The mrand48() and jrand48() functions shall return signed long integers uniformly distributed over the interval [-231,231).

The srand48(), seed48(), and lcong48() functions 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 shall be supplied automatically if drand48(), lrand48(), or mrand48() is called without a prior call to an initialization entry point.) The erand48(), nrand48(), and jrand48() functions 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() is 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 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 data item 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; that is why the application shall ensure that these are 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. That is why these routines 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; that is, the sequence of numbers in each stream shall _not_depend upon 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 low-order 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. The low-order 16 bits of Xi are set to the low-order 16 bits of _seed16v_[_0_]. The mid-order 16 bits of Xi are set to the low-order 16 bits of _seed16v_[_1_]. The high-order 16 bits of Xi are set to the low-order 16 bits of_seed16v_[2_]. 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 initializer 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 Xi , _param_[_3-5_] 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() shall restore the standard multiplier and addend values, a and _c,_specified above.

The drand48(), lrand48(), and mrand48() functions need not be reentrant. A function that is not required to be reentrant is not required to be thread-safe.

The following sections are informative.

End of informative text.