These functions generate pseudo-random numbers using the linear congruential algorithm and 48-bit integer arithmetic.
The drand48() and erand48() functions return non-negative double-precision floating-point values uniformly distributed between [0.0, 1.0).
The lrand48() and nrand48() functions return non-negative long integers uniformly distributed between 0 and 2^31.
The mrand48() and jrand48() functions return signed long integers uniformly distributed between -2^31 and 2^31.
The srand48(), seed48() and lcong48() functions are initialization functions, one of which should be called before using drand48(), lrand48() or mrand48(). The functions erand48(), nrand48() and jrand48() do not require an initialization function to be called first.
All the functions work by generating a sequence of 48-bit integers, Xi, according to the linear congruential formula:
Xn+1 = (aXn + c) mod m, where n >= 0
The parameter m = 2^48, hence 48-bit integer arithmetic is performed. Unless lcong48() is called, a and c are given by:
a = 0x5DEECE66D
c = 0xB
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, is copied from the high-order 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. The functions erand48(), nrand48() and jrand48() require the calling program to provide storage for the successive Xi values in the array argument xsubi. The functions are initialized by placing the initial value of Xi into the array before calling the function for the first time.
The initializer function srand48() sets the high order 32-bits of Xi to the argument seedval. The low order 16-bits are set to the arbitrary value 0x330E.
The initializer function seed48() sets the value of Xi to the 48-bit value specified in the array argument seed16v. The previous value of Xi is copied into an internal buffer and a pointer to this buffer is returned by seed48().
The initialization function lcong48() allows the user to specify initial values for Xi, a and c. Array argument elements param[0-2] specify Xi, param[3-5] specify a, and param specifies c. After lcong48() has been called, a subsequent call to either srand48() or seed48() will restore the standard values of a and c.
These functions are declared obsolete by SVID 3, which states that rand(3) should be used instead.