Generate uniformly distributed pseudo-random numbers

#include <stdlib.h>double drand48(void);double erand48(unsigned shortxsubi[3]);long int lrand48(void);long int nrand48(unsigned shortxsubi[3]);long int mrand48(void);long int jrand48(unsigned shortxsubi[3]);void srand48(long intseedval);unsigned short *seed48(unsigned shortseed16v[3]);void lcong48(unsigned shortparam[7]);

Feature Test Macro Requirements for glibc (see **feature_test_macros**(7)):

All functions shown above: _SVID_SOURCE || _XOPEN_SOURCE

These functions generate pseudo-random numbers using the linear congruential algorithm and 48-bit integer arithmetic.

The **drand48**() and **erand48**() functions return nonnegative double-precision floating-point values uniformly distributed between [0.0, 1.0).

The **lrand48**() and **nrand48**() functions return nonnegative 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 = 0x5DEECE66Dc = 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[6]* specifies *c*. After **lcong48**() has been called, a subsequent call to either **srand48**() or **seed48**() will restore the standard values of *a* and *c*.

The **drand48**(), **erand48**(), **lrand48**(), **nrand48**(), **mrand48**(), **jrand48**(), **srand48**(), **seed48**(), and **lcong48**() functions record global state information for the random number generator, so they are not thread-safe.

SVr4, POSIX.1-2001.

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