Content-type: text/html Man page of drand48

drand48

Section: C Library Functions (3)
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NAME

drand48, drand48_r, erand48, erand48_r, jrand48, jrand48_r, lcong48, lcong48_r, lrand48, lrand48_r, mrand48, mrand48_r, nrand48, nrand48_r, seed48, seed48_r, srand48, srand48_r - Generate uniformly distributed pseudorandom number sequences  

LIBRARY

Standard C Library (libc.so, libc.a)  

SYNOPSIS

#include <stdlib.h>

double drand48(void);

int drand48_r(        struct drand48_data *dp,
       double *randval);

double erand48(        unsigned short xsubi[3]);

int erand48_r(        unsigned short xsubi[3],
       struct drand48_data *dp,
       double *randval);

long jrand48(        unsigned short xsubi[3]);

int jrand48_r(        unsigned short xsubi[3],
       struct drand48_data *dp,
       long *randval);

void lcong48(        unsigned short param[7]);

int lcong48_r(        unsigned short param[7],
       struct drand48_data *dp);

long lrand48(void);

int lrand48_r(        struct drand48_data *dp,
       long *randval);

long mrand48(void);

int mrand48_r(        struct drand48_data *dp,
       long *randval);

long nrand48(        unsigned short xsubi[3]);

int nrand48_r(        unsigned short xsubi[3],
       struct drand48_data *dp,
       long *randval);

unsigned short *seed48(        unsigned short seed_16v[3]);

int seed48_r(        unsigned short seed_16v[3],
       struct drand48_data *dp);

void srand48 (        long seed_val);

int srand48_r(        long seed_val,
       struct drand48_data *dp);
 

STANDARDS

Interfaces documented on this reference page conform to industry standards as follows:

drand48(), erand48(), jrand48(), lcong48(), lrand48(), mrand48(), nrand48(), seed48(), srand48():  XPG4, XPG4-UNIX

drand48_r(), erand48_r(), jrand48_r(), lcong48_r(), lrand48_r(), mrand48_r(), nrand48_r(), seed48_r(), srand48_r():  POSIX.1c

Refer to the standards(5) reference page for more information about industry standards and associated tags.  

PARAMETERS

Specifies an array of three shorts, which, when concatenated together, form a 48-bit integer. Specifies the initialization value to begin randomization. Changing this value changes the randomization pattern. Specifies another seed value; an array of three unsigned shorts that form a 48-bit seed value. Specifies an array that in turn specifies the initial X[i], the multiplier value a, and the addend value c. [POSIX]  Points to a structure which contains data for the reentrant routines. [POSIX]  Points at the returned nonnegative pseudorandom numbers.  

DESCRIPTION

This family of functions generates pseudorandom 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 over the range of y values such that 0 ≤ y < 1.0.

The lrand48() and nrand48() functions return nonnegative long integers uniformly distributed over the range of y values such that 0 ≤ y < 2^31.

The mrand48() and jrand48() functions return signed long integers uniformly distributed over the range of y values such that -2^31 ≤ y < 2^31.

The srand48(), seed48(), and lcong48() functions initialize the random-number generator. Programs should invoke one of them before calling the drand48(), lrand48(), or the mrand48() functions. (Although it is not recommended practice, constant default initializer values are supplied automatically if the drand48(), lrand48(), or mrand48() functions are called without first calling an initialization function.) The erand48(), nrand48(), and jrand48() functions do not require that an initialization function be called first.

All the functions 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 equals 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 (hex) = 273673163155 (octal) c = B (hex) = 13 (octal)

The values returned by the drand48(), erand48(), lrand48(), nrand48(), mrand48(), and jrand48() functions are computed by first generating the next 48-bit X[i] in the sequence. Then the appropriate bits, according to the type of data item to be returned, are copied from the high-order (most-significant) bits of X[i] and transformed into the returned value.

The drand48(), lrand48(), and mrand48() functions store the last 48-bit X[i] generated into an internal buffer, which is why they must be initialized prior to being invoked.

The erand48(), nrand48(), and jrand48() functions require that the calling program provide storage for the successive X[i] values in the array pointed to by the xsubi parameter. This is why these routines do not have to be initialized; the calling program merely has to place the desired initial value of X[i] into the array and pass it as a parameter.

By using different parameters, the erand48(), nrand48(), and jrand48() functions allow separate modules of a large program to generate several independent sequences of pseudorandom numbers; that is, the sequence of numbers that one module generates does not depend upon how many times the functions are called by other modules.

The initializer function srand48() sets the high-order 32 bits of X[i] to the LONG_BIT bits contained in its parameter. The low-order 16 bits of X[i] are set to the arbitrary hexadecimal value 330E.

The initializer function seed48() sets the value of X[i] to the 48-bit value specified in the array pointed to by the seed_16v parameter. In addition, seed48() returns a pointer to a 48-bit internal buffer that contains the previous value of X[i] which is used only by seed48(). The returned pointer allows you to restart the pseudorandom sequence at a given point. Use the pointer to copy the previous X[i] value into a temporary array. To resume where the original sequence left off, you can call seed48() with a pointer to this array.

The lcong48() function specifies the initial X[i] value, the multiplier value a, and the addend value c. The param 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() restores the standard a and c as specified previously.

[POSIX]  The drand48_r(), erand48_r(), jrand48_r(), lcong48_r(), lrand48_r(), mrand48_r(), nrand48_r()l, seed48_r(), and srand_r() functions are the reentrant versions of drand48(), erand48(), jrand48(), lcong48(), lrand48(), mrand48(), nrand48(), seed48(), and srand48().  

NOTES

[POSIX]   Note that if the reentrant initialization functions are not used prior to calling the random number generators, the init field of the struct drand48_data structure (passed to the random number generator functions) must be cleared prior to calling them.

[POSIX]   Note also, that the 48-bit internal buffer that used to be returned by the seed48() function is now placed in the DRAND48D structure in the lastx field by the seed48_r() function.  

RETURN VALUES

The drand48() and erand48() functions return nonnegative, double-precision, floating-point values.

The lrand48() and nrand48() functions return signed long integers uniformly distributed over the range 0 ≤ y < 2^31.

The mrand48() and jrand48() functions return signed long integers uniformly distributed over the range -2^31 ≤ y < 2^31.

The seed48() function returns a pointer to a 48-bit internal buffer.

The lcong48() and srand48() functions do not return a value.

[POSIX]  Upon success, both the drand48_r() and erand48_r() functions return a pointer (randval) to the returned double value, and return a value of 0 (zero). Upon success, both the mrand48_r() and jrand48_r() functions have randval at the returned long value, and return a value of 0 (zero). Upon success, the lcong48_r(), seed48_r(), and srand48_r() return a value of 0 (zero). The lrand48_r() and rand48_r() functions have randval point at the returned long value, and return a value of 0 (zero). Upon error, the reentrant functions return a value of -1, and set errno to [EFAULT].  

ERRORS

[POSIX]  Upon error, the drand48_r(), erand48_r(), jrand48_r, lcong48_r, lrand48_r, mrand48_r, nrand48_r, seed48_r, and srand_r functions return a value of -1, and errno will be set to the following value: The dp, randval, xsubi, param, or seed_16v value is invalid.  

RELATED INFORMATION

Functions: rand(3), random(3)

Standards: standards(5) delim off delim off


 

Index

NAME
LIBRARY
SYNOPSIS
STANDARDS
PARAMETERS
DESCRIPTION
NOTES
RETURN VALUES
ERRORS
RELATED INFORMATION

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Time: 02:42:35 GMT, October 02, 2010