seed48() — Pseudo-random number initializer

Standards

Standards / Extensions C or C++ Dependencies
XPG4
XPG4.2
Single UNIX Specification, Version 3
 both  

Format

#define _XOPEN_SOURCE
#include <stdlib.h>

unsigned short int *seed48(unsigned short int seed16v[3]);

General description

The drand48(), erand48(), jrand48(), lrand48(), mrand48() and nrand48() functions generate uniformly distributed pseudo-random numbers using a linear congruential algorithm and 48-bit integer arithmetic.

The lcong48(), seed48(), and srand48() functions are initialization functions, one of which should be invoked before either the drand48(), lrand48() or mrand48() function is called.

The drand48(), lrand48() and mrand48() functions generate a sequence of 48-bit integer values, X(i), according to the linear congruential formula: 
   X(n+1) = (aX(n) + c)mod(2**48)     n>=0
The initial values of X, a, and c are: 
   X(0)= 1
   a   = 5deece66d (base 16)
   c   = b         (base 16)

C/370 provides storage to save the most recent 48-bit integer value of the sequence, X(i). This storage is shared by the drand48(), lrand48() and mrand48() functions. The seed48() function is used to reinitialize the most recent 48-bit value in this storage. The seed48() function replaces the low-order (rightmost) 16 bits of this storage with seed16v[0], the middle-order 16 bits with seed16v[1], and the high-order 16 bits with seed16v[2].

The values a and c, may be changed by calling the lcong48() function. The seed48() function restores the initial values of a and c.

Special behavior for z/OS® UNIX Services: You can make the seed48() function and other functions in the drand48 family thread-specific by setting the environment variable _RAND48 to the value THREAD before calling any function in the drand48 family.

If you do not request thread-specific behavior for the drand48 family, C/370 serializes access to the storage for X(n), a and c by functions in the drand48 family when they are called by a multithreaded application.

If thread-specific behavior is requested, calls to the drand48(), lrand48() and mrand48() functions from thread t generate a sequence of 48-bit integer values, X(t,i), according to the linear congruential formula: 
   X(t,n+1) = (a(t)X(t,n) + c(t))mod(2**48)     n>=0

C/370 provides thread-specific storage to save the most recent 48-bit integer value of the sequence, X(t,i). When the seed48()function is called from thread t, it reinitializes the most recent 48-bit value in this storage. The seed48() function replaces the low-order (rightmost) 16 bits of this storage with seed16v[0], the middle-order 16 bits with seed16v[1], and the high-order 16 bits with seed16v[2].

The values of a(t) and c(t) may be changed by calling the lcong48() function from thread t. When the seed48()function is called from this thread, it restores the initial values of a(t) and c(t) for the thread which are: 
   a(t)   = 5deece66d (base 16)
   c(t)   = b         (base 16)

Returned value

When seed48() is called, it saves the most recent 48-bit integer value in the sequence, X(i), in an array of unsigned short ints provided by C/370 before reinitializing storage for the most recent value in the sequence, X(i). seed48() returns a pointer to the array containing the saved value.

Special behavior for z/OS UNIX Services: If thread-specific behavior is requested for the drand48 family and seed48() is called on thread t, it saves the most recent 48-bit integer value in the sequence, X(t,i), for the thread in a thread-specific array of unsigned short ints before reinitializing storage for the most recent value in the sequence, X(t,i). seed48() returns a pointer to this thread-specific array containing the saved value.

Related information