seed48() — Pseudo-random number initializer

Standards

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.

   X(n+1) = (aX(n) + c)mod(2**48)     n>=0

   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.

   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].

   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

• stdlib.h
• drand48() — Pseudo-random number generator
• erand48() — Pseudo-random number generator
• jrand48() — Pseudo-random number generator
• lcong48() — Pseudo-random number initializer
• lrand48() — Pseudo-random number generator
• mrand48() — Pseudo-random number generator
• nrand48() — Pseudo-random number generator
• srand48() — Pseudo-random number initializer