man pages section 3: Basic Library Functions

Updated: Wednesday, July 27, 2022

drand48(3C)

Name

drand48, erand48, lrand48, nrand48, mrand48, jrand48, srand48, seed48, lcong48 - generate uniformly distributed pseudo-random numbers

Synopsis

```#include <stdlib.h>

double drand48(void)```
`double erand48(unsigned short xsubi[3]);`
`long lrand48(void)`
`long nrand48(unsigned short xsubi[3]);`
`long mrand48(void)`
`long jrand48(unsigned short xsubi[3]);`
`void srand48(long seedval);`
`unsigned short *seed48(unsigned short seed16v[3]);`
`void lcong48(unsigned short param[7]);`
`int drand48_r(struct drand48_data *buffer, double *result);`
```int erand48_r(unsigned short xsubi[3],
struct drand48_data *buf, double *result);```
`int lrand48_r(struct drand48_data *buf, long int *result);`
```int nrand48_r(unsigned short int xsubi[3],
struct drand48_data *buf, long int *result);```
`int mrand48_r(struct drand48_data *buf, long int *result);`
```int jrand48_r(unsigned short int xsubi[3],
struct drand48_data *buf, long int *result);```
`int srand48_r(long int seedval, struct drand48_data *buf);`
```int seed48_r(unsigned short int seed16v[3],
struct drand48_data *buf);```
```int lcong48_r(unsigned short int param[7],
struct drand48_data *buf);```

Description

This family of functions generates pseudo-random numbers using the well-known linear congruential algorithm and 48-bit integer arithmetic.

Functions drand48() and erand48() return non-negative double-precision floating-point values uniformly distributed over the interval [0.0, 1.0).

Functions lrand48() and nrand48() return non-negative long integers uniformly distributed over the interval [0, 2 31 ].

Functions mrand48() and jrand48() return signed long integers uniformly distributed over the interval [-2 31 , 2 31 ].

Functions srand48(), seed48(), and lcong48() are initialization entry points, one of which should be invoked before either drand48(), lrand48(), or mrand48() is called. (Although it is not recommended practice, constant default initializer values will be supplied automatically if drand48(), lrand48(), or mrand48() is called without a prior call to an initialization entry point.) Functions erand48(), nrand48(), and jrand48() do not require an initialization entry point to be called first.

All the routines work by generating a sequence of 48-bit integer values, Xi, according to the linear congruential formula

Xn+1= (aX n+c)mod m n>=0.

The parameter m = 248; 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 = 5DEECE66D16 = 2736731631558
• c = B16 = 138

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, are copied from the high-order (leftmost) 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. Xi must be initialized prior to being invoked. The functions erand48(), nrand48(), and jrand48() require the calling program to provide storage for the successive Xi values in the array specified as an argument when the functions are invoked. These routines do not have to be initialized; the calling program must place the desired initial value of Xi into the array and pass it as an argument. By using different arguments, functions erand48(), nrand48(), and jrand48() allow separate modules of a large program to generate several independent streams of pseudo-random numbers, that is, the sequence of numbers in each stream will not depend upon how many times the routines have been called to generate numbers for the other streams.

The initializer function srand48() sets the high-order 32 bits of Xi to the 32 bits contained in its argument. The low-order 16 bits of Xi are set to the arbitrary value 330E16 .

The initializer function seed48() sets the value of Xi to the 48-bit value specified in the argument array. In addition, the previous value of Xi is copied into a 48-bit internal buffer, used only by seed48(), and a pointer to this buffer is the value returned by seed48(). This returned pointer, which can just be ignored if not needed, is useful if a program is to be restarted from a given point at some future time — use the pointer to get at and store the last Xi value, and then use this value to reinitialize using seed48() when the program is restarted.

The initialization function lcong48() allows the user to specify the initial Xi the multiplier value a, and the addend value c. Argument array elements param[0-2] specify Xi, 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() will restore the “standard” multiplier and addend values, a and c, specified above.

Functions drand48_r(), erand48_r(), jrand48_r(), lrand48_r(), mrand_r(), and nrand_r() are reentrant versions of drand48(), erand48(), jrand48(), lrand48(), mrand48(), and nrand48() functions. They use the supplied buffer, buf, to store the random number generator state, as opposed to the global buffers used by the non-reentrant functions. The buffer must be first allocated, and then initialized either by filling with zeroes, or by calling one of the srand48_r(), seed48_r(), or lcong48_r() functions.

Functions srand48_r(), seed48_r(), and lcong48_r() are reentrant versions of the functions srand48(), seed48(), and lcong48(). In addition, they can be used to initialize the supplied buffer, buf.

Usage

Programmers should use /dev/urandom or /dev/random for most random-number generation, especially for cryptographic purposes. See getrandom(2) and random(4D).

Attributes

See attributes(7) for descriptions of the following attributes:

ATTRIBUTE TYPE
ATTRIBUTE VALUE
Interface Stability
Committed
MT-Level
Safe
Standard