drand48(3)drand48(3)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
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,
double *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] ); long 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 );
LIBRARY
Standard C Library (libc)
STANDARDS
Interfaces documented on this reference page conform to industry stan‐
dards 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 lin‐
ear congruential algorithm and 48-bit integer arithmetic.
The drand48() and erand48() functions return nonnegative, double-preci‐
sion, 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 uni‐
formly 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 val‐
ues, 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 per‐
formed. 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 ini‐
tialized 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 parame‐
ter.
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] speci‐
fies the 16-bit addend c. After lcong48() has been called, a subse‐
quent 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 genera‐
tor 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 struc‐
ture in the lastx field by the seed48_r() function.
RETURN VALUES
The drand48() and erand48() functions return nonnegative, double-preci‐
sion, floating-point values.
The lrand48() and nrand48() functions return signed long integers uni‐
formly distributed over the range 0 <=y < 2^31.
The mrand48() and jrand48() functions return signed long integers uni‐
formly 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 func‐
tions 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.
SEE ALSO
Functions: rand(3), random(3)
Standards: standards(5)drand48(3)
