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NAME
drand48, erand48, jrand48, lcong48, lrand48, mrand48, nrand48, seed48, srand48 — generate
uniformly distributed pseudo-random numbers
SYNOPSIS
#include <stdlib.h>
double drand48(void);
double erand48(unsigned short xsubi[3]);
long jrand48(unsigned short xsubi[3]);
void lcong48(unsigned short param[7]);
long lrand48(void);
long mrand48(void);
long nrand48(unsigned short xsubi[3]);
unsigned short *seed48(unsigned short seed16v[3]);
void srand48(long seedval);
DESCRIPTION
This family of functions shall generate pseudo-random numbers using a linear congruential
algorithm and 48-bit integer arithmetic.
The drand48() and erand48() functions shall return non-negative, double-precision,
floating-point values, uniformly distributed over the interval [0.0,1.0).
The lrand48() and nrand48() functions shall return non-negative, long integers, uniformly
distributed over the interval [0,231).
The mrand48() and jrand48() functions shall return signed long integers uniformly
distributed over the interval [-231,231).
The srand48(), seed48(), and lcong48() functions 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 shall be
supplied automatically if drand48(), lrand48(), or mrand48() is called without a prior
call to an initialization entry point.) The erand48(), nrand48(), and jrand48() functions
do not require an initialization entry point to be called first.
All the routines work by generating a sequence of 48-bit integer values, X_i , according
to the linear congruential formula:
Xn+1 = (aX_n +c)mod m n≥ 0
The parameter m=2^48; hence 48-bit integer arithmetic is performed. Unless lcong48() is
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 drand48(), erand48(), jrand48(), lrand48(), mrand48(), or
nrand48() functions is computed by first generating the next 48-bit X_i 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 X_i and transformed into the returned
value.
The drand48(), lrand48(), and mrand48() functions store the last 48-bit X_i generated in
an internal buffer; that is why the application shall ensure that these are initialized
prior to being invoked. The erand48(), nrand48(), and jrand48() functions require the
calling program to provide storage for the successive X_i values in the array specified as
an argument when the functions are invoked. That 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 an argument. By using different arguments, 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 shall
not depend upon how many times the routines are called to generate numbers for the other
streams.
The initializer function srand48() sets the high-order 32 bits of X_i to the low-order 32
bits contained in its argument. The low-order 16 bits of X_i are set to the arbitrary
value 330E_16 .
The initializer function seed48() sets the value of X_i to the 48-bit value specified in
the argument array. The low-order 16 bits of X_i are set to the low-order 16 bits of
seed16v[0]. The mid-order 16 bits of X_i are set to the low-order 16 bits of seed16v[1].
The high-order 16 bits of X_i are set to the low-order 16 bits of seed16v[2]. In
addition, the previous value of X_i 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 X_i value, and then use this value to reinitialize via seed48() when the program is
restarted.
The initializer function lcong48() allows the user to specify the initial X_i , the
multiplier value a, and the addend value c. Argument 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() is called, a subsequent call to either srand48() or seed48() shall restore the
standard multiplier and addend values, a and c, specified above.
The drand48(), lrand48(), and mrand48() functions need not be thread-safe.
RETURN VALUE
As described in the DESCRIPTION above.
ERRORS
No errors are defined.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
These functions should be avoided whenever non-trivial requirements (including safety)
have to be fulfilled.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
initstate(), rand()
The Base Definitions volume of POSIX.1‐2017, <stdlib.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
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