drand48(3) - Linux manual page (original) (raw)

drand48(3) Library Functions Manual drand48(3)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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]);**

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

   All functions shown above:
       _XOPEN_SOURCE
           || /* glibc >= 2.19: */ _DEFAULT_SOURCE
           || /* glibc <= 2.19: */ _SVID_SOURCE

DESCRIPTION top

   These functions generate pseudo-random numbers using the linear
   congruential algorithm and 48-bit integer arithmetic.

   The **drand48**() and **erand48**() functions return nonnegative double-
   precision floating-point values uniformly distributed over the
   interval [0.0, 1.0).

   The **lrand48**() and **nrand48**() functions return nonnegative long
   integers uniformly distributed over the interval [0, 2^31).

   The **mrand48**() and **jrand48**() functions return signed long integers
   uniformly distributed over the interval [-2^31, 2^31).

   The **srand48**(), **seed48**(), and **lcong48**() functions are
   initialization functions, one of which should be called before
   using **drand48**(), **lrand48**(), or **mrand48**().  The functions
   **erand48**(), **nrand48**(), and **jrand48**() do not require an
   initialization function to be called first.

   All the functions work by generating a sequence of 48-bit
   integers, _Xi_, according to the linear congruential formula:

       **Xn+1 = (aXn + c) mod m, where n >= 0**

   The parameter _m_ = 2^48, hence 48-bit integer arithmetic is
   performed.  Unless **lcong48**() is called, _a_ and _c_ are given by:

       **a = 0x5DEECE66D**
       **c = 0xB**

   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, is copied from the high-order 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.  The functions
   **erand48**(), **nrand48**(), and **jrand48**() require the calling program to
   provide storage for the successive _Xi_ values in the array argument
   _xsubi_.  The functions are initialized by placing the initial value
   of _Xi_ into the array before calling the function for the first
   time.

   The initializer function **srand48**() sets the high order 32-bits of
   _Xi_ to the argument _seedval_.  The low order 16-bits are set to the
   arbitrary value 0x330E.

   The initializer function **seed48**() sets the value of _Xi_ to the
   48-bit value specified in the array argument _seed16v_.  The
   previous value of _Xi_ is copied into an internal buffer and a
   pointer to this buffer is returned by **seed48**().

   The initialization function **lcong48**() allows the user to specify
   initial values for _Xi_, _a_, and _c_.  Array argument elements
   _param[0-2]_ specify _Xi_, _param[3-5]_ specify _a_, and _param[6]_
   specifies _c_.  After **lcong48**() has been called, a subsequent call
   to either **srand48**() or **seed48**() will restore the standard values
   of _a_ and _c_.

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌─────────────────────────────────┬───────────────┬──────────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├─────────────────────────────────┼───────────────┼──────────────┤
   │ **drand48**(), **erand48**(),           │ Thread safety │ MT-Unsafe    │
   │ **lrand48**(), **nrand48**(),           │               │ race:drand48 │
   │ **mrand48**(), **jrand48**(),           │               │              │
   │ **srand48**(), **seed48**(), **lcong48**()  │               │              │
   └─────────────────────────────────┴───────────────┴──────────────┘

   The above functions record global state information for the random
   number generator, so they are not thread-safe.

STANDARDS top

   POSIX.1-2008.

HISTORY top

   POSIX.1-2001, SVr4.

SEE ALSO top

   [rand(3)](../man3/rand.3.html), [random(3)](../man3/random.3.html)

COLOPHON top

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Linux man-pages 6.10 2024-07-23 drand48(3)

Pages that refer to this page:arc4random(3), drand48_r(3), rand(3), random(3), random_r(3)