rand, rand_r, srand - Generate pseudorandom numbers
#include <stdlib.h>
int rand(
void ); int rand_r(
unsigned int *seedptr ); void srand(
unsigned int seed );
The following function does not conform to current standards
and is supported only for backward compatibility:
int rand_r(
unsigned int *seedptr,
int *randval );
Standard C Library (libc)
Berkeley Compatibility Library (libbsd.a)
Interfaces documented on this reference page conform to
industry standards as follows:
rand_r(): POSIX.1c
rand(), srand(): XPG4, XPG4-UNIX
Refer to the standards(5) reference page for more information
about industry standards and associated tags.
Specifies an initial seed value. Points to a seed value,
updated at each call. Points to a place to store the random
number.
The rand() function returns successive pseudorandom numbers
in the range from 0 (zero) to RAND_MAX. The sequence
of values returned depends on the seed value set with the
srand() function. If rand() is called before any calls to
srand() have been made, the same sequence will be generated
as when srand() is first called with a seed value of
1.
The srand() function resets the random-number generator to
a random starting point. The generator is initially seeded
with a value of 1.
The rand() function is a very simple random-number generator.
Its spectral properties, the mathematical measurement
of how random the number sequence is, are somewhat
weak.
[POSIX] The rand_r() function is the reentrant version of
the rand() function. The rand_r() function places the
seed value at the address pointed to by seedptr, and
returns the random number.
[Tru64 UNIX] The obsolete version of the rand_r() function
places the seed value at the address pointed to by
seedptr, and places the random number at the address
pointed to by randval.
See the drand48() and random() functions for more elaborate
random-number generators that have better spectral
properties.
The rand() function is not supported for multithreaded
applications. Instead, its reentrant equivalent, rand_r(),
should be used with multiple threads.
The BSD version of the rand() function returns a number in
the range 0 to (2^31)-1, rather than 0 to (2^15)-1, and
can be used by compiling the code with the Berkeley Compatibility
Library (libbsd.a).
There are better random number generators, as noted above;
however, the rand() and srand() functions are the interfaces
defined for the ANSI C library.
The following functions define the semantics of the rand()
and srand() functions, and are included here to facilitate
porting applications from different implementations:
static unsigned int next = 1; int myrand(void) {
next = next * 1103515245 + 12345;
return ( (next >>16) & RAND_MAX); } void mysrand
(unsigned int seed) { next = seed }
The rand() function returns the next pseudorandom number
in the sequence.
[POSIX] The rand_r function returns the next random number
in the sequence.
[Tru64 UNIX] Upon successful completion, the obsolete
version of the rand_r() function returns a value of 0
(zero). Otherwise, -1 is returned and errno is set to
indicate the error.
The srand() function returns no value.
[Tru64 UNIX] If the rand_r() function fails, errno may be
set to the following value: Either seedptr or randval is a
null pointer.
Functions: drand48(3), random(3)
Standards: standards(5)
rand(3)
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