# rand_r(3c) [opensolaris man page]

rand(3C) Standard C Library Functions rand(3C)NAME

rand, srand, rand_r - simple random-number generatorSYNOPSIS

#include <stdlib.h> int rand(void); void srand(unsigned int seed); int rand_r(unsigned int *seed);DESCRIPTION

The rand() function uses a multiplicative congruential random-number generator with period 2^32 that returns successive pseudo-random num- bers in the range of 0 to RAND_MAX (defined in <stdlib.h>). The srand() function uses the argument seed as a seed for a new sequence of pseudo-random numbers to be returned by subsequent calls to rand(). If srand() is then called with the same seed value, the sequence of pseudo-random numbers will be repeated. 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 rand_r() function has the same functionality as rand() except that a pointer to a seed seed must be supplied by the caller. If rand_r() is called with the same initial value for the object pointed to by seed and that object is not modified between successive calls to rand_r(), the same sequence as that produced by calls to rand() will be generated. The rand() and srand() functions provide per-process pseudo-random streams shared by all threads. The same effect can be achieved if all threads call rand_r() with a pointer to the same seed object. The rand_r() function allows a thread to generate a private pseudo-random stream by having the seed object be private to the thread.USAGE

The spectral properties of rand() are limited. The drand48(3C) function provides a better, more elaborate random-number generator. When compiling multithreaded applications, the _REENTRANT flag must be defined on the compile line. This flag should be used only in mul- tithreaded applications.ATTRIBUTES

See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ |MT-Level |Safe | +-----------------------------+-----------------------------+SEE ALSO

drand48(3C), attributes(5), standards(5)SunOS 5.1119 May 2004 rand(3C)

## Check Out this Related Man Page

rand(3) Library Functions Manual rand(3)delim $$NAME

rand, rand_r, srand - Generates pseudorandom numbersLIBRARY

Standard C Library (libc.so, libc.a) Berkeley Compatibility Library (libbsd.a)SYNOPSIS

#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);STANDARDS

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.PARAMETERS

Specifies an initial seed value. Points to a seed value, updated at each call. Points to a place to store the random number.DESCRIPTION

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 num- ber 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 ran- dom 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.NOTES

[POSIX] 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 }RETURN VALUES

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,is returned and errno is set to indicate the error. The srand() function returns no value.-1ERRORS

[Tru64 UNIX] If the rand_r() function fails, errno may be set to the following value: Either seedptr or randval is a null pointer. delim offRELATED INFORMATION

Functions: drand48(3), random(3) Standards: standards(5) delim off rand(3)