# srand(3) [osf1 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)

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RAND(3) Linux Programmer's Manual RAND(3)NAME

rand, rand_r, srand - pseudo-random number generatorSYNOPSIS

#include <stdlib.h> int rand(void); int rand_r(unsigned int *seedp); void srand(unsigned int seed); Feature Test Macro Requirements for glibc (see feature_test_macros(7)): rand_r(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCEDESCRIPTION

The rand() function returns a pseudo-random integer in the range [0, RAND_MAX]. The srand() function sets its argument as the seed for a new sequence of pseudo-random integers to be returned by rand(). These sequences are repeatable by calling srand() with the same seed value. If no seed value is provided, the rand() function is automatically seeded with a value of 1. The function rand() is not reentrant or thread-safe, since it uses hidden state that is modified on each call. This might just be the seed value to be used by the next call, or it might be something more elaborate. In order to get reproducible behavior in a threaded applica- tion, this state must be made explicit. The function rand_r() is supplied with a pointer to an unsigned int, to be used as state. This is a very small amount of state, so this function will be a weak pseudo-random generator. Try drand48_r(3) instead.RETURN VALUE

The rand() and rand_r() functions return a value between 0 and RAND_MAX. The srand() function returns no value.CONFORMING TO

The functions rand() and srand() conform to SVr4, 4.3BSD, C89, C99, POSIX.1-2001. The function rand_r() is from POSIX.1-2001. POSIX.1-2008 marks rand_r() as obsolete.NOTES

The versions of rand() and srand() in the Linux C Library use the same random number generator as random(3) and srandom(3), so the lower- order bits should be as random as the higher-order bits. However, on older rand() implementations, and on current implementations on dif- ferent systems, the lower-order bits are much less random than the higher-order bits. Do not use this function in applications intended to be portable when good randomness is needed. (Use random(3) instead.)EXAMPLE

POSIX.1-2001 gives the following example of an implementation of rand() and srand(), possibly useful when one needs the same sequence on two different machines. static unsigned long next = 1; /* RAND_MAX assumed to be 32767 */ int myrand(void) { next = next * 1103515245 + 12345; return((unsigned)(next/65536) % 32768); } void mysrand(unsigned seed) { next = seed; }SEE ALSO

drand48(3), random(3)COLOPHON

This page is part of release 3.25 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. 2008-08-29 RAND(3)