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rand(3) Library Functions Manual rand(3) delim $$ NAME rand, rand_r, srand - Generates pseudorandom numbers LIBRARY 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 stan- dards as follows: rand_r(): POSIX.1c rand(), srand(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry 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 de- pends 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 spec- tral 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. [Digital] 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-num- ber generators that have better spectral properties. NOTES [POSIX] The rand() function is not supported for multithreaded applica- tions. 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 se- quence. [POSIX] The rand_r function returns the next random number in the se- quence. [Digital] Upon successful completion, the obsolete version of the rand_r() function returns a value of 0 (zero). Otherwise, -1 is re- turned and errno is set to indicate the error. The srand() function returns no value. ERRORS [Digital] If the rand_r() function fails, errno may be set to the fol- lowing value: Either seedptr or randval is a null pointer. delim off RELATED INFORMATION Functions: drand48(3), random(3) Standards: standards(5) delim off rand(3)