RAND(3) Library Functions Manual RAND(3) NAME rand, srand, rand_r -- bad random number generator LIBRARY Standard C Library (libc, -lc) SYNOPSIS #include <stdlib.h> void srand(unsigned seed); int rand(void); int rand_r(unsigned *ctx); DESCRIPTION The functions described in this manual page are not cryptographically se- cure. Applications which require unpredictable random numbers should use arc4random(3) instead. The rand() function computes a sequence of pseudo-random integers in the range of 0 to RAND_MAX, inclusive. The srand() function seeds the algorithm with the seed parameter. Repeat- able sequences of rand() output may be obtained by calling srand() with the same seed. rand() is implicitly initialized as if srand(1) had been in- voked explicitly. In FreeBSD 13, rand() is implemented using the same 128-byte state LFSR generator algorithm as random(3). However, the legacy rand_r() function is not (and can not be, because of its limited *ctx size). rand_r() imple- ments the historical, poor-quality Park-Miller 32-bit LCG and should not be used in new designs. IMPLEMENTATION NOTES Since FreeBSD 13, rand() is implemented with the same generator as ran- dom(3), so the low-order bits should no longer be significantly worse than the high-order bits. SEE ALSO arc4random(3), random(3), random(4) STANDARDS The rand() and srand() functions conform to ISO/IEC 9899:1990 ("ISO C90"). The rand_r() function is not part of ISO/IEC 9899:1990 ("ISO C90") and is marked obsolescent in IEEE Std 1003.1-2008 ("POSIX.1"). It may be removed in a future revision of POSIX. CAVEATS Prior to FreeBSD 13, rand() used the historical Park-Miller generator with 32 bits of state and produced poor quality output, especially in the lower bits. rand() in earlier versions of FreeBSD, as well as other standards- conforming implementations, may continue to produce poor quality output. These functions should not be used in portable applications that want a high quality or high performance pseudorandom number generator. One possi- ble replacement, random(3), is portable to Linux a but it is not especially fast, nor standardized. If broader portability or better performance is desired, any of the widely available and permissively licensed SFC64/32, JSF64/32, PCG64/32, or Split- Mix64 algorithm implementations may be embedded in your application. These algorithms have the benefit of requiring less space than random(3) and be- ing quite fast (in header inline implementations). FreeBSD 15.1 February 1, 2020 RAND(3)
NAME | LIBRARY | SYNOPSIS | DESCRIPTION | IMPLEMENTATION NOTES | SEE ALSO | STANDARDS | CAVEATS
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