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.Dd May 1, 2020 .Dt GETENTROPY 3 .Os .Sh NAME .Nm getentropy .Nd fill a buffer with high quality random data .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn getentropy "void *buf" "size_t buflen" n limits.h .Li #define GETENTROPY_MAX 256 .Sh DESCRIPTION
p The .Fn getentropy function fills a buffer with high quality random data, suitable for seeding cryptographically secure psuedorandom number generators.
p .Fn getentropy is only intended for seeding random number generators and is not intended for use by regular code which simply needs secure random data. For this purpose, please use .Xr arc4random 3 .
p The maximum value for .Li buflen is 256 bytes. .Sh IMPLEMENTATION NOTES .Fn getentropy reads from the .Xr sysctl 7 variable .Li kern.arandom . .Sh RETURN VALUES .Rv -std getentropy .Sh ERRORS .Fn getentropy will succeed unless: l -tag -width Er t Bq Er EFAULT The .Fa buf argument points to an invalid memory address. t Bq Er EINVAL More than 256 bytes were requested. .Sh SEE ALSO .Xr arc4random 3 , .Xr rnd 4 .Sh STANDARDS The .Fn getentropy function is non-standard. However, it is likely to be included in the next revision of POSIX. .Sh HISTORY The .Fn getentropy function first appeared in .Ox 5.6 , then in .Fx 12.0 , and .Nx 10 .