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.Dd February 28, 2023 .Dt GETENTROPY 3 .Os .Sh NAME .Nm getentropy .Nd generate uniform random seeds from system entropy for cryptography .Sh LIBRARY .Lb libc .Sh SYNOPSIS n unistd.h .Ft int .Fn getentropy "void *buf" "size_t buflen" n limits.h
p .Li #define GETENTROPY_MAX 256 .Sh DESCRIPTION
p The .Nm function fills .Fa buf with exactly .Fa buflen independent uniform random bytes derived from the system's entropy pool.
p The output of .Nm is meant to be unpredictable to an adversary and fit for use in cryptography. See .Sx CAVEATS below.
p .Nm is meant for seeding random number generators, not for direct use by applications; most applications should use .Xr arc4random 3 .
p .Fa buflen must be at most 256. .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. .El .Sh CAVEATS Security can only be guaranteed relative to whatever unpredictable physical processes or secret seed material are available to the system; see .Xr entropy 7 .
p On systems which have no hardware random number generator and which have not had secret seed material loaded, .Nx makes a reasonable effort to incorporate samples from various physical processes available to it that might be unpredictable from random jitter in timing.
p However, the .Nm interface alone can make no security guarantees without a physical system configuration that includes random number generation hardware or secret seed material from such hardware on another machine. .Sh SEE ALSO .Xr arc4random 3 , .Xr rnd 4 , .Xr entropy 7 .Sh STANDARDS The .Fn getentropy function is nonstandard. 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 in .Nx 10.0 .