getrandom, getentropy: Mention the crypto/gc-random module.

Suggested by Simon Josefsson in
<https://lists.gnu.org/archive/html/bug-gnulib/2020-05/msg00400.html>.

* doc/glibc-functions/getrandom.texi: Mention the quality issues and the
crypto/gc-random module.
* doc/glibc-functions/getentropy.texi: Likewise.

2 files changed, 13 insertions, 1 deletions

diff --git a/doc/glibc-functions/getentropy.texi b/doc/glibc-functions/getentropy.texi
index b7717e514b..998bcf46d6 100644
--- a/doc/glibc-functions/getentropy.texi
+++ b/doc/glibc-functions/getentropy.texi
@@ -31,3 +31,9 @@ Mac OS X 10.13, Solaris 11.4, Android 9.0.
 Portability problems not fixed by Gnulib:
 @itemize
 @end itemize
+
+Note: This function does not provides high-quality random numbers, as needed
+by some crypto applications.  If you want such high-quality random numbers,
+use the function @code{getrandom} with the @code{GRND_RANDOM} flag or (better)
+use the @samp{crypto/gc-random} module and configure with
+@samp{--with-libgcrypt}.
diff --git a/doc/glibc-functions/getrandom.texi b/doc/glibc-functions/getrandom.texi
index 3baf3905e2..7488f6faaa 100644
--- a/doc/glibc-functions/getrandom.texi
+++ b/doc/glibc-functions/getrandom.texi
@@ -29,6 +29,12 @@ Solaris 11.4.
 
 Portability problems not fixed by Gnulib:
 @itemize
-This function cannot produce truly random numbers on some platforms:
+This function cannot produce truly random numbers, even when the
+@code{GRND_RANDOM} flag is given, on some platforms:
 GNU/Hurd, Mac OS X, GNU/kFreeBSD, FreeBSD 12.0, OpenBSD 6.5, Minix 3.3, AIX 7.1, Haiku, mingw, MSVC 14.
 @end itemize
+
+Note: This function does not provides high-quality random numbers, as needed
+by some crypto applications, even when the @code{GRND_RANDOM} flag is given.
+If you want such high-quality random numbers, use the @samp{crypto/gc-random}
+module and configure with @samp{--with-libgcrypt}.