diff --git a/man2/posix_fadvise.2 b/man2/posix_fadvise.2
index 0077a86e2..eed5d9c7d 100644
--- a/man2/posix_fadvise.2
+++ b/man2/posix_fadvise.2
@@ -192,7 +192,9 @@ The name of the wrapper function in the C library is
 The underlying system call is called
 .BR fadvise64 ()
 (or, on some architectures,
-.BR fadvise64_64 ()).
+.BR fadvise64_64 ()); the difference between the two is that former syscall
+assumes that the type of the \fIlen\fP argument is \fBsize_t\fP,
+while the latter expects \fBloff_t\fP there.
 .SS Architecture-specific variants
 Some architectures require
 64-bit arguments to be aligned in a suitable pair of registers (see