=pod

=head1 NAME

RAND_load_file, RAND_write_file, RAND_file_name - PRNG seed file

=head1 SYNOPSIS

 #include <openssl/rand.h>

 int RAND_load_file(const char *filename, long max_bytes);

 int RAND_write_file(const char *filename);

 const char *RAND_file_name(char *buf, size_t num);

=head1 DESCRIPTION

RAND_load_file() reads a number of bytes from file B<filename> and
adds them to the PRNG. If B<max_bytes> is nonnegative,
up to B<max_bytes> are read;
if B<max_bytes> is -1, the complete file is read (unless the file
is not a regular file, in that case a fixed number of bytes,
256 in the current implementation, is attempted to be read).
RAND_load_file() can read less than the complete file or the requested number
of bytes if it doesn't fit in the return value type.
Do not load the same file multiple times unless its contents have
been updated by RAND_write_file() between reads.
Also, note that B<filename> should be adequately protected so that an
attacker cannot replace or examine the contents.
If B<filename> is not a regular file, then user is considered to be
responsible for any side effects, e.g. non-anticipated blocking or
capture of controlling terminal.

RAND_write_file() writes a number of random bytes (currently 128) to
file B<filename> which can be used to initialize the PRNG by calling
RAND_load_file() in a later session.

RAND_file_name() generates a default path for the random seed
file. B<buf> points to a buffer of size B<num> in which to store the
filename.

On all systems, if the environment variable B<RANDFILE> is set, its
value will be used as the seed filename.
Otherwise, the file is called C<.rnd>, found in platform dependent locations:

=over 4

=item On Windows (in order of preference)

 %HOME%, %USERPROFILE%, %SYSTEMROOT%, C:\

=item On VMS

 SYS$LOGIN:

=item On all other systems

 $HOME

=back

If C<$HOME> (on non-Windows and non-VMS system) is not set either, or
B<num> is too small for the pathname, an error occurs.

=head1 RETURN VALUES

RAND_load_file() returns the number of bytes read or -1 on error.

RAND_write_file() returns the number of bytes written, or -1 if the
bytes written were generated without appropriate seeding.

RAND_file_name() returns a pointer to B<buf> on success, and NULL on
error.

=head1 SEE ALSO

L<RAND_add(3)>,
L<RAND_bytes(3)>,
L<RAND(7)>

=head1 COPYRIGHT

Copyright 2000-2025 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the Apache License 2.0 (the "License").  You may not use
this file except in compliance with the License.  You can obtain a copy
in the file LICENSE in the source distribution or at
L<https://www.openssl.org/source/license.html>.

=cut
