RAND_LOAD_FILE(3ossl)

NAME
RAND_load_file, RAND_write_file, RAND_file_name - PRNG seed file

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);

DESCRIPTION
RAND_load_file() reads a number of bytes from file filename and adds them to the PRNG. If max_bytes is nonnegative, up to max_bytes are read; if 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 filename should be adequately protected so that an attacker cannot replace or examine the contents. If 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 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. buf points to a buffer of size num in which to store the filename.

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

On Windows (in order of preference)
     %HOME%, %USERPROFILE%, %SYSTEMROOT%, C:\

On VMS
     SYS$LOGIN:

On all other systems
     $HOME

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

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 buf on success, and NULL on error.

SEE ALSO
RAND_add(3), RAND_bytes(3), RAND(7)

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 <https://www.openssl.org/source/license.html>.