Function svn_config_write_auth_data 

pub unsafe extern "C" fn svn_config_write_auth_data(
    hash: *mut apr_hash_t,
    cred_kind: *const c_char,
    realmstring: *const c_char,
    config_dir: *const c_char,
    pool: *mut apr_pool_t,
) -> *mut svn_error_t

Use @a cred_kind and @a realmstring to create or overwrite a file within the ~/.subversion/auth/ area. Write the contents of @a hash into the file. If @a config_dir is not NULL it specifies a directory to read the config overriding all other sources.

Also, add @a realmstring to the file, with key @c SVN_CONFIG_REALMSTRING_KEY. This allows programs (or users) to verify exactly which set credentials live within the file.

The hashtable must contain const char * keys and svn_string_t * values.