#include <openssl/conf.h> void OPENSSL_config(const char *config_name); void OPENSSL_no_config(void);
OPENSSL_no_config() disables configuration. If called before OPENSSL_config() no configuration takes place.
It is also possible to automatically call OPENSSL_config() when an application calls OPENSSL_add_all_algorithms() by compiling an application with the preprocessor symbol OPENSSL_LOAD_CONF #define'd. In this way configuration can be added without source changes.
The environment variable OPENSSL_CONF can be set to specify the location of the configuration file.
Currently ASN1 OBJECTs and ENGINE configuration can be performed future versions of OpenSSL will add new configuration options.
There are several reasons why calling the OpenSSL configuration routines is advisable. For example new ENGINE functionality was added to OpenSSL 0.9.7. In OpenSSL 0.9.7 control functions can be supported by ENGINEs, this can be used (among other things) to load dynamic ENGINEs from shared libraries (DSOs). However very few applications currently support the control interface and so very few can load and use dynamic ENGINEs. Equally in future more sophisticated ENGINEs will require certain control operations to customize them. If an application calls OPENSSL_config() it doesn't need to know or care about ENGINE control operations because they can be performed by editing a configuration file.
Applications should free up configuration at application closedown by calling CONF_modules_free().
It is however much better than nothing. Applications which need finer control over their configuration functionality should use the configuration functions such as CONF_load_modules() directly.