#include <openssl/bio.h> #define BIO_set_callback(b,cb) ((b)->callback=(cb)) #define BIO_get_callback(b) ((b)->callback) #define BIO_set_callback_arg(b,arg) ((b)->cb_arg=(char *)(arg)) #define BIO_get_callback_arg(b) ((b)->cb_arg) long BIO_debug_callback(BIO *bio,int cmd,const char *argp,int argi, long argl,long ret); typedef long (*callback)(BIO *b, int oper, const char *argp, int argi, long argl, long retvalue);
BIO_set_callback_arg() and BIO_get_callback_arg() are macros which can be used to set and retrieve an argument for use in the callback.
BIO_debug_callback() is a standard debugging callback which prints out information relating to each BIO operation. If the callback argument is set if is interpreted as a BIO to send the information to, otherwise stderr is used.
callback() is the callback function itself. The meaning of each argument is described below.
The BIO the callback is attached to is passed in b.
oper is set to the operation being performed. For some operations the callback is called twice, once before and once after the actual operation, the latter case has oper or'ed with BIO_CB_RETURN.
The meaning of the arguments argp, argi and argl depends on the value of oper, that is the operation being performed.
retvalue is the return value that would be returned to the application if no callback were present. The actual value returned is the return value of the callback itself. In the case of callbacks called before the actual BIO operation 1 is placed in retvalue, if the return value is not positive it will be immediately returned to the application and the BIO operation will not be performed.
The callback should normally simply return retvalue when it has finished processing, unless if specifically wishes to modify the value returned to the application.