HP Open Source Security for OpenVMS Volume 2: HP SSL for OpenVMS > CRYPTO Application
Programming Interface (API) Reference
BIO_set_callback, BIO_get_callback, BIO_set_callback_arg, BIO_get_callback_arg, BIO_debug_callback — BIO callback functions
#define BIO_set_callback_arg(b,arg) ((b)->cb_arg=(char
#define BIO_get_callback_arg(b) ((b)->cb_arg)
BIO_debug_callback(BIO *bio,int cmd,const char *argp,int argi, long
argl,long ret); typedef long callback(BIO *b,
const char *argp,
int argi, long argl, long retvalue);
BIO_set_callback() and BIO_get_callback() set and retrieve
the BIO callback, they are both macros. The callback is called during
most high level BIO operations. It can be used for debugging purposes
to trace operations on a BIO or to modify its operation.
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
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
The meaning of the arguments argp, argi and argl depends
on the value of oper, that is the operation
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.
callback(b, BIO_CB_FREE, NULL, 0L, 0L, 1L) is called before
the free operation.
BIO_read(b, out, outl)
callback(b, BIO_CB_READ, out, outl, 0L, 1L) is called before
the read and callback(b, BIO_CB_READ|BIO_CB_RETURN, out,
outl, 0L, retvalue) after.
BIO_write(b, in, inl)
callback(b, BIO_CB_WRITE, in, inl, 0L, 1L) is called before
the write and callback(b, BIO_CB_WRITE|BIO_CB_RETURN, in,
inl, 0L, retvalue) after.
BIO_gets(b, out, outl)
callback(b, BIO_CB_GETS, out, outl, 0L, 1L) is called before
the operation and callback(b, BIO_CB_GETS|BIO_CB_RETURN,
out, outl, 0L, retvalue) after.
callback(b, BIO_CB_WRITE, in, 0, 0L, 1L) is called before
the operation and callback(b, BIO_CB_WRITE|BIO_CB_RETURN,
in, 0, 0L, retvalue) after.
BIO_ctrl(BIO *b, int cmd, long larg, void
callback(b,BIO_CB_CTRL,parg,cmd,larg,1L) is called before
the call and callback(b,BIO_CB_CTRL|BIO_CB_RETURN,parg,cmd,
The BIO_debug_callback() function is a good example, its source
is in crypto/bio/bio_cb.c