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       BIO_set_callback, BIO_get_callback, BIO_set_callback_arg,
       BIO_get_callback_arg, BIO_debug_callback	- BIO callback functions

	#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() 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 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.

	   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.

       BIO_puts(b, in)
	   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 *parg)
	   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, larg,ret)	after.

       The BIO_debug_callback()	function is a good example, its	source is in


1.1.0-pre5			  2016-09-09		   BIO_SET_CALLBACK(3)


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