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BIO_set_callback(3)			     OpenSSL			      BIO_set_callback(3)

       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 relat-
       ing 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

       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 oper-
       ation 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 call-
	   back(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 call-
	   back(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 call-
	   back(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 crypto/bio/bio_cb.c


0.9.7a					    2000-09-14			      BIO_set_callback(3)
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