BIO_get_callback man page on MacOSX

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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 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


1.0.1f				  2014-01-06		   BIO_set_callback(3)

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