BIO_set_callback(3)

     BIO_set_callback(3)    0.9.6h (2000-09-14)    BIO_set_callback(3)

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

     SYNOPSIS
           #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);

     DESCRIPTION
          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

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     BIO_set_callback(3)    0.9.6h (2000-09-14)    BIO_set_callback(3)

          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 OPERATIONS
          BIO_free(b)
              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.

     EXAMPLE
          The BIO_debug_callback() function is a good example, its
          source is in crypto/bio/bio_cb.c

     SEE ALSO
          TBA

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