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BIO_set_callback(3)                 OpenSSL                BIO_set_callback(3)

       BIO_set_callback_ex, BIO_get_callback_ex, BIO_set_callback,
       BIO_get_callback, BIO_set_callback_arg, BIO_get_callback_arg,
       BIO_debug_callback, BIO_callback_fn_ex, BIO_callback_fn - BIO callback

       libcrypto, -lcrypto

        #include <openssl/bio.h>

        typedef long (*BIO_callback_fn_ex)(BIO *b, int oper, const char *argp,
                                           size_t len, int argi,
                                           long argl, int ret, size_t *processed);
        typedef long (*BIO_callback_fn)(BIO *b, int oper, const char *argp, int argi,
                                        long argl, long ret);

        void BIO_set_callback_ex(BIO *b, BIO_callback_fn_ex callback);
        BIO_callback_fn_ex BIO_get_callback_ex(const BIO *b);

        void BIO_set_callback(BIO *b, BIO_callback_fn cb);
        BIO_callback_fn BIO_get_callback(BIO *b);
        void BIO_set_callback_arg(BIO *b, char *arg);
        char *BIO_get_callback_arg(const BIO *b);

        long BIO_debug_callback(BIO *bio, int cmd, const char *argp, int argi,
                                long argl, long ret);

       BIO_set_callback_ex() and BIO_get_callback_ex() set and retrieve the
       BIO callback. 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() and BIO_get_callback() set and retrieve the old
       format BIO callback. New code should not use these functions, but they
       are retained for backwards compatibility. Any callback set via
       BIO_set_callback_ex() will get called in preference to any set by

       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 it is interpreted as a BIO to send the information to, otherwise
       stderr is used.

       BIO_callback_fn_ex() is the type of the callback function and
       BIO_callback_fn() is the type of the old format callback function. The
       meaning of each argument is described below:

       b   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.

       len The length of the data requested to be read or written. This is
           only useful if oper is BIO_CB_READ, BIO_CB_WRITE or BIO_CB_GETS.

       argp argi argl
           The meaning of the arguments argp, argi and argl depends on the
           value of oper, that is the operation being performed.

           processed is a pointer to a location which will be updated with the
           amount of data that was actually read or written. Only used for

       ret ret 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 ret, 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 ret when it has finished
       processing, unless it specifically wishes to modify the value returned
       to the application.

       In the notes below, callback defers to the actual callback function
       that is called.

            callback_ex(b, BIO_CB_FREE, NULL, 0, 0, 0L, 1L, NULL)


            callback(b, BIO_CB_FREE, NULL, 0L, 0L, 1L)

           is called before the free operation.

       BIO_read_ex(b, data, dlen, readbytes)
            callback_ex(b, BIO_CB_READ, data, dlen, 0, 0L, 1L, NULL)


            callback(b, BIO_CB_READ, data, dlen, 0L, 1L)

           is called before the read and

            callback_ex(b, BIO_CB_READ | BIO_CB_RETURN, data, dlen, 0, 0L, retvalue,


            callback(b, BIO_CB_READ|BIO_CB_RETURN, data, dlen, 0L, retvalue)


       BIO_write(b, data, dlen, written)
            callback_ex(b, BIO_CB_WRITE, data, dlen, 0, 0L, 1L, NULL)


            callback(b, BIO_CB_WRITE, datat, dlen, 0L, 1L)

           is called before the write and

            callback_ex(b, BIO_CB_WRITE | BIO_CB_RETURN, data, dlen, 0, 0L, retvalue,


            callback(b, BIO_CB_WRITE|BIO_CB_RETURN, data, dlen, 0L, retvalue)


       BIO_gets(b, buf, size)
            callback_ex(b, BIO_CB_GETS, buf, size, 0, 0L, 1, NULL, NULL)


            callback(b, BIO_CB_GETS, buf, size, 0L, 1L)

           is called before the operation and

            callback_ex(b, BIO_CB_GETS | BIO_CB_RETURN, buf, size, 0, 0L, retvalue,


            callback(b, BIO_CB_GETS|BIO_CB_RETURN, buf, size, 0L, retvalue)


       BIO_puts(b, buf)
            callback_ex(b, BIO_CB_PUTS, buf, 0, 0, 0L, 1L, NULL);


            callback(b, BIO_CB_PUTS, buf, 0, 0L, 1L)

           is called before the operation and

            callback_ex(b, BIO_CB_PUTS | BIO_CB_RETURN, buf, 0, 0, 0L, retvalue, &written)


            callback(b, BIO_CB_PUTS|BIO_CB_RETURN, buf, 0, 0L, retvalue)


       BIO_ctrl(BIO *b, int cmd, long larg, void *parg)
            callback_ex(b, BIO_CB_CTRL, parg, 0, cmd, larg, 1L, NULL)


            callback(b, BIO_CB_CTRL, parg, cmd, larg, 1L)

           is called before the call and

            callback_ex(b, BIO_CB_CTRL | BIO_CB_RETURN, parg, 0, cmd, larg, ret, NULL)


            callback(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, cmd, larg, ret)


           Note: cmd == BIO_CTRL_SET_CALLBACK is special, because parg is not
           the argument of type BIO_info_cb itself.  In this case parg is a
           pointer to the actual call parameter, see BIO_callback_ctrl.

       BIO_get_callback_ex() and BIO_get_callback() return the callback
       function previously set by a call to BIO_set_callback_ex() and
       BIO_set_callback() respectively.

       BIO_get_callback_arg() returns a char pointer to the value previously
       set via a call to BIO_set_callback_arg().

       BIO_debug_callback() returns 1 or ret if it's called after specific BIO

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

       Copyright 2000-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2020-12-10               BIO_set_callback(3)