BIO_SET_CALLBACK(3SSL) OpenSSL BIO_SET_CALLBACK(3SSL)
NAME
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_debug_callback_ex, BIO_callback_fn_ex,
BIO_callback_fn - BIO callback functions
SYNOPSIS
#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);
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_arg(BIO *b, char *arg);
char *BIO_get_callback_arg(const BIO *b);
long BIO_debug_callback_ex(BIO *bio, int oper, const char *argp, size_t len,
int argi, long argl, int ret, size_t *processed);
The following functions have been deprecated since OpenSSL 3.0, and can
be hidden entirely by defining OPENSSL_API_COMPAT with a suitable
version value, see openssl_user_macros(7):
typedef long (*BIO_callback_fn)(BIO *b, int oper, const char *argp, int argi,
long argl, long ret);
void BIO_set_callback(BIO *b, BIO_callback_fn cb);
BIO_callback_fn BIO_get_callback(const BIO *b);
long BIO_debug_callback(BIO *bio, int cmd, const char *argp, int argi,
long argl, long ret);
typedef struct bio_mmsg_cb_args_st {
BIO_MSG *msg;
size_t stride, num_msg;
uint64_t flags;
size_t *msgs_processed;
} BIO_MMSG_CB_ARGS;
DESCRIPTION
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().
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_ex() 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. The BIO_debug_callback() function is the deprecated
version of the same callback for use with the old callback format
BIO_set_callback() function.
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
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
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
BIO_CB_READ, BIO_CB_WRITE, BIO_CB_GETS and BIO_CB_PUTS.
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.
CALLBACK OPERATIONS
In the notes below, callback defers to the actual callback function that
is called.
BIO_free(b)
callback_ex(b, BIO_CB_FREE, NULL, 0, 0, 0L, 1L, NULL)
or
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)
or
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,
&readbytes)
or
callback(b, BIO_CB_READ|BIO_CB_RETURN, data, dlen, 0L, retvalue)
after.
BIO_write(b, data, dlen, written)
callback_ex(b, BIO_CB_WRITE, data, dlen, 0, 0L, 1L, NULL)
or
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,
&written)
or
callback(b, BIO_CB_WRITE|BIO_CB_RETURN, data, dlen, 0L, retvalue)
after.
BIO_gets(b, buf, size)
callback_ex(b, BIO_CB_GETS, buf, size, 0, 0L, 1, NULL, NULL)
or
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,
&readbytes)
or
callback(b, BIO_CB_GETS|BIO_CB_RETURN, buf, size, 0L, retvalue)
after.
BIO_puts(b, buf)
callback_ex(b, BIO_CB_PUTS, buf, 0, 0, 0L, 1L, NULL);
or
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)
or
callback(b, BIO_CB_PUTS|BIO_CB_RETURN, buf, 0, 0L, retvalue)
after.
BIO_ctrl(BIO *b, int cmd, long larg, void *parg)
callback_ex(b, BIO_CB_CTRL, parg, 0, cmd, larg, 1L, NULL)
or
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)
or
callback(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, cmd, larg, ret)
after.
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_sendmmsg(BIO *b, BIO_MSG *msg, size_t stride, size_t num_msg,
uint64_t flags, size_t *msgs_processed)
callback_ex(b, BIO_CB_SENDMMSG, args, 0, 0, 0, 1, NULL)
or
callback(b, BIO_CB_SENDMMSG, args, 0, 0, 1)
is called before the call and
callback_ex(b, BIO_CB_SENDMMSG | BIO_CB_RETURN, args, ret, 0, 0, ret, NULL)
or
callback(b, BIO_CB_SENDMMSG | BIO_CB_RETURN, args, ret, 0, 0, ret)
after.
args is a pointer to a BIO_MMSG_CB_ARGS structure containing the
arguments passed to BIO_sendmmsg(). ret is the return value of the
BIO_sendmmsg() call. The return value of BIO_sendmmsg() is altered
to the value returned by the BIO_CB_SENDMMSG | BIO_CB_RETURN call.
BIO_recvmmsg(BIO *b, BIO_MSG *msg, size_t stride, size_t num_msg,
uint64_t flags, size_t *msgs_processed)
See the documentation for BIO_sendmmsg(). BIO_recvmmsg() works
identically except that BIO_CB_RECVMMSG is used instead of
BIO_CB_SENDMMSG.
RETURN VALUES
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
operations.
EXAMPLES
The BIO_debug_callback_ex() function is an example, its source is in
crypto/bio/bio_cb.c
HISTORY
The BIO_debug_callback_ex() function was added in OpenSSL 3.0.
BIO_set_callback(), BIO_get_callback(), and BIO_debug_callback() were
deprecated in OpenSSL 3.0. Use the non-deprecated _ex functions instead.
COPYRIGHT
Copyright 2000-2021 The OpenSSL Project Authors. All Rights Reserved.
Licensed under the Apache License 2.0 (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
<https://www.openssl.org/source/license.html>.
3.5.7 2026-06-09 BIO_SET_CALLBACK(3SSL)
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