.\" $OpenBSD: BIO_set_callback.3,v 1.9 2018/03/29 20:42:17 schwarze Exp $ .\" full merge up to: OpenSSL 99d63d46 Oct 26 13:56:48 2016 -0400 .\" selective merge up to: OpenSSL 61f805c1 Jan 16 01:01:46 2018 +0800 .\" .\" This file is a derived work. .\" The changes are covered by the following Copyright and license: .\" .\" Copyright (c) 2018 Ingo Schwarze .\" .\" Permission to use, copy, modify, and distribute this software for any .\" purpose with or without fee is hereby granted, provided that the above .\" copyright notice and this permission notice appear in all copies. .\" .\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES .\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF .\" MERCHANTABILITY AND FITNESS. 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IN NO EVENT SHALL THE OpenSSL PROJECT OR .\" ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, .\" SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; .\" LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, .\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) .\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED .\" OF THE POSSIBILITY OF SUCH DAMAGE. .\" .Dd $Mdocdate: March 29 2018 $ .Dt BIO_SET_CALLBACK 3 .Os .Sh NAME .Nm BIO_set_callback , .Nm BIO_get_callback , .Nm BIO_set_callback_arg , .Nm BIO_get_callback_arg , .Nm BIO_debug_callback .Nd BIO callback functions .Sh SYNOPSIS .In openssl/bio.h .Ft void .Fo BIO_set_callback .Fa "BIO *b" .Fa "BIO_callback_fn cb" .Fc .Ft BIO_callback_fn .Fo BIO_get_callback .Fa "BIO *b" .Fc .Ft void .Fo BIO_set_callback_arg .Fa "BIO *b" .Fa "char *arg" .Fc .Ft char * .Fo BIO_get_callback_arg .Fa "const BIO *b" .Fc .Ft long .Fo BIO_debug_callback .Fa "BIO *bio" .Fa "int oper" .Fa "const char *argp" .Fa "int argi" .Fa "long argl" .Fa "long ret" .Fc .Ft typedef long .Fo "(*BIO_callback_fn)" .Fa "BIO *b" .Fa "int oper" .Fa "const char *argp" .Fa "int argi" .Fa "long argl" .Fa "long ret" .Fc .Sh DESCRIPTION .Fn BIO_set_callback and .Fn BIO_get_callback 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. .Pp .Fn BIO_set_callback_arg and .Fn BIO_get_callback_arg set and retrieve an argument for use in the callback. .Pp .Fn 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. .Pp .Fn BIO_callback_fn is the type of the callback function. The meaning of each argument is described below. .Pp The BIO the callback is attached to is passed in .Fa b . .Pp .Fa 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 .Fa oper or'ed with .Dv BIO_CB_RETURN . .Pp The meaning of the arguments .Fa argp , .Fa argi and .Fa argl depends on the value of .Fa oper (i.e. the operation being performed). .Pp When .Fa oper does not include .Dv BIO_CB_RETURN , i.e. when the callback is invoked before an operation, the value passed into the callback via .Fa ret is always 1. In this case, if the callback returns a negative value, the library aborts the requested operation and instead returns the negative return value from the callback to the application. If the callback returns a non-negative value, that return value is ignored by the library, and the operation is performed normally. .Pp When .Fa oper includes .Dv BIO_CB_RETURN , i.e. when the callback is invoked after an operation, the value passed into the callback via .Fa ret is the return value that the operation would return to the application if no callback were present. When a callback is present, the operation only passes this value to the callback and instead of it returns the return value of the callback to the application. .Pp The callback should normally simply return .Fa ret when it has finished processing, unless it specifically wishes to abort the operation or to modify the value returned to the application. .Ss Callback operations .Bl -tag -width Ds .It Fn BIO_free b .Fn callback b BIO_CB_FREE NULL 0L 0L 1L is called before the free operation. .It Fn BIO_read b out outl .Fn callback b BIO_CB_READ out outl 0L 1L is called before the read and .Fn callback b BIO_CB_READ|BIO_CB_RETURN out outl 0L ret after. .It Fn BIO_write b in inl .Fn callback b BIO_CB_WRITE in inl 0L 1L is called before the write and .Fn callback b BIO_CB_WRITE|BIO_CB_RETURN in inl 0L ret after. .It Fn BIO_gets b out outl .Fn callback b BIO_CB_GETS out outl 0L 1L is called before the operation and .Fn callback b BIO_CB_GETS|BIO_CB_RETURN out outl 0L ret after. .It Fn BIO_puts b in .Fn callback b BIO_CB_PUTS in 0 0L 1L is called before the operation and .Fn callback b BIO_CB_PUTS|BIO_CB_RETURN in 0 0L ret after. .It Fn BIO_ctrl b oper larg parg .Fn callback b BIO_CB_CTRL parg oper larg 1L is called before the call and .Fn callback b BIO_CB_CTRL|BIO_CB_RETURN parg oper larg ret after. .El .Sh RETURN VALUES .Fn BIO_get_callback returns a pointer to the function .Fa cb previously installed with .Fn BIO_set_callback , or .Dv NULL if no callback was installed. .Pp .Fn BIO_get_callback_arg returns a pointer to the .Fa arg previously set with .Fn BIO_set_callback_arg , or .Dv NULL if no such argument was set. .Pp .Fn BIO_debug_callback returns .Fa ret if the bit .Dv BIO_CB_RETURN is set in .Fa cmd , or 1 otherwise. .Sh EXAMPLES The .Fn BIO_debug_callback function is a good example. Its source is in the file .Pa crypto/bio/bio_cb.c . .Sh SEE ALSO .Xr BIO_new 3 .Sh HISTORY .Fn BIO_set_callback , .Fn BIO_get_callback , .Fn BIO_set_callback_arg , and .Fn BIO_debug_callback first appeared in SSLeay 0.6.0. .Fn BIO_get_callback_arg first appeared in SSLeay 0.8.0. All these functions have been available since .Ox 2.4 .