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.Dd $Mdocdate: April 7 2023 $
.Dt BIO_SET_CALLBACK 3
.Os
.Sh NAME
.Nm BIO_callback_fn_ex ,
.Nm BIO_set_callback_ex ,
.Nm BIO_get_callback_ex ,
.Nm BIO_callback_fn ,
.Nm BIO_set_callback ,
.Nm BIO_get_callback ,
.Nm BIO_set_callback_arg ,
.Nm BIO_get_callback_arg ,
.Nm BIO_debug_callback
.\" The following three macros are intentionally undocumented because
.\" they are unused and would only cause obfuscation if they were used.
.\" .Nm BIO_CB_return
.\" .Nm BIO_cb_pre
.\" .Nm BIO_cb_post
.Nd BIO callback functions
.Sh SYNOPSIS
.In openssl/bio.h
.Ft typedef long
.Fo (*BIO_callback_fn_ex)
.Fa "BIO *b"
.Fa "int oper"
.Fa "const char *argp"
.Fa "size_t len"
.Fa "int argi"
.Fa "long argl"
.Fa "int ret"
.Fa "size_t *processed"
.Fc
.Ft void
.Fo BIO_set_callback_ex
.Fa "BIO *b"
.Fa "BIO_callback_fn_ex cb_ex"
.Fc
.Ft BIO_callback_fn_ex
.Fo BIO_get_callback_ex
.Fa "const BIO *b"
.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
.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 *pointer"
.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
.Sh DESCRIPTION
.Fn BIO_set_callback_ex
and
.Fn 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.
.Pp
.Fn BIO_set_callback
and
.Fn BIO_get_callback
are deprecated functions that set and retrieve the old-style BIO callback,
which is only used if no new-style callback is set with
.Fn BIO_set_callback_ex .
.Pp
.Fn BIO_set_callback_arg
stores the
.Fa pointer
internally in
.Fa b
and
.Fn BIO_get_callback_arg
retrieves it from
.Fa b .
The name of these two functions is badly misleading: the
.Fa pointer
is never passed as an argument to any callback function.
But of course, callback functions can call
.Fn BIO_get_callback_arg
and access the pointer, just like any other code can.
.Pp
.Fn BIO_debug_callback
is a standard debugging callback which prints
out information related to each BIO operation.
If
.Fn BIO_set_callback_arg
was called with a
.Pf non- Dv NULL
argument, information is sent to the BIO pointed to by the
.Fa pointer ;
otherwise, standard error output is used.
.Pp
The arguments of the callback functions are as follows:
.Bl -tag -width Ds
.It Fa b
The BIO the callback is attached to.
.It Fa oper
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 .
.It Fa argp , argi , argl
The meaning of these three arguments depends on the value of
.Fa oper ,
that is on the operation being performed.
.It Fa len
The length of the data requested to be read or written.
This is only useful if
.Fa oper
is
.Dv BIO_CB_READ ,
.Dv BIO_CB_WRITE ,
or
.Dv BIO_CB_GETS .
.It Fa ret
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.
.It Fa processed
The location pointed to is updated with the number of bytes
actually read or written.
Only used for
.Dv BIO_CB_READ ,
.Dv BIO_CB_WRITE ,
.Dv BIO_CB_GETS ,
and
.Dv BIO_CB_PUTS .
.El
.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.
.Pp
The callbacks are called as follows:
.Bl -tag -width 1n
.It \&In Fn BIO_free "BIO *b" :
.Bd -literal
before the free operation:
cb_ex(b, BIO_CB_FREE, NULL, 0, 0, 0, 1, NULL)
or cb(b, BIO_CB_FREE, NULL,    0, 0, 1)
.Ed
.It \&In Fn BIO_read "BIO *b" "void *out" "int outl" :
.Bd -literal
before the read operation:
cb_ex(b, BIO_CB_READ, out, outl, 0, 0, 1, NULL)
or cb(b, BIO_CB_READ, out, outl,    0, 1)

after the read operation:
cb_ex(b, BIO_CB_READ|BIO_CB_RETURN, out, outl, 0, 0, ret, &bytes)
or cb(b, BIO_CB_READ|BIO_CB_RETURN, out, outl,    0, ret)
.Ed
.It \&In Fn BIO_write "BIO *b" "const void *in" "int inl" :
.Bd -literal
before the write operation:
cb_ex(b, BIO_CB_WRITE, in, inl, 0, 0, 1, NULL)
or cb(b, BIO_CB_WRITE, in, inl,    0, 1)

after the write operation:
cb_ex(b, BIO_CB_WRITE|BIO_CB_RETURN, in, inl, 0, 0, ret, &bytes)
or cb(b, BIO_CB_WRITE|BIO_CB_RETURN, in, inl,    0, ret)
.Ed
.It \&In Fn BIO_gets "BIO *b" "char *out" "int outl" :
.Bd -literal
before the read operation:
cb_ex(b, BIO_CB_GETS, out, outl, 0, 0, 1, NULL)
or cb(b, BIO_CB_GETS, out, outl,    0, 1)

after the read operation:
cb_ex(b, BIO_CB_GETS|BIO_CB_RETURN, out, outl, 0, 0, ret, &bytes)
or cb(b, BIO_CB_GETS|BIO_CB_RETURN, out, outl,    0, ret)
.Ed
.It \&In Fn BIO_puts "BIO *b" "const char *in" :
.Bd -literal
before the write operation:
cb_ex(b, BIO_CB_PUTS, in, 0, 0, 0, 1, NULL)
or cb(b, BIO_CB_PUTS, in,    0, 0, 1)

after the write operation:
cb_ex(b, BIO_CB_PUTS|BIO_CB_RETURN, in, 0, 0, 0, ret, &bytes)
or cb(b, BIO_CB_PUTS|BIO_CB_RETURN, in,    0, 0, ret)
.Ed
.It \&In Fn BIO_ctrl "BIO *b" "int cmd" "long larg" "void *parg" :
.Bd -literal
before the control operation:
cb_ex(b, BIO_CB_CTRL, parg, 0, cmd, larg, 1, NULL)
or cb(b, BIO_CB_CTRL, parg,    cmd, larg, 1)

after the control operation:
cb_ex(b, BIO_CB_CTRL|BIO_CB_RETURN, parg, 0, cmd, larg, ret, NULL)
or cb(b, BIO_CB_CTRL|BIO_CB_RETURN, parg,    cmd, larg, ret)
.Ed
.It \&In Fn BIO_callback_ctrl "BIO *b" "int cmd" "BIO_info_cb *fp" :
.Bd -literal
before the control operation:
cb_ex(b, BIO_CB_CTRL, fp, 0, cmd, 0, 1, NULL)
or cb(b, BIO_CB_CTRL, fp,    cmd, 0, 1)

after the control operation:
cb_ex(b, BIO_CB_CTRL|BIO_CB_RETURN, fp, 0, cmd, 0, ret, NULL)
or cb(b, BIO_CB_CTRL|BIO_CB_RETURN, fp,    cmd, 0, ret)
.Ed
.El
.Sh RETURN VALUES
.Fn BIO_get_callback_ex
returns a pointer to the function
.Fa cb_ex
previously installed with
.Fn BIO_set_callback_cb ,
or
.Dv NULL
if no such callback was installed.
.Pp
.Fn BIO_get_callback
returns a pointer to the function
.Fa cb
previously installed with
.Fn BIO_set_callback ,
or
.Dv NULL
if no such callback was installed.
.Pp
.Fn BIO_get_callback_arg
returns the
.Fa pointer
previously set with
.Fn BIO_set_callback_arg ,
or
.Dv NULL
if no such pointer 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.
These functions have been available since
.Ox 2.4 .
.Pp
.Fn BIO_callback_fn
first appeared in OpenSSL 1.1.0.
.Fn BIO_callback_fn_ex ,
.Fn BIO_set_callback_ex ,
and
.Fn BIO_get_callback_ex
first appeared in OpenSSL 1.1.1.
These functions have been available since
.Ox 7.1 .