.\" Copyright (c) 2018 Yubico AB. All rights reserved.
.\" Use of this source code is governed by a BSD-style
.\" license that can be found in the LICENSE file.
.\"
.Dd $Mdocdate: August 11 2020 $
.Dt FIDO_DEV_SET_IO_FUNCTIONS 3
.Os
.Sh NAME
.Nm fido_dev_set_io_functions
.Nd FIDO 2 device I/O interface
.Sh SYNOPSIS
.In fido.h
.Bd -literal
typedef void *fido_dev_io_open_t(const char *);
typedef void  fido_dev_io_close_t(void *);
typedef int   fido_dev_io_read_t(void *, unsigned char *, size_t, int);
typedef int   fido_dev_io_write_t(void *, const unsigned char *, size_t);
typedef int   fido_dev_io_rx_t(struct fido_dev *, uint8_t, unsigned char *, size_t, int);
typedef int   fido_dev_io_tx_t(struct fido_dev *, uint8_t, const unsigned char *, size_t);

typedef struct fido_dev_io {
	fido_dev_io_open_t  *open;
	fido_dev_io_close_t *close;
	fido_dev_io_read_t  *read;
	fido_dev_io_write_t *write;
	fido_dev_io_rx_t    *rx;
	fido_dev_io_tx_t    *tx;
} fido_dev_io_t;
.Ed
.Ft int
.Fn fido_dev_set_io_functions "fido_dev_t *dev" "const fido_dev_io_t *io"
.Sh DESCRIPTION
The
.Nm
interface defines the I/O and transmission handlers used to talk to
.Fa dev .
Its usage is optional.
By default,
.Em libfido2
will use the operating system's native HID interface to talk CTAP2 to
a FIDO device.
.Pp
A
.Vt fido_dev_io_open_t
function is expected to return a non-NULL opaque pointer on success,
and NULL on error.
The returned opaque pointer is never dereferenced by
.Em libfido2 .
.Pp
A
.Vt fido_dev_io_close_t
function receives the opaque handle obtained from
.Vt fido_dev_io_open_t .
It is not expected to be idempotent.
.Pp
A
.Vt fido_dev_io_read_t
function reads a single HID report from
.Fa dev .
The first parameter taken is the opaque handle obtained from
.Vt fido_dev_io_open_t .
The read buffer is pointed to by the second parameter, and the
third parameter holds its size.
The last argument passed to
.Vt fido_dev_io_read_t
is the number of milliseconds the caller is willing to sleep,
should the call need to block.
If this value holds -1,
.Vt fido_dev_io_read_t
may block indefinitely.
The number of bytes read is returned.
On error, -1 is returned.
.Pp
A
.Vt fido_dev_io_write_t
function writes a single HID report to
.Fa dev .
The first parameter taken is the opaque handle returned by
.Vt fido_dev_io_open_t .
The write buffer is pointed to by the second parameter, and the
third parameter holds its size.
A
.Vt fido_dev_io_write_t
function may block.
The number of bytes written is returned.
On error, -1 is returned.
.Pp
A
.Vt fido_dev_io_rx_t
function receives a complete CTAP2 message from
.Fa dev .
The first parameter taken is a pointer to
.Fa dev .
The second parameter holds the expected CTAP2 command byte.
The read buffer is pointed to by the third parameter, and the
fourth parameter holds its size.
The last argument passed to
.Vt fido_dev_io_rx_t
is the number of milliseconds the caller is willing to sleep,
should the call need to block.
If this value holds -1,
.Vt fido_dev_io_rx_t
may block indefinitely.
The number of bytes read is returned.
On error, -1 is returned.
.Pp
A
.Vt fido_dev_io_tx_t
function transmits a complete CTAP2 message to
.Fa dev .
The first parameter taken is a pointer to
.Fa dev .
The second parameter holds the CTAP2 command byte.
The write buffer is pointed to by the third parameter, and the
fourth parameter holds its size.
A
.Vt fido_dev_io_tx_t
function may block.
On success, 0 is returned.
On error, -1 is returned.
.Pp
When calling
.Fn fido_dev_set_io_functions ,
the
.Fa open ,
.Fa close ,
.Fa read
and
.Fa write
fields of
.Fa io
may not be NULL.
Either
.Fa rx
or
.Fa tx
may be NULL, in which case
.Em libfido2
uses its corresponding CTAP2 HID transport method.
.Pp
No references to
.Fa io
are held by
.Fn fido_dev_set_io_functions .
.Sh RETURN VALUES
On success,
.Fn fido_dev_set_io_functions
returns
.Dv FIDO_OK .
On error, a different error code defined in
.In fido/err.h
is returned.