FIDO_DEV_SET_IO_FUNCTIONS(3) | Library Functions Manual | FIDO_DEV_SET_IO_FUNCTIONS(3)

# NAME

`fido_dev_set_io_functions`, `fido_dev_set_sigmask`, `fido_dev_set_timeout`, `fido_dev_set_transport_functions`, `fido_dev_io_handle` — FIDO2 device I/O interface

# SYNOPSIS

library “libfido2” libcbor libcrypto libz
`#include <fido.h>`

    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 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_t;

    #ifdef _WIN32
    typedef int fido_sigset_t;
    #else
    typedef sigset_t fido_sigset_t;
    #endif

    typedef int   fido_dev_rx_t(struct fido_dev *,
                      uint8_t, unsigned char *, size_t, int);
    typedef int   fido_dev_tx_t(struct fido_dev *,
                      uint8_t, const unsigned char *, size_t);

    typedef struct fido_dev_transport {
    	fido_dev_rx_t *rx;
    	fido_dev_tx_t *tx;
    } fido_dev_transport_t;

int
`fido_dev_set_io_functions`(fido_dev_t *dev, const fido_dev_io_t *io);

int
`fido_dev_set_sigmask`(fido_dev_t *dev, const fido_sigset_t *sigmask);

int
`fido_dev_set_timeout`(fido_dev_t *dev, int ms);

int
`fido_dev_set_transport_functions`(fido_dev_t *dev, const fido_dev_transport_t *t);

void *
`fido_dev_io_handle`(const fido_dev_t *dev);

# DESCRIPTION

The `fido_dev_set_io_functions`() function sets the I/O handlers used by libfido2 to talk to dev. By default, these handlers are set to the operating system's native HID or NFC interfaces. They are defined as follows:

fido_dev_open_t
  Receives a const char * holding a path and opens the corresponding device, returning a non-NULL opaque pointer on success and NULL on error.
fido_dev_close_t
  Receives the opaque pointer returned by fido_dev_open_t and closes the device.
fido_dev_read_t
  Reads a single transmission unit (HID report, APDU) from a device. The first parameter is the opaque pointer returned by fido_dev_open_t. The second parameter is the read buffer, and the third parameter is the read buffer size. The fourth parameter is the number of milliseconds the caller is willing to sleep, should the call need to block. If this value holds -1, fido_dev_read_t may block indefinitely. On success, the number of bytes read is returned. On error, -1 is returned.
fido_dev_write_t
  Writes a single transmission unit (HID report, APDU) to dev. The first parameter is the opaque pointer returned by fido_dev_open_t. The second parameter is the write buffer, and the third parameter is the number of bytes to be written. A fido_dev_write_t may block. On success, the number of bytes written is returned. On error, -1 is returned.

When calling `fido_dev_set_io_functions`(), the open, close, read, and write fields of io may not be NULL.

No references to io are held by `fido_dev_set_io_functions`().

The `fido_dev_set_sigmask`() function may be used to specify a non-NULL signal mask sigmask to be used while libfido2's default I/O handlers wait on dev. On UNIX-like operating systems, fido_sigset_t is defined as sigset_t. On Windows, fido_sigset_t is defined as int and `fido_dev_set_sigmask`() is a no-op.

No references to sigmask are held by `fido_dev_set_sigmask`().

The `fido_dev_set_timeout`() function informs libfido2 not to block for more than ms milliseconds while communicating with dev. If a timeout occurs, the corresponding fido_dev_* function will fail with `FIDO_ERR_RX`. If ms is -1, then libfido2 may block indefinitely. This is the default behaviour. When using the Windows Hello backend, ms is used as a guidance and may be overwritten by the platform.

The `fido_dev_set_transport_functions`() function sets the transport functions used by libfido2 to talk to dev. While the I/O handlers are responsible for sending and receiving transmission units of initialization and continuation packets already formatted by libfido2, the transport handlers are responsible for sending and receiving the CTAPHID commands and data directly, as defined in the FIDO Client to Authenticator Protocol (CTAP) standard. They are defined as follows:

fido_dev_tx_t
  Receives a device, a CTAPHID command to transmit, a data buffer to transmit, and the length of the data buffer. On success, 0 is returned. On error, -1 is returned.
fido_dev_rx_t
  Receives a device, a CTAPHID command whose response the caller expects to receive, a data buffer to receive into, the size of the data buffer determining the maximum length of a response, and the maximum number of milliseconds to wait for a response. On success, the number of bytes read into the data buffer is returned. On error, -1 is returned.

When transport functions are specified, libfido2 will use them instead of the `read` and `write` functions of the I/O handlers. However, the I/O handlers must still be specified to open and close the device.

The `fido_dev_io_handle`() function returns the opaque pointer returned by the `open` function of the I/O handlers. This is useful mainly for the transport functions, which unlike the I/O handlers are passed the fido_dev_t pointer instead of the opaque I/O handle.

# RETURN VALUES

On success, `fido_dev_set_io_functions`(), `fido_dev_set_transport_functions`(), `fido_dev_set_sigmask`(), and `fido_dev_set_timeout`() return `FIDO_OK`. On error, a different error code defined in `<fido/err.h>` is returned.

# SEE ALSO

fido_dev_info_manifest(3), fido_dev_open(3)

Client to Authenticator Protocol (CTAP), https://fidoalliance.org/specs/fido-v2.1-ps-20210615/fido-client-to-authenticator-protocol-v2.1-ps-20210615.html, FIDO Alliance, 2021-06-15, Proposed Standard, Version 2.1.

July 9, 2025 | Debian