spi_xchange()

The spi_xchange() function exchanges data between a specific device and the SPI master. The prototype for this function is:

int spi_xchange( int fd,
                 uint32_t device,
                 void *wbuf,
                 void *rbuf,
                 int len );

The arguments are:

fd

The file descriptor returned by spi_open().

device

The device ID with at most one of the following flags optionally ORed in:

• SPI_DEV_LOCK
• SPI_DEV_UNLOCK

wbuf

A pointer to the send buffer.

rbuf

A pointer to the receive buffer.

len

The length, in bytes, of the data to be exchanged.

The function returns the number of bytes of data that it successfully exchanged between the device and the SPI master. If an error occurred, the function returns -1 and sets errno:

EIO

The write to the device failed, or a hardware error occurred.

EINVAL

The device ID is invalid, or you're trying to unlock a device that isn't locked.

ENOMEM

Insufficient memory.

EPERM

The device is locked by another connection.

An SPI driver typically considers it to be an error if the number of bytes returned by this function isn't the same as the number of bytes it asked the function to exchange.