spi_cmdread()

The spi_cmdread() function sends a command to, and then reads data from, a specific device on SPI bus. The prototype for this function is:

int spi_cmdread( int fd,
                 uint32_t device,
                 void *cbuf,
                 int16_t clen,
                 void *rbuf,
                 int rlen );

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

cbuf

A pointer to the command buffer.

clen

The command length, in bytes.

rbuf

A pointer to the receive buffer.

rlen

The read length, in bytes.

The function returns the number of bytes of data that it successfully read. 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 read.