---
name: "struct_i2c_msg"
section: "9"
platform: "linux"
package: "linux-manual-3.2"
version: "3.2.78-1"
release: "Debian 7 (Wheezy)"
platforms: ["linux"]
url: "https://new.carta.tech/struct_i2c_msg"
---

# struct\_i2c\_msg(9)

an I2C transaction segment beginning with START

## NAME

struct\_i2c\_msg - an I2C transaction segment beginning with START

## SYNOPSIS

```
struct i2c_msg {
  __u16 addr;
  __u16 flags;
#define I2C_M_TEN		0x0010
#define I2C_M_RD		0x0001
#define I2C_M_NOSTART		0x4000
#define I2C_M_REV_DIR_ADDR	0x2000
#define I2C_M_IGNORE_NAK	0x1000
#define I2C_M_NO_RD_ACK		0x0800
#define I2C_M_RECV_LEN		0x0400
  __u16 len;
  __u8 * buf;
};
```

## MEMBERS

addr

Slave address, either seven or ten bits. When this is a ten bit address, I2C\_M\_TEN must be set in *flags* and the adapter must support I2C\_FUNC\_10BIT\_ADDR.

flags

I2C\_M\_RD is handled by all adapters. No other flags may be provided unless the adapter exported the relevant I2C\_FUNC\_\* flags through **i2c\_check\_functionality**.

len

Number of data bytes in *buf* being read from or written to the I2C slave address. For read transactions where I2C\_M\_RECV\_LEN is set, the caller guarantees that this buffer can hold up to 32 bytes in addition to the initial length byte sent by the slave (plus, if used, the SMBus PEC); and this value will be incremented by the number of block data bytes received.

buf

The buffer into which data is read, or from which it's written.

## DESCRIPTION

An i2c\_msg is the low level representation of one segment of an I2C transaction. It is visible to drivers in the *i2c\_transfer*() procedure, to userspace from i2c-dev, and to I2C adapter drivers through the *i2c\_adapter*.*master\_xfer*() method.

Except when I2C “protocol mangling” is used, all I2C adapters implement the standard rules for I2C transactions. Each transaction begins with a START. That is followed by the slave address, and a bit encoding read versus write. Then follow all the data bytes, possibly including a byte with SMBus PEC. The transfer terminates with a NAK, or when all those bytes have been transferred and ACKed. If this is the last message in a group, it is followed by a STOP. Otherwise it is followed by the next *i2c\_msg* transaction segment, beginning with a (repeated) START.

Alternatively, when the adapter supports I2C\_FUNC\_PROTOCOL\_MANGLING then passing certain *flags* may have changed those standard protocol behaviors. Those flags are only for use with broken/nonconforming slaves, and with adapters which are known to support the specific mangling options they need (one or more of IGNORE\_NAK, NO\_RD\_ACK, NOSTART, and REV\_DIR\_ADDR).

## COPYRIGHT