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.Dd March 26, 2015
.Dt CONNECTX 2
.Os Darwin
.Sh NAME
.Nm connectx
.Nd initiate a connection on a socket
.Sh SYNOPSIS
.Fd #include <sys/socket.h>
.Ft int
.Fo connectx
.Fa "int socket"
.Fa "const sa_endpoints_t *endpoints"
.Fa "sae_associd_t associd"
.Fa "unsigned int flags"
.Fa "const struct iovec *iov"
.Fa "unsigned int iovcnt"
.Fa "size_t *len"
.Fa "sae_connid_t *connid"
.Fc
.Sh DESCRIPTION
The parameter
.Fa socket
is a socket.
In general,
.Fn connectx
may be used as a substitute for cases when
.Xr bind 2
and
.Xr connect 2
are issued in succession, as well as a mechanism to transmit data
at connection establishment time.
.Pp
The
.Fn connectx
system call uses a
.Fa sa_endpoints
structure to minimize the number of directly supplied arguments. This structure
has the following form, as defined in
.In sys/socket.h :
.Pp
.Bd -literal
typedef struct sa_endpoints {
	unsigned int     sae_srcif;      /* optional source interface   */
	struct sockaddr *sae_srcaddr;    /* optional source address     */
	socklen_t        sae_srcaddrlen; /* size of source address      */
	struct sockaddr *sae_dstaddr;    /* destination address         */
	socklen_t        sae_dstaddrlen; /* size of destination address */
}sa_endpoints_t;
.Ed
.Pp
When the optional source address
.Fa sae_srcaddr
parameter is specified,
.Fn connectx
binds the connection to the address, as if
.Xr bind 2
is used.  The length of
.Fa sae_srcaddr
buffer is specified by
.Fa sae_srcaddrlen .
.\" This buffer may hold more than one addresses, where each successive address
.\" immediately follows the previous one.
The source address can be obtained by calling
.Xr getifaddrs 3 .
.Pp
The optional parameter
.Fa sae_srcif
may also be specified, in order to force the connection to use the interface
whose interface index equals to
.Fa sae_srcif .
The value for
.Fa sae_srcif
may be obtained by issuing a call to
.Xr if_nametoindex 3 .
If only
.Fa sae_srcif
is specified, the communication domain will choose a source address on that
interface for communicating to the peer socket.  Both
.Fa sae_srcaddr
and
.Fa sae_srcif
parameters may also be specified in order to add more constraints to the connection, and
.Fn connectx
will fail unless the address is currently assigned to that interface.
.Pp
A destination address must be specified in the
.Fa sae_dstaddr
parameter.  The
.Fa sae_dstaddrlen
specifies the length of that buffer.
.\" When more than one addresses
.\" is specified, each successive address immediately follows the previous one.
.\" Each communication domain interprets the
.\" .Fa sae_srcaddr
.\" and
.\" .Fa sae_dstaddr
.\" parameters in its own way.
.\" When multiple addresses are specified, one of the addresses will be chosen.
.\" The rules used in selecting the eligible addresses as well as their address family requirements vary between communication domains.
.\" .Pp
.\" Changes related to the connection state may be monitored by registering for the
.\" .Dv NOTE_CONNINFO_UPDATED
.\" .Xr kqueue 2
.\" event, using the predefined system filter
.\" .Dv EVFILT_SOCK .
.\" Details regarding the event may be retrieved by calling
.\" .Xr getconninfo 3 .
.\" .Sh MULTIPATH
.\" On a multipath socket,
.\" .Fn connectx
.\" may be used multiple times, in order to establish the initial session
.\" association with the peer socket upon the first connection, and to further
.\" establish additional connections related to that association on subsequent
.\" ones.
.\" .Pp
.\" The parameter
.\" .Fa associd
.\" specifies the association identifier.  When
.\" .Fn connectx
.\" is initially called to establish an associtation, the association identifier
.\" is not yet known, and
.\" .Dv ASSOCID_ANY
.\" must be specified.  After the initial connection is established, the
.\" association identifier may be retrieved using
.\" .Xr getassocids 3 ,
.\" and the value may then be used on subsequent
.\" .Fn connectx
.\" calls.
.\" .Pp
.\" An association representing one or more connections, or a single connection
.\" may be dissolved by calling
.\" .Xr disconnectx 2 .
.\" .Sh NON-MULTIPATH
.\" On non-multipath socket,
.\" .Fn connectx
.\" behaves much like a combination of
.\" .Xr bind 2
.\" and
.\" .Xr connect 2 .
.Pp
Data to be transmitted may optionally be defined via the
.Fa iovcnt
buffers specified by members of the
.Fa iov
array, along with a non-NULL
.Fa len
parameter, which upon success, indicates the number of bytes enqueued for
transmission.
.Pp
When the
.Fa iov
and
.Fa len 
parameters are non-NULL, the communication domain will copy the data to the
socket send buffer. The communication domain may impose a limit on the amount of data allowed to be buffered before connection establishment.
.Pp
When the flags parameter is set to CONNECT_RESUME_ON_READ_WRITE and an
.Fa iov
is not passed in, the communication domain will trigger the actual connection
establishment upon the first read or write following the
.Xr connectx 2
system call. This flag is ignored if the iov is specified in the
.Xr connectx 2
call itself.
.Pp
The flags parameter may also be set to CONNECT_DATA_IDEMPOTENT to indicate to
the communication domain that the data is idempotent. For example, this will
trigger TCP Fast Open (RFC 7413) with SOCK_STREAM type. The data must be passed in the
.Fa iov 
parameter in
.Xr connectx 2 
, or passed in with the first write call such as with the
.Xr writev 2
or similar system call if the CONNECT_RESUME_ON_READ_WRITE is also set.
.Pp
In general, the communication domain makes the final decision on the amount of
data that may get transmitted at connection establishment time.  If the socket
requires the data be sent atomically and the data size makes this impossible,
EMSGSIZE will be returned and the state of the socket is left unchanged as if
.Fn connectx
was not called.
.Pp
The parameter
.Fa associd
is reserved for future use, and must always be set to
.Dv SAE_ASSOCID_ANY .
The parameter
.Fa connid
is also reserved for future use and should be set to NULL.
.Sh NOTES
.Fn connectx
is currently supported only on AF_INET and AF_INET6 sockets of type SOCK_DGRAM
and SOCK_STREAM.
.Pp
Generally,
.\" non-multipath
connection-oriented sockets may successfully
.Fn connectx
only once.  Connectionless sockets may use
.Fn connectx
to create an association to the peer socket, and it may call
.Xr disconnectx 2
to dissolve any existing association.  Unlike connection-oriented sockets,
connectionless sockets may call
.Fn connectx
again afterwards to associate to another peer socket.
.Pp
If CONNECT_RESUME_ON_READ_WRITE is set without data 
supplied,
.Fn connectx
will immediately return success, assuming the rest of the parameters are valid.
.Xr select 2
will indicate that the socket is ready for writing, and the actual connection
establishment is attempted once the initial data is written to the socket via
.Xr writev 2
or similar.  Subsequent attempts to write more data will fail until the existing
connection establishment attempt is successful.  The error status of the socket
may be retrieved via the SO_ERROR option using
.Xr getsockopt 2 .
.Sh RETURN VALUES
Upon successful completion, a value of 0 is returned.
.\" and an opaque value may be returned through the
.\" .Fa connid
.\" parameter.  
The number of bytes from
.Fa iov
array which were enqueued for transmission is returned via
.Fa len .
.\" If the initial connection establishes an association with a peer socket, the association identifier may be retrieved by calling
.\" .Xr getassocids 2 .
.\" Both of these identifiers are unique
.\" on a per
.\" .Fa socket
.\" basis.
Upon failure, a value of -1 is returned and the global integer
variable
.Va errno
is set to indicate the error.
.Sh ERRORS
The
.Fn connectx
system call will fail if:
.Bl -tag -width Er
.\" ==========
.It Bq Er EADDRINUSE
The address specified in
.Fa sae_srcaddr
parameter is already in use.
.\" ==========
.It Bq Er EADDRNOTAVAIL
The specified in
.Fa sae_srcaddr
parameter is not available on this machine, or is not assigned to the interface specified by
.Fa sae_srcif .
.\" ==========
.It Bq Er EAFNOSUPPORT
The
.Fa socket
cannot find any usable addresses of a specific address family
as required by the communication domain.
.\" ==========
.It Bq Er EALREADY
A previous connection attempt has not yet been completed.
.\" ==========
.It Bq Er EBADF
.Fa socket
is not a valid descriptor.
.\" ==========
.It Bq Er ECONNREFUSED
The attempt to connect was ignored
(because the target is not listening for connections)
or explicitly rejected.
.\" ==========
.It Bq Er EFAULT
Part of
.Fa iov
or data to be written to
.Fa socket
points outside the process's allocated address space.
.\" ==========
.It Bq Er EHOSTUNREACH
The target host cannot be reached (e.g., down, disconnected).
.\" ==========
.It Bq Er EINPROGRESS
The connection cannot be completed immediately.
It is possible to
.Xr select 2
for completion by selecting the
.Fa socket
for writing.
.\" ==========
.It Bq Er EINTR
Its execution was interrupted by a signal.
.\" ==========
.It Bq Er EMSGSIZE
The size of the message exceeds the available send buffer space in the
.Fa socket .
.\" ==========
.It Bq Er EINVAL
An invalid argument was detected
(e.g.,
.Fa sae_dstaddrlen
is not valid, the contents of
.Fa sae_srcaddr
or
.Fa sae_dstaddr,
buffer is invalid, etc.)
.\" ==========
.It Bq Er EISCONN
The socket is already connected.
.\" ==========
.It Bq Er ENETDOWN
The local network interface is not functioning.
.\" ==========
.It Bq Er ENETUNREACH
The network isn't reachable from this host.
.\" ==========
.It Bq Er ENOBUFS
The system call was unable to allocate a needed memory buffer.
.\" ==========
.It Bq Er ENOTSOCK
.Fa socket
is not a file descriptor for a socket.
.\" ==========
.It Bq Er EOPNOTSUPP
Because
.Fa socket
is listening, no connection is allowed.
.\" ==========
.\".It Bq Er EPROTOTYPE
.\".Fa address
.\"has a different type than the socket
.\"that is bound to the specified peer address.
.\" ==========
.It Bq Er ETIMEDOUT
Connection establishment timed out without establishing a connection.
.\" ==========
.El
.Sh SEE ALSO
.\".Xr accept 2 ,
.Xr bind 2 ,
.Xr connect 2 ,
.Xr disconnectx 2 ,
.Xr disconnectx 2 ,
.Xr getsockopt 2 ,
.\".Xr kqueue 2 ,
.\".Xr shutdown 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.\".Xr getassocids 3 ,
.\".Xr getconnids 3 ,
.\".Xr getconninfo 3 ,
.Xr writev 2 ,
.Xr compat 5
.Sh HISTORY
The
.Fn connectx
function call appeared in Darwin 15.0.0