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.\"     @(#)recv.2	8.3 (Berkeley) 2/21/94
.\"
.Dd March 18, 2015
.Dt RECV 2
.Os
.Sh NAME
.Nm recv ,
.Nm recvfrom ,
.Nm recvmsg
.Nd receive a message from a socket
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/socket.h
.Ft ssize_t
.Fo recv
.Fa "int socket"
.Fa "void *buffer"
.Fa "size_t length"
.Fa "int flags"
.Fc
.Ft ssize_t
.Fo recvfrom
.Fa "int socket"
.Fa "void *restrict buffer"
.Fa "size_t length"
.Fa "int flags"
.Fa "struct sockaddr *restrict address"
.Fa "socklen_t *restrict address_len"
.Fc
.Ft ssize_t
.Fo recvmsg
.Fa "int socket"
.Fa "struct msghdr *message"
.Fa "int flags"
.Fc
.Sh DESCRIPTION
The
.Fn recvfrom
and
.Fn recvmsg
system calls
are used to receive messages from a socket,
and may be used to receive data on a socket whether or not
it is connection-oriented.
.Pp
If
.Fa address
is not a null pointer
and the socket is not connection-oriented,
the source address of the message is filled in.
The
.Fa address_len
argument
is a value-result argument, initialized to the size of
the buffer associated with
.Fa address ,
and modified on return to indicate the actual size of the
address stored there.
.Pp
The
.Fn recv
function is normally used only on a
.Em connected
socket (see
.Xr connect 2 
or
.Xr connectx 2 )
and is identical to
.Fn recvfrom
with a
null pointer passed as its
.Fa address
argument.
As it is redundant, it may not be supported in future releases.
.Pp
All three routines return the length of the message on successful
completion.
If a message is too long to fit in the supplied buffer,
excess bytes may be discarded depending on the type of socket
the message is received from (see
.Xr socket 2 ) .
.Pp
If no messages are available at the socket, the
receive call waits for a message to arrive, unless
the socket is nonblocking (see
.Xr fcntl 2 )
in which case the value
-1 is returned and the external variable
.Va errno
set to
.Er EAGAIN .
The receive calls normally return any data available,
up to the requested amount,
rather than waiting for receipt of the full amount requested;
this behavior is affected by the socket-level options
.Dv SO_RCVLOWAT
and
.Dv SO_RCVTIMEO
described in
.Xr getsockopt 2 .
.Pp
The
.Xr select 2
system call may be used to determine when more data arrive.
.Pp
If no messages are available to be received and the peer has
performed an orderly shutdown, the value 0 is returned.
.Pp
The
.Fa flags
argument to a
.Fn recv
function is formed by
.Em or Ap ing
one or more of the values:
.Bl -column MSG_WAITALL -offset indent
.It Dv MSG_OOB Ta process out-of-band data
.It Dv MSG_PEEK Ta peek at incoming message
.It Dv MSG_WAITALL Ta wait for full request or error
.El
.Pp
The
.Dv MSG_OOB
flag requests receipt of out-of-band data
that would not be received in the normal data stream.
Some protocols place expedited data at the head of the normal
data queue, and thus this flag cannot be used with such protocols.
The
.Dv MSG_PEEK
flag causes the receive operation to return data
from the beginning of the receive queue without removing that
data from the queue.
Thus, a subsequent receive call will return the same data.
The
.Dv MSG_WAITALL
flag requests that the operation block until
the full request is satisfied.
However, the call may still return less data than requested
if a signal is caught, an error or disconnect occurs,
or the next data to be received is of a different type than that returned.
.Pp
The
.Fn recvmsg
system call uses a
.Fa msghdr
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
struct msghdr {
	void		*msg_name;      /* optional address */
	socklen_t	msg_namelen;    /* size of address */
	struct		iovec *msg_iov; /* scatter/gather array */
	int		msg_iovlen;     /* # elements in msg_iov */
	void		*msg_control;   /* ancillary data, see below */
	socklen_t	msg_controllen; /* ancillary data buffer len */
	int		msg_flags;      /* flags on received message */
};
.Ed
.Pp
Here
.Fa msg_name
and
.Fa msg_namelen
specify the destination address if the socket is unconnected;
.Fa msg_name
may be given as a null pointer if no names are desired or required.
.Pp
The
.Fa msg_iov
and
.Fa msg_iovlen
arguments
describe scatter gather locations, as discussed in
.Xr read 2 .
.Fa msg_iovlen 
shall be set to the dimension of this array. In each 
.Fa iovec 
structure, the 
.Fa iov_base 
field specifies a storage area and the 
.Fa iov_len 
field gives its size in bytes. Each storage area indicated by 
.Fa msg_iov 
is filled with received data in turn until all of the received data 
is stored or all of the areas have been filled.
.Pp
The
.Fa msg_control
argument,
which has length
.Fa msg_controllen ,
points to a buffer for other protocol control related messages
or other miscellaneous ancillary data.
The messages are of the form:
.Bd -literal
struct cmsghdr {
	u_int	cmsg_len;	/* data byte count, including hdr */
	int	cmsg_level;	/* originating protocol */
	int	cmsg_type;	/* protocol-specific type */
/* followed by
	u_char	cmsg_data[]; */
};
.Ed
.Pp
As an example, one could use this to learn of changes
in the data-stream in XNS/SPP,
or in ISO, to obtain user-connection-request data by requesting a
.Fn recvmsg
with no data buffer provided immediately after an
.Fn accept
system call.
.Pp
Open file descriptors are now passed as ancillary data for
.Dv AF_UNIX
domain sockets, with
.Fa cmsg_level
set to
.Dv SOL_SOCKET
and
.Fa cmsg_type
set to
.Dv SCM_RIGHTS .
.Pp
The
.Fa msg_flags
field is set on return according to the message received.
.Dv MSG_EOR
indicates end-of-record;
the data returned completed a record.
.\" (generally used with sockets of type
.\".Dv SOCK_SEQPACKET ) .
.Dv MSG_TRUNC
indicates that
the trailing portion of a datagram was discarded
because the datagram was larger than the buffer supplied.
.Dv MSG_CTRUNC
indicates that some control data were discarded
due to lack of space in the buffer for ancillary data.
.Dv MSG_OOB
is returned to indicate that expedited or out-of-band data were received.
.Sh RETURN VALUES
These calls return the number of bytes received, or -1
if an error occurred.
.Pp
For TCP sockets, the return value 0 means the peer has closed its 
half side of the connection.
.Sh ERRORS
The calls fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EAGAIN
The socket is marked non-blocking, and the receive operation
would block, or
a receive timeout had been set,
and the timeout expired before data were received.
.\" ===========
.It Bq Er EBADF
The argument
.Fa socket
is an invalid descriptor.
.\" ===========
.It Bq Er ECONNRESET
The connection is closed by the peer
during a receive attempt on a socket.
.\" ===========
.It Bq Er EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.\" ===========
.It Bq Er EINTR
The receive was interrupted by delivery of a signal before
any data were available.
.\" ===========
.It Bq Er EINVAL
MSG_OOB is set, but no out-of-band data is available.
.\" ===========
.It Bq Er ENOBUFS
An attempt to allocate a memory buffer fails.
.\" ===========
.It Bq Er ENOTCONN
The socket is associated with a connection-oriented protocol
and has not been connected (see
.Xr connect 2,
.Xr connectx 2,
and
.Xr accept 2 ) .
.\" ===========
.It Bq Er ENOTSOCK
The argument
.Fa socket
does not refer to a socket.
.\" ===========
.It Bq Er EOPNOTSUPP
The type and/or protocol of
.Fa socket
do not support the option(s) specified in
.Fa flags . 
.\" ===========
.It Bq Er ETIMEDOUT
The connection timed out.
.El
.Pp
The
.Fn recvfrom
call may also fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EINVAL
The total of the iov_len values overflows a ssize_t.
.El
.Pp
The
.Fn recvmsg
call may also fail if:
.Bl -tag -width Er
.\" ===========
.It Bq Er EMSGSIZE
The 
.Fa msg_iovlen 
member of the 
.Fa msghdr 
structure pointed to by message is less than or equal to 0, or is greater than 
.Dv IOV_MAX .
.\" ===========
.It Bq Er ENOMEM
Insufficient memory is available.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr socket 2
.Sh HISTORY
The
.Fn recv
function appeared in
.Bx 4.2 .