libc/sys/recv.2

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.\"     @(#)recv.2	8.3 (Berkeley) 2/21/94
.\" $FreeBSD$
.\"
.Dd March 19, 2013
.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/types.h
.In sys/socket.h
.Ft ssize_t
.Fn recv "int s" "void *buf" "size_t len" "int flags"
.Ft ssize_t
.Fn recvfrom "int s" "void *buf" "size_t len" "int flags" "struct sockaddr * restrict from" "socklen_t * restrict fromlen"
.Ft ssize_t
.Fn recvmsg "int s" "struct msghdr *msg" "int flags"
.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 from
is not a null pointer
and the socket is not connection-oriented,
the source address of the message is filled in.
The
.Fa fromlen
argument
is a value-result argument, initialized to the size of
the buffer associated with
.Fa from ,
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 )
and is identical to
.Fn recvfrom
with a
null pointer passed as its
.Fa from
argument.
.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 non-blocking (see
.Xr fcntl 2 )
in which case the value
\-1 is returned and the global variable
.Va errno
is 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 arrives.
.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 ".Dv MSG_CMSG_CLOEXEC" -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
.It Dv MSG_DONTWAIT Ta do not block
.It Dv MSG_CMSG_CLOEXEC Ta set received fds close-on-exec
.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.
The
.Dv MSG_DONTWAIT
flag requests the call to return when it would block otherwise.
If no data is available,
.Va errno
is set to
.Er EAGAIN .
This flag is not available in strict
.Tn ANSI
or C99 compilation mode.
.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 :
.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.
The
.Fa msg_iov
and
.Fa msg_iovlen
arguments
describe scatter gather locations, as discussed in
.Xr read 2 .
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 {
	socklen_t  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 .
The close-on-exec flag on received descriptors is set according to the
.Dv MSG_CMSG_CLOEXEC
flag passed to
.Fn recvmsg .
.Pp
Process credentials can also be passed as ancillary data for
.Dv AF_UNIX
domain sockets using a
.Fa cmsg_type
of
.Dv SCM_CREDS .
In this case,
.Fa cmsg_data
should be a structure of type
.Fa cmsgcred ,
which is defined in
.In sys/socket.h
as follows:
.Bd -literal
struct cmsgcred {
	pid_t	cmcred_pid;		/* PID of sending process */
	uid_t	cmcred_uid;		/* real UID of sending process */
	uid_t	cmcred_euid;		/* effective UID of sending process */
	gid_t	cmcred_gid;		/* real GID of sending process */
	short	cmcred_ngroups;		/* number or groups */
	gid_t	cmcred_groups[CMGROUP_MAX];	/* groups */
};
.Ed
.Pp
If a sender supplies ancillary data with enough space for the above struct
tagged as
.Dv SCM_CREDS
control message type to the
.Fn sendmsg
system call, then kernel will fill in the credential information of the
sending process and deliver it to the receiver.
Since receiver usually has no control over a sender, this method of retrieving
credential information isn't reliable.
For reliable retrieval of remote side credentials it is advised to use the
.Dv LOCAL_CREDS
socket option on the receiving socket.
See
.Xr unix 4
for details.
.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.
.Sh ERRORS
The calls fail if:
.Bl -tag -width Er
.It Bq Er EBADF
The argument
.Fa s
is an invalid descriptor.
.It Bq Er ECONNRESET
The remote socket end is forcibly closed.
.It Bq Er ENOTCONN
The socket is associated with a connection-oriented protocol
and has not been connected (see
.Xr connect 2
and
.Xr accept 2 ) .
.It Bq Er ENOTSOCK
The argument
.Fa s
does not refer to a socket.
.It Bq Er EMSGSIZE
The
.Fn recvmsg
system call
was used to receive rights (file descriptors) that were in flight on the
connection.
However, the receiving program did not have enough free file
descriptor slots to accept them.
In this case the descriptors are
closed, any pending data can be returned by another call to
.Fn recvmsg .
.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 EINTR
The receive was interrupted by delivery of a signal before
any data were available.
.It Bq Er EFAULT
The receive buffer pointer(s) point outside the process's
address space.
.El
.Sh SEE ALSO
.Xr fcntl 2 ,
.Xr getsockopt 2 ,
.Xr read 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr unix 4
.Sh HISTORY
The
.Fn recv
function appeared in
.Bx 4.2 .