| RECV(2) | System Calls Manual | RECV(2) | 
recv, recvfrom,
  recvmsg, recvmmsg —
#include <sys/socket.h>
ssize_t
  
  recv(int
    s, void *buf,
    size_t len,
    int flags);
ssize_t
  
  recvfrom(int
    s, void * restrict
    buf, size_t len,
    int flags,
    struct sockaddr * restrict
    from, socklen_t *
    restrict fromlen);
ssize_t
  
  recvmsg(int
    s, struct msghdr
    *msg, int
  flags);
int
  
  recvmmsg(int
    s, struct mmsghdr
    *mmsg, unsigned int
    vlen, unsigned int
    flags, struct timespec
    *timeout);
recvfrom(), recvmsg() and
  recvmmsg() 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.
If from is non-nil, and the socket is not connection-oriented, the source address of the message is filled in. fromlen is a value-result parameter, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there.
The recvmmsg() call can be used to receive
    multiple messages in the same call using an array of
    mmsghdr elements with the following form, as defined
    in <sys/socket.h>:
struct mmsghdr {
	struct msghdr	msg_hdr;	/* the message to be sent */
	unsigned int	msg_len;	/* number of bytes received */
};
The msg_len member contains the number of
    bytes received for each msg_hdr member. The array has
    vlen elements, which is limited to
    1024. If there is an error, a number fewer than
    vlen may be returned, and the error may be retrieved
    using getsockopt(2) with
    SO_ERROR. If the flag
    MSG_WAITFORONE is set in flags
    then the recvmmsg() call will wait for one message,
    and set MSG_DONTWAIT for the rest. If the
    timeout parameter is not NULL,
    then recvmmsg() will return if that time is
    exceeded.
The recv() call is normally used only on a
    connected socket (see
    connect(2)) and is identical
    to recvfrom() with a nil from
    parameter. As it is redundant, it may not be supported in future
  releases.
recv(), recvfrom()
    and recvmsg() routines return the length of the
    message on successful completion. recvmmsg() returns
    the number of messages received. 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
    socket(2)).
If no messages are available at the socket, the receive call waits
    for a message to arrive, unless the socket is nonblocking (see
    fcntl(2)) in which case the
    value -1 is returned and the external variable errno
    set to EAGAIN. If no data is available and the
    remote peer was shut down, 0 is returned. 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 SO_RCVLOWAT and
    SO_RCVTIMEO described in
    getsockopt(2).
The select(2) or poll(2) call may be used to determine when more data arrives.
The flags argument to a recv call is formed by or'ing one or more of the values:
| MSG_CMSG_CLOEXEC | set the close on exec property for passed file descriptors | 
| MSG_OOB | process out-of-band data | 
| MSG_PEEK | peek at incoming message | 
| MSG_WAITALL | wait for full request or error | 
The 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
    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
    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 recvmsg() call uses a
    msghdr structure to minimize the number of directly
    supplied parameters. This structure has the following form, as defined in
    <sys/socket.h>:
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 */
};
Here msg_name and msg_namelen specify the source address if the socket is unconnected; msg_name may be given as a null pointer if no names are desired or required. If the socket is connected, msg_name and msg_namelen are ignored. msg_iov and msg_iovlen describe scatter gather locations, as discussed in read(2). msg_control, which has length msg_controllen, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form:
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[]; */
};
Open file descriptors are now passed as ancillary data for
    AF_LOCAL domain sockets, with
    cmsg_level set to SOL_SOCKET
    and cmsg_type set to
    SCM_RIGHTS.
The msg_flags field is set on return
    according to the message received. MSG_EOR indicates
    end-of-record; the data returned completed a record (generally used with
    sockets of type SOCK_SEQPACKET).
    MSG_TRUNC indicates that the trailing portion of a
    datagram was discarded because the datagram was larger than the buffer
    supplied. MSG_CTRUNC indicates that some control
    data were discarded due to lack of space in the buffer for ancillary data.
    MSG_OOB is returned to indicate that expedited or
    out-of-band data were received.
recv(), recvfrom() and
  recvmsg() calls return the number of bytes received,
  or -1 if an error occurred. For connected sockets whose remote peer was shut
  down, 0 is returned when no more data is available. The
  recvmmsg() call returns the number of messages
  received, or -1 if an error occurred.
EAGAIN]EBADF]EFAULT]EINTR]EINVAL]ENOBUFS]ENOTCONN]ENOTSOCK]recvmsg() will also fail if:
EMSGSIZE]IOV_MAX}.recv() function call appeared in
  4.2BSD. The recvmmsg()
  function call appeared in Linux 2.6.32 and NetBSD 7.0.
| March 19, 2018 | NetBSD 10.0 |