send, sendto, sendmsg - send a message on a socket

#include <sys/socket.h>
ssize_t send(int sockfd, const void buf[.len], size_t len, int flags);
ssize_t sendto(int sockfd, const void buf[.len], size_t len, int flags,
 const struct sockaddr *dest_addr, socklen_t addrlen);
ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

The system calls send(), sendto(), and sendmsg() are used to transmit a message to another socket.

The send() call may be used only when the socket is in a connected state (so that the intended recipient is known). The only difference between send() and write(2) is the presence of flags. With a zero flags argument, send() is equivalent to write(2). Also, the following call

send(sockfd, buf, len, flags);

is equivalent to

sendto(sockfd, buf, len, flags, NULL, 0);

The argument sockfd is the file descriptor of the sending socket.

If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the arguments dest_addr and addrlen are ignored (and the error EISCONN may be returned when they are not NULL and 0), and the error ENOTCONN is returned when the socket was not actually connected. Otherwise, the address of the target is given by dest_addr with addrlen specifying its size. For sendmsg(), the address of the target is given by msg.msg_name, with msg.msg_namelen specifying its size.

For send() and sendto(), the message is found in buf and has length len. For sendmsg(), the message is pointed to by the elements of the array msg.msg_iov. The sendmsg() call also allows sending ancillary data (also known as control information).

If the message is too long to pass atomically through the underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted.

No indication of failure to deliver is implicit in a send(). Locally detected errors are indicated by a return value of -1.

When the message does not fit into the send buffer of the socket, send() normally blocks, unless the socket has been placed in nonblocking I/O mode. In nonblocking mode it would fail with the error EAGAIN or EWOULDBLOCK in this case. The select(2) call may be used to determine when it is possible to send more data.

The flags argument

The flags argument is the bitwise OR of zero or more of the following flags.

MSG_CONFIRM (since Linux 2.3.15)

Tell the link layer that forward progress happened: you got a successful reply from the other side. If the link layer doesn't get this it will regularly reprobe the neighbor (e.g., via a unicast ARP). Valid only on SOCK_DGRAM and SOCK_RAW sockets and currently implemented only for IPv4 and IPv6. See arp(7) for details.

MSG_DONTROUTE

Don't use a gateway to send out the packet, send to hosts only on directly connected networks. This is usually used only by diagnostic or routing programs. This is defined only for protocol families that route; packet sockets don't.

MSG_DONTWAIT (since Linux 2.2)

Enables nonblocking operation; if the operation would block, EAGAIN or EWOULDBLOCK is returned. This provides similar behavior to setting the O_NONBLOCK flag (via the fcntl(2) F_SETFL operation), but differs in that MSG_DONTWAIT is a per-call option, whereas O_NONBLOCK is a setting on the open file description (see open(2)), which will affect all threads in the calling process as well as other processes that hold file descriptors referring to the same open file description.

MSG_EOR (since Linux 2.2)

Terminates a record (when this notion is supported, as for sockets of type SOCK_SEQPACKET).

MSG_MORE (since Linux 2.4.4)

The caller has more data to send. This flag is used with TCP sockets to obtain the same effect as the TCP_CORK socket option (see tcp(7)), with the difference that this flag can be set on a per-call basis.

Since Linux 2.6, this flag is also supported for UDP sockets, and informs the kernel to package all of the data sent in calls with this flag set into a single datagram which is transmitted only when a call is performed that does not specify this flag. (See also the UDP_CORK socket option described in udp(7).)

MSG_NOSIGNAL (since Linux 2.2)

Don't generate a SIGPIPE signal if the peer on a stream-oriented socket has closed the connection. The EPIPE error is still returned. This provides similar behavior to using sigaction(2) to ignore SIGPIPE, but, whereas MSG_NOSIGNAL is a per-call feature, ignoring SIGPIPE sets a process attribute that affects all threads in the process.

MSG_OOB

Sends out-of-band data on sockets that support this notion (e.g., of type SOCK_STREAM); the underlying protocol must also support out-of-band data.

MSG_FASTOPEN (since Linux 3.7)

Attempts TCP Fast Open (RFC7413) and sends data in the SYN like a combination of connect(2) and write(2), by performing an implicit connect(2) operation. It blocks until the data is buffered and the handshake has completed. For a non-blocking socket, it returns the number of bytes buffered and sent in the SYN packet. If the cookie is not available locally, it returns EINPROGRESS, and sends a SYN with a Fast Open cookie request automatically. The caller needs to write the data again when the socket is connected. On errors, it sets the same errno as connect(2) if the handshake fails. This flag requires enabling TCP Fast Open client support on sysctl net.ipv4.tcp_fastopen.

Refer to TCP_FASTOPEN_CONNECT socket option in tcp(7) for an alternative approach.

sendmsg()

The definition of the msghdr structure employed by sendmsg() is as follows:

struct msghdr {
    void         *msg_name;       /* Optional address */
    socklen_t     msg_namelen;    /* Size of address */
    struct iovec *msg_iov;        /* Scatter/gather array */
    size_t        msg_iovlen;     /* # elements in msg_iov */
    void         *msg_control;    /* Ancillary data, see below */
    size_t        msg_controllen; /* Ancillary data buffer len */
    int           msg_flags;      /* Flags (unused) */
};

The msg_name field is used on an unconnected socket to specify the target address for a datagram. It points to a buffer containing the address; the msg_namelen field should be set to the size of the address. For a connected socket, these fields should be specified as NULL and 0, respectively.

The msg_iov and msg_iovlen fields specify scatter-gather locations, as for writev(2).

You may send control information (ancillary data) using the msg_control and msg_controllen members. The maximum control buffer length the kernel can process is limited per socket by the value in /proc/sys/net/core/optmem_max; see socket(7). For further information on the use of ancillary data in various socket domains, see unix(7) and ip(7).

The msg_flags field is ignored.

On success, these calls return the number of bytes sent. On error, -1 is returned, and errno is set to indicate the error.

An example of the use of sendto() is shown in getaddrinfo(3).

fcntl(2), getsockopt(2), recv(2), select(2), sendfile(2), sendmmsg(2), shutdown(2), socket(2), write(2), cmsg(3), ip(7), ipv6(7), socket(7), tcp(7), udp(7), unix(7)