| SEND(2) | System Calls Manual | SEND(2) | 
send, sendto,
  sendmsg, sendmmsg —
#include <sys/socket.h>
ssize_t
  
  send(int
    s, const void *msg,
    size_t len,
    int flags);
ssize_t
  
  sendto(int
    s, const void *msg,
    size_t len,
    int flags,
    const struct sockaddr
    *to, socklen_t
    tolen);
ssize_t
  
  sendmsg(int
    s, const struct msghdr
    *msg, int
  flags);
int
  
  sendmmsg(int
    s, struct mmsghdr
    *mmsg, unsigned int
    vlen, unsigned int
    flags);
send(), sendto(),
  sendmsg(), and sendmmsg() are
  used to transmit a message to another socket. send()
  may be used only when the socket is in a connected state,
  while sendto(), sendmsg() and
  sendmmsg() may be used at any time.
The sendmmsg() call can be used to send
    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 transmitted */
};
The msg_len member contains the number of
    bytes sent 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.
The address of the target is given by to,
    with tolen specifying its size. The length of the
    message is given by len. 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.
If no messages space is available at the socket to hold the
    message to be transmitted, then send() normally
    blocks, unless the socket has been placed in non-blocking I/O mode. The
    select(2) or
    poll(2) call may be used to
    determine when it is possible to send more data. Unfortunately this does not
    work when the interface queue which is used to send the message is full, and
    the call returns ENOBUFS.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x0001 /* process out-of-band data */ #define MSG_PEEK 0x0002 /* peek at incoming message */ #define MSG_DONTROUTE 0x0004 /* bypass routing, use direct interface */ #define MSG_EOR 0x0008 /* data completes record */ #define MSG_NOSIGNAL 0x0400 /* do not generate SIGPIPE on EOF */
The flag MSG_OOB is used to send
    “out-of-band” data on sockets that support this notion (e.g.
    SOCK_STREAM); the underlying protocol must also
    support “out-of-band” data. MSG_EOR is
    used to indicate a record mark for protocols which support the concept.
    MSG_DONTROUTE is usually used only by diagnostic or
    routing programs.
See recv(2) for a
    description of the msghdr structure.
    MSG_NOSIGNAL is used to prevent
    SIGPIPE generation when writing a socket that may be
    closed.
send(), sendto(), and
  sendmsg() calls return the number of characters sent,
  or -1 if an error occurred. The sendmmsg() call
  returns the number of messages sent, or -1 if an error occured.
send(), sendto(),
  sendmsg(), and sendmmsg() fail
  if:
EACCES]EAFNOSUPPORT]EAGAIN|EWOULDBLOCK]EBADF]EDSTADDRREQ]EFAULT]EHOSTDOWN]EHOSTUNREACH]EINVAL]EMSGSIZE]ENOBUFS]An alternative reason: the output queue for a network interface was full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion.
ENOTSOCK]EPIPE]sendto() will also fail if:
EISCONN]sendmsg() and
    sendmmsg() will also fail if:
EMSGSIZE]IOV_MAX}.send() function call appeared in
  4.2BSD. The sendmmsg()
  function call appeared in Linux 3.0 and NetBSD 7.0.
| June 22, 2012 | NetBSD 9.4 |