SEND(2) BSD System Calls Manual SEND(2)NAME
send, sendmsg, sendto — send a message from a socket
SYNOPSIS
#include <sys/socket.h>
ssize_t
send(int socket, const void *buffer, size_t length, int flags);
ssize_t
sendmsg(int socket, const struct msghdr *message, int flags);
ssize_t
sendto(int socket, const void *buffer, size_t length, int flags,
const struct sockaddr *dest_addr, socklen_t dest_len);
DESCRIPTIONSend(), sendto(), and sendmsg() are used to transmit a message to another
socket. Send() may be used only when the socket is in a connected state,
while sendto() and sendmsg() may be used at any time.
The address of the target is given by dest_addr with dest_len specifying
its size. The length of the message is given by length. 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) call may be used to
determine when it is possible to send more data.
The flags parameter may include one or more of the following:
#define MSG_OOB 0x1 /* process out-of-band data */
#define MSG_DONTROUTE 0x4 /* bypass routing, use direct interface */
The flag MSG_OOB is used to send “out-of-band” data on sockets that sup‐
port this notion (e.g. SOCK_STREAM); the underlying protocol must also
support “out-of-band” data. MSG_DONTROUTE is usually used only by diag‐
nostic or routing programs.
The sendmsg() system call uses a msghdr structure to minimize the number
of directly supplied arguments. The msg_iov and msg_iovlen fields of
message specify zero or more buffers containing the data to be sent.
msg_iov points to an array of iovec structures; msg_iovlen shall be set
to the dimension of this array. In each iovec structure, the iov_base
field specifies a storage area and the iov_len field gives its size in
bytes. Some of these sizes can be zero. The data from each storage area
indicated by msg_iov is sent in turn. See recv(2) for a complete
description of the msghdr structure.
RETURN VALUES
Upon successful completion, the number of bytes which were sent is
returned. Otherwise, -1 is returned and the global variable errno is set
to indicate the error.
ERRORS
The send(), sendmsg(), and sendto() system calls will fail if:
[EACCES] The SO_BROADCAST option is not set on the socket and a
broadcast address is given as the destination.
[EAGAIN] The socket is marked non-blocking and the requested
operation would block.
[EBADF] An invalid descriptor is specified.
[ECONNRESET] A connection is forcibly closed by a peer.
[EFAULT] An invalid user space address is specified for a
parameter.
[EHOSTUNREACH] The destination address specifies an unreachable host.
[EINTR] A signal interrupts the system call before any data is
transmitted.
[EMSGSIZE] The socket requires that message be sent atomically,
and the size of the message to be sent makes this
impossible. IOV_MAX.
[ENETDOWN] The local network interface used to reach the destina‐
tion is down.
[ENETUNREACH] No route to the network is present.
[ENOBUFS] The system is unable to allocate an internal buffer.
The operation may succeed when buffers become avail‐
able.
[ENOBUFS] The output queue for a network interface is full.
This generally indicates that the interface has
stopped sending, but may be caused by transient con‐
gestion.
[ENOTSOCK] The argument socket is not a socket.
[EOPNOTSUPP] socket does not support (some of) the option(s) speci‐
fied in flags.
[EPIPE] The socket is shut down for writing or the socket is
connection-mode and is no longer connected. In the
latter case, and if the socket is of type SOCK_STREAM,
the SIGPIPE signal is generated to the calling thread.
The sendmsg() and sendto() system calls will fail if:
[EAFNOSUPPORT] Addresses in the specified address family cannot be
used with this socket.
[EDESTADDRREQ] The socket is not connection-mode and does not have
its peer address set, and no destination address is
specified.
[EISCONN] A destination address was specified and the socket is
already connected.
[ENOENT] A component of the pathname does not name an existing
file or the path name is an empty string.
[ENOMEM] Insufficient memory is available to fulfill the
request.
[ENOTCONN] The socket is connection-mode, but is not connected.
[ENOTDIR] A component of the path prefix of the pathname in the
socket address is not a directory.
The send() system call will fail if:
[EDESTADDRREQ] The socket is not connection-mode and no peer address
is set.
[ENOTCONN] The socket is not connected or otherwise has not had
the peer pre-specified.
The sendmsg() system call will fail if:
[EINVAL] The sum of the iov_len values overflows an ssize_t.
[EMSGSIZE] The socket requires that message be sent atomically,
and the size of the message to be sent makes this
impossible, or the msg_iovlen member of the msghdr
structure pointed to by message is less than or equal
to 0 or is greater than IOV_MAX.
LEGACY SYNOPSIS
#include <sys/types.h>
#include <sys/socket.h>
The include file <sys/types.h> is necessary.
SEE ALSOfcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2),
compat(5)HISTORY
The send() function call appeared in 4.2BSD.
4.2 Berkeley Distribution February 21, 1994 4.2 Berkeley Distribution
