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# send() - Unix, Linux System Call

# NAME

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

# SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>

ssize_t send(int s, const void *buf, size_t len, int flags);
ssize_t sendto(int s, const void *buf, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);
ssize_t sendmsg(int s, const struct msghdr *msg, int flags);

# DESCRIPTION

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() is the presence of flags. With zero flags parameter, send() is equivalent to write(). Also, send(s,buf,len,flags) is equivalent to sendto(s,buf,len,flags,NULL,0).

The parameter s is the file descriptor of the sending socket.

If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the parameters to and tolen 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 to with tolen 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 non-blocking I/O mode. In non-blocking mode it would return EAGAIN in this case. The select(2) call may be used to determine when it is possible to send more data.

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

Tag | Description
--- | ---
MSG_CONFIRM (Linux 2.3+ only)
| 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 neighbour (e.g. via a unicast ARP). Only valid on SOCK_DGRAM and SOCK_RAW sockets and currently only implemented for IPv4 and IPv6. See arp(7) for details.
MSG_DONTROUTE
| Don’t use a gateway to send out the packet, only send to hosts on directly connected networks. This is usually used only by diagnostic or routing programs. This is only defined for protocol families that route; packet sockets don’t.
MSG_DONTWAIT
| Enables non-blocking operation; if the operation would block, EAGAIN is returned (this can also be enabled using the O_NONBLOCK with the F_SETFL fcntl(2)).
MSG_EOR
| 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 only transmitted when a call is performed that does not specify this flag. (See also the UDP_CORK socket option described in udp(7).)
MSG_NOSIGNAL
| Requests not to send SIGPIPE on errors on stream oriented sockets when the other end breaks the connection. The EPIPE error is still returned.
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.

The definition of the msghdr structure follows. See recv(2) and below for an exact description of its fields.

    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 */
        socklen_t     msg_controllen; /* ancillary data buffer len */
        int           msg_flags;      /* flags on received message */
    };

You may send control information using the msg_control and msg_controllen members. The maximum control buffer length the kernel can process is limited per socket by the net.core.optmem_max sysctl; see socket(7).

# RETURN VALUE

On success, these calls return the number of characters sent. On error, -1 is returned, and errno is set appropriately.

# ERRORS

These are some standard errors generated by the socket layer. Additional errors may be generated and returned from the underlying protocol modules; see their respective manual pages.

Tag | Description
--- | ---
EACCES | (For Unix domain sockets, which are identified by pathname) Write permission is denied on the destination socket file, or search permission is denied for one of the directories the path prefix. (See path_resolution(2).)
EAGAIN or EWOULDBLOCK
| The socket is marked non-blocking and the requested operation would block.
EBADF | An invalid descriptor was specified.
ECONNRESET
| Connection reset by peer.
EDESTADDRREQ
| The socket is not connection-mode, and no peer address is set.
EFAULT | An invalid user space address was specified for a parameter.
EINTR | A signal occurred before any data was transmitted.
EINVAL | Invalid argument passed.
EISCONN
| The connection-mode socket was connected already but a recipient was specified. (Now either this error is returned, or the recipient specification is ignored.)
EMSGSIZE
| The socket type requires that message be sent atomically, and the size of the message to be sent made this impossible.
ENOBUFS
| 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. (Normally, this does not occur in Linux. Packets are just silently dropped when a device queue overflows.)
ENOMEM | No memory available.
ENOTCONN
| The socket is not connected, and no target has been given.
ENOTSOCK
| The argument s is not a socket.
EOPNOTSUPP
| Some bit in the flags argument is inappropriate for the socket type.
EPIPE | The local end has been shut down on a connection oriented socket. In this case the process will also receive a SIGPIPE unless MSG_NOSIGNAL is set.

# CONFORMING TO

4.4BSD, SVr4, POSIX.1-2001. These function calls appeared in 4.2BSD.

POSIX.1-2001 only describes the MSG_OOB and MSG_EOR flags. The MSG_CONFIRM flag is a Linux extension.

# NOTES

The prototypes given above follow the Single Unix Specification, as glibc2 also does; the flags argument was ‘int’ in 4.x BSD, but ‘unsigned int’ in libc4 and libc5; the len argument was ‘int’ in 4.x BSD and libc4, but ‘size_t’ in libc5; the tolen argument was ‘int’ in 4.x BSD and libc4 and libc5. See also accept(2).

According to POSIX.1-2001, the msg_controllen field of the msghdr structure should be typed as socklen_t, but glibc currently (2.4) types it as size_t.

# BUGS

Linux may return EPIPE instead of ENOTCONN.

# SEE ALSO

- fcntl (2)

- getsockopt (2)

- recv (2)

- select (2)

- sendfile (2)

- shutdown (2)

- socket (2)

- write (2)

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