Manpage of SEND


Section: Linux Programmer's Manual (2)
Updated: 2016-03-15


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


#include <sys/types.h>#include <sys/socket.h>ssize_t send(int sockfd, const void *buf, size_t len, int flags);ssize_t sendto(int sockfd, const void *buf, 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 connectedstate (so that the intended recipient is known). The only difference between send() and write(2) is the presence of flags. With a zero flagsargument, 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 sockfdis the file descriptor of the sending socket.

If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the arguments dest_addrand addrlenare ignored (and the error EISCONNmay be returned when they are not NULL and 0), and the error ENOTCONNis returned when the socket was not actually connected. Otherwise, the address of the target is given by dest_addrwith addrlenspecifying its size. For sendmsg(), the address of the target is given by msg.msg_name, with msg.msg_namelenspecifying its size.

For send() and sendto(), the message is found in bufand 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 EMSGSIZEis 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 EAGAINor EWOULDBLOCKin this case. The select(2) call may be used to determine when it is possible to send more data.  

The flags argument

The flagsargument 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). Only valid on SOCK_DGRAMand SOCK_RAWsockets and currently implemented only for IPv4 and IPv6. See arp(7) for details.
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, EAGAINor EWOULDBLOCKis returned. This provides similar behavior to setting the O_NONBLOCKflag (via the fcntl(2) F_SETFLoperation), but differs in that MSG_DONTWAITis a per-call option, whereas O_NONBLOCKis a setting on the open file description (see open(2)), which will affect all threads in the calling process and 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_CORKsocket 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_CORKsocket option described in udp(7).)

MSG_NOSIGNAL (since Linux 2.2)
Don't generate a SIGPIPEsignal if the peer on a stream-oriented socket has closed the connection. The EPIPEerror is still returned. This provides similar behavior to using sigaction(2) to ignore SIGPIPE, but, whereas MSG_NOSIGNALis a per-call feature, ignoring SIGPIPEsets a process attribute that affects all threads in the process.
Sends out-of-banddata on sockets that support this notion (e.g., of type SOCK_STREAM); the underlying protocol must also support out-of-banddata.


The definition of the msghdrstructure 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_namefield is used on an unconnected socket to specify the target address for a datagram. It points to a buffer containing the address; the msg_namelenfield 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_iovand msg_iovlenfields specify scatter-gather locations, as for writev(2).

You may send control information using the msg_controland msg_controllenmembers. 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).

The msg_flagsfield is ignored.  


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


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.
(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(7).)

(For UDP sockets) An attempt was made to send to a network/broadcast address as though it was a unicast address.

The socket is marked nonblocking and the requested operation would block. POSIX.1-2001 allows either error to be returned for this case, and does not require these constants to have the same value, so a portable application should check for both possibilities.
(Internet domain datagram sockets) The socket referred to by sockfdhad not previously been bound to an address and, upon attempting to bind it to an ephemeral port, it was determined that all port numbers in the ephemeral port range are currently in use. See the discussion of /proc/sys/net/ipv4/ip_local_port_rangein ip(7).
sockfdis not a valid open file descriptor.
Connection reset by peer.
The socket is not connection-mode, and no peer address is set.
An invalid user space address was specified for an argument.
A signal occurred before any data was transmitted; see signal(7).
Invalid argument passed.
The connection-mode socket was connected already but a recipient was specified. (Now either this error is returned, or the recipient specification is ignored.)
The socket type requires that message be sent atomically, and the size of the message to be sent made this impossible.
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.)
No memory available.
The socket is not connected, and no target has been given.
The file descriptor sockfddoes not refer to a socket.
Some bit in the flagsargument is inappropriate for the socket type.
The local end has been shut down on a connection oriented socket. In this case, the process will also receive a SIGPIPEunless MSG_NOSIGNALis set.


4.4BSD, SVr4, POSIX.1-2001. These interfaces first appeared in 4.2BSD.

POSIX.1-2001 describes only the MSG_OOBand MSG_EORflags. POSIX.1-2008 adds a specification of MSG_NOSIGNAL. The MSG_CONFIRMflag is a Linux extension.  


According to POSIX.1-2001, the msg_controllenfield of the msghdrstructure should be typed as socklen_t, but glibc currently types it as size_t.

See sendmmsg(2) for information about a Linux-specific system call that can be used to transmit multiple datagrams in a single call.  


Linux may return EPIPEinstead of ENOTCONN.  


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), socket(7), tcp(7), udp(7)



The flags argument

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