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PROLOG |NAME |SYNOPSIS |DESCRIPTION |RETURN VALUE |ERRORS |EXAMPLES |APPLICATION USAGE |RATIONALE |FUTURE DIRECTIONS |SEE ALSO |COPYRIGHT | |
SENDMSG(3P) POSIX Programmer's ManualSENDMSG(3P)This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on Linux.
sendmsg — send a message on a socket using a message structure
#include <sys/socket.h> ssize_t sendmsg(intsocket, const struct msghdr *message, intflags);
Thesendmsg() function shall send a message through a connection- mode or connectionless-mode socket. If the socket is a connectionless-mode socket, the message shall be sent to the address specified bymsghdrif no pre-specified peer address has been set. If a peer address has been pre-specified, either the message shall be sent to the address specified inmsghdr (overriding the pre-specified peer address), or the function shall return -1 and seterrno to[EISCONN]. If the socket is connection-mode, the destination address inmsghdrshall be ignored. Thesendmsg() function takes the following arguments:socket Specifies the socket file descriptor.message Points to amsghdrstructure, containing both the destination address and the buffers for the outgoing message. The length and format of the address depend on the address family of the socket. Themsg_flags member is ignored.flags Specifies the type of message transmission. The application may specify 0 or the following flag: MSG_EOR Terminates a record (if supported by the protocol). MSG_OOB Sends out-of-band data on sockets that support out-of-bound data. The significance and semantics of out-of- band data are protocol-specific. MSG_NOSIGNAL Requests not to send the SIGPIPE signal if an attempt to send is made on a stream-oriented socket that is no longer connected. The[EPIPE]error shall still be returned. Themsg_iov andmsg_iovlen fields ofmessage specify zero or more buffers containing the data to be sent.msg_iov points to an array ofiovecstructures;msg_iovlen shall be set to the dimension of this array. In eachiovecstructure, theiov_base field specifies a storage area and theiov_len field gives its size in bytes. Some of these sizes can be zero. The data from each storage area indicated bymsg_iov is sent in turn. Successful completion of a call tosendmsg() does not guarantee delivery of the message. A return value of -1 indicates only locally-detected errors. If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does not have O_NONBLOCK set, thesendmsg() function shall block until space is available. If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does have O_NONBLOCK set, thesendmsg() function shall fail. If the socket protocol supports broadcast and the specified address is a broadcast address for the socket protocol,sendmsg() shall fail if the SO_BROADCAST option is not set for the socket. The socket in use may require the process to have appropriate privileges to use thesendmsg() function.
Upon successful completion,sendmsg() shall return the number of bytes sent. Otherwise, -1 shall be returned anderrno set to indicate the error.
Thesendmsg() function shall fail if:EAGAINorEWOULDBLOCK The socket's file descriptor is marked O_NONBLOCK and the requested operation would block.EAFNOSUPPORT Addresses in the specified address family cannot be used with this socket.EBADFThesocket argument is not a valid file descriptor.ECONNRESET A connection was forcibly closed by a peer.EINTRA signal interruptedsendmsg() before any data was transmitted.EINVALThe sum of theiov_len values overflows anssize_t.EMSGSIZE The message is too large to be sent all at once (as the socket requires), or themsg_iovlen member of themsghdr structure pointed to bymessage is less than or equal to 0 or is greater than {IOV_MAX}.ENOTCONN The socket is connection-mode but is not connected.ENOTSOCK Thesocket argument does not refer to a socket.EOPNOTSUPP Thesocket argument is associated with a socket that does not support one or more of the values set inflags.EPIPEThe 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 or SOCK_SEQPACKET and the MSG_NOSIGNAL flag is not set, the SIGPIPE signal is generated to the calling thread. If the address family of the socket is AF_UNIX, thensendmsg() shall fail if:EIOAn I/O error occurred while reading from or writing to the file system.ELOOPA loop exists in symbolic links encountered during resolution of the pathname in the socket address.ENAMETOOLONG The length of a component of a pathname is longer than {NAME_MAX}.ENOENTA component of the pathname does not name an existing file or the path name is an empty string.ENOTDIR A component of the path prefix of the pathname in the socket address names an existing file that is neither a directory nor a symbolic link to a directory, or the pathname in the socket address contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory. Thesendmsg() function may fail if:EACCESSearch permission is denied for a component of the path prefix; or write access to the named socket is denied.EDESTADDRREQ The socket is not connection-mode and does not have its peer address set, and no destination address was specified.EHOSTUNREACH The destination host cannot be reached (probably because the host is down or a remote router cannot reach it).EIOAn I/O error occurred while reading from or writing to the file system.EISCONN A destination address was specified and the socket is already connected.ENETDOWN The local network interface used to reach the destination is down.ENETUNREACH No route to the network is present.ENOBUFS Insufficient resources were available in the system to perform the operation.ENOMEMInsufficient memory was available to fulfill the request. If the address family of the socket is AF_UNIX, thensendmsg() may fail if:ELOOPMore than {SYMLOOP_MAX} symbolic links were encountered during resolution of the pathname in the socket address.ENAMETOOLONG The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.The following sections are informative.Done.
Theselect() andpoll() functions can be used to determine when it is possible to send more data.
None.
None.
getsockopt(3p),poll(3p),pselect(3p),recv(3p),recvfrom(3p),recvmsg(3p),send(3p),sendto(3p),setsockopt(3p),shutdown(3p),socket(3p) The Base Definitions volume of POSIX.1‐2017,sys_socket.h(0p)
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online athttp://www.opengroup.org/unix/online.html . Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, seehttps://www.kernel.org/doc/man-pages/reporting_bugs.html .IEEE/The Open Group 2017SENDMSG(3P)Pages that refer to this page:netinet_in.h(0p), sys_socket.h(0p), pselect(3p), recv(3p), recvfrom(3p), recvmsg(3p), send(3p), sendto(3p), socket(3p)
HTML rendering created 2025-09-06 byMichael Kerrisk, author ofThe Linux Programming Interface. For details of in-depthLinux/UNIX system programming training courses that I teach, lookhere. Hosting byjambit GmbH. | |