sendmsg() — Send messages on a socket

X/Open:

#define _XOPEN_SOURCE_EXTENDED 1
#include <sys/socket.h>

ssize_t sendmsg(int socket, struct msghdr *msg, int flags);

Berkeley sockets:

#define _OE_SOCKETS
#include <sys/socket.h>

int sendmsg(int socket, struct msghdr *msg, int flags);

The sendmsg() function sends messages on a socket with a socket descriptor passed in an array of message headers.

Parameter

Description

socket

The socket descriptor.

msg

An array of message headers from which messages are sent.

flags

Specifying one or more of the following flags. If more than one flag is specified, the logical OR operator ( | ) must be used to separate them.

MSG_OOB

Sends out-of-band data on the socket. Only SOCK_STREAM sockets support out-of-band data. The out-of-band data is a single byte.

Before out-of-band data can be sent between two programs, there must be some coordination of effort. If the data is intended to not be read inline, the recipient of the out-of-band data must specify the recipient of the SIGURG signal that is generated when the out-of-band data is sent. If no recipient is set, no signal is sent. The recipient is set by setting the action parameter of the fcntl() function to F_SETOWN and specifying either a PID or GID. For more information on setting a recipient for out-of-band data, see fcntl() — Control open file descriptors.

The recipient of the data determines whether to receive out-of-band data inline or not inline by the setting of the SO_OOBINLINE socket option using the setsockopt() function. For more information on receiving out-of-band data, see setsockopt() — Set options associated with a socket, recv() — Receive data on a socket, recvfrom() — Receive messages on a socket, and recvmsg() — Receive messages on a socket and store in an array of message headers.

MSG_DONTROUTE

The SO_DONTROUTE socket option is turned on for the duration of the operation. This flag is typically used by diagnostic or routing programs.

A message header is defined by the msghdr structure, which can be found in the sys/socket.h header file and contains the following elements:

Element

Description

msg_iov

An array of iovec buffers containing the message.

msg_iovlen

The number of elements in the msg_iov array.

msg_name

An optional pointer to the buffer containing the recipient's address.

msg_namelen

The size of the address buffer.

caddr_t msg_accrights

Access rights sent or received (ignored if specified by the user). This field is ignored by z/OS® UNIX services.

int msg_accrightslen

Length of access rights data (ignored if specified by the user). This field is ignored by z/OS UNIX services.

msg_control

Ancillary data.

msg_controllen

Ancillary data buffer length.

msg_flags

Flags on received message.

The sys/socket.h header file defines the cmsghdr structure that includes at least the following elements:

Element

Description

cmsg_len

Data byte count, including header.

cmsg_level

Originating protocol.

cmsg_type

Protocol-specific type.

The following ancillary data are available at the IPv4 level:

Ancillary data

Description

IP_PKTINFO

(RAW and UDP) Specifies the interface packets are sent over and the IP address used as the packet source IP. The data is passed in an in_pktinfo structure as defined in netinet/in.h.

The following ancillary data are available at the IPv6 level:

Ancillary data

Description

IPV6_HOPLIMIT

(RAW, TCP, and UDP) Specifies the maximum hop limit for an outgoing packet. The data is passed in a structure as defined in netinet/in.h.

IPV6_PATHMTU

(RAW and UDP) Specifies the path MTU value for the destination of a connected socket. The data is passed in a structure as defined in netinet/in.h.

IPV6_PKTINFO

(RAW and UDP) Specifies the interface packets are sent over and the IP address used as the packet source IP. The data is passed in an in6_pktinfo structure as defined in netinet/in.h.

The following ancillary data are available at the socket level:

Ancillary data

Description

SCM_RIGHTS

Specifies the data array that contains the access rights to be sent or received. This ancillary data is valid only for the AF_UNIX domain. The data is passed in a structure as defined in sys/socket.h.

The sys/socket.h header file defines the following macros to gain access to the data arrays in the ancillary data associated with a message header:

CMSG_DATA(cmsg)

If the argument is a pointer to a cmsghdr structure, this macro returns an unsigned character pointer to the data array associated with the cmsghdr structure.

CMSG_NXTHDR(mhdr,cmsg)

If the first argument is a pointer to a msghdr structure and the second argument is a pointer to a cmsghdr structure in the ancillary data (pointed to by the msg_control field of that msghdr structure), this macro returns a pointer to the next cmsghdr structure or a NULL pointer if this structure is the last cmsghdr structure in the ancillary data.

CMSG_FIRSTHDR(mhdr)

If the argument is a pointer to a msghdr structure, this macro returns a pointer to the first cmsghdr structure in the ancillary data associated with this msghdr structure, or a NULL pointer if there is no ancillary data associated with the msghdr structure.

If unsuccessful, sendmsg() returns -1 and sets errno to one of the following values:

Error Code

Description

EADDRNOTAVAIL

The ipi6_addr is not available for use on the ipi6_ifindex interface.

EAFNOSUPPORT

The address family is not supported (it is not AF_UNIX, AF_INET, or AF_INET6).

EBADF

socket is not a valid socket descriptor.

ECONNREFUSED

The attempt to connect was rejected.

ECONNRESET

A connection was forcibly closed by a peer.

EFAULT

Using msg would result in an attempt to access storage outside the caller's address space.

EHOSTUNREACH

No route to the destination exists over the interface specified by ifi6_index.

EINTR

A signal interrupted sendmsg() before any data was transmitted.

EINVAL

msg_namelength is not the size of a valid address for the specified address family.

EIO

There has been a network or transport failure.

EMSGSIZE

The message was too big to be sent as a single datagram. The default is large-envelope-size. (Envelopes are used to hold datagrams and fragments during TCP/IP processing. Large envelopes hold UDP datagrams greater than 2KB while they are processed for output, and when they are waiting for an application program to receive them on input.)

ENETDOWN

The interface specified by ipi6_ifindex is not enabled for IPv6 use.

ENOBUFS

Buffer space is not available to send the message.

ENOTCONN

The socket is not connected.

ENOTSOCK

The descriptor is for a file, not for a socket.

ENXIO

The interface specified by ipi6_ifindex does not exist.

EOPNOTSUPP

The socket argument is associated with a socket that does not support one or more of the values set in flags.

EPIPE

For a connected stream socket the connection to the peer socket has been lost. A SIGPIPE signal is sent to the calling process.

EWOULDBLOCK

socket is in nonblocking mode and no data buffers are available or the SO_SNDTIMEO timeout value was reached before buffers became available.

The following are for AF_UNIX only:

Error Code

Description

EACCES

Search permission is denied for a component of the path prefix, or write access to the named socket is denied.

EIO

An I/O error occurred while reading from or writing to the file system.

ELOOP

Too many symbolic links were encountered in translating the pathname in the socket address.

ENAMETOOLONG

A component of a pathname exceeded NAME_MAX characters, or an entire pathname exceeded PATH_MAX characters.

ENOENT

A component of the pathname does not name an existing file or the pathname is an empty string.

ENOTDIR

A component of the path prefix of the pathname in the socket address is not a directory.