sendmsg()

Send a message and its header to a socket

Synopsis:

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

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

Arguments:

s

The descriptor for the socket; see socket().

msg

A pointer to the message that you want to send. For a description of the msghdr structure, see recvmsg().

flags

A combination of the following:

• MSG_OOB — process out-of-band data. Use this bit when you send “out-of-band” data on sockets that support this notion (e.g. SOCK_STREAM). The underlying protocol must also support out-of-band data.
• MSG_DONTROUTE — bypass routing; create a direct interface. You normally use this bit only in diagnostic or routing programs.
• MSG_NOSIGNAL — don't raise a SIGPIPE signal when the other end breaks the connection.

Library:

libsocket

Use the -l socket option to qcc to link against this library.

Description:

The sendmsg() function is used to transmit a message to another socket. You can use send() only when the socket is in a connected state; you can use sendmsg() at any time.

No indication of failure to deliver is implicit in a sendmsg(). Locally detected errors are indicated by a return value of -1.

If no message space is available at the socket to hold the message to be transmitted, then sendmsg() normally blocks, unless the socket has been placed in nonblocking I/O mode. You can use select() to determine when it's possible to send more data.

Returns:

The number of bytes sent, or -1 if an error occurs (errno is set).

Errors:

EACCES

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

EAGAIN

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.

EBADF

An invalid descriptor was specified.

ECONNRESET

A connection was forcibly closed by a peer.

EDESTADDRREQ

The socket isn't connection-mode and doesn't have its peer address set, and no destination address was specified.

EFAULT

An invalid user space address was specified for a parameter.

EHOSTUNREACH

The destination host can't be reached (probably because the host is down, or a remote router can't reach it).

EINTR

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

EINVAL

The sum of the iov_len values overflows an ssize_t.

EIO

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

EISCONN

A destination address was specified and the socket is already connected.

EMSGSIZE

The message is too large to be sent all at once (as the socket requires), 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.

ENETDOWN

The local network interface used to reach the destination is down.

ENETUNREACH

No route to the network is present.

ENOBUFS

The system couldn't allocate an internal buffer. The operation may succeed when buffers become available.

ENOMEM

Insufficient memory was available to fulfill the request.

ENOTCONN

The socket is connection-mode but isn't connected.

ENOTSOCK

The argument s isn't a socket.

EOPNOTSUPP

The s argument is associated with a socket that doesn't support one or more of the values set 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, a SIGPIPE signal is generated to the calling thread.

EWOULDBLOCK

The socket is marked nonblocking and the requested operation would block.

If the address family of the socket is AF_UNIX, sendmsg() fails if:

EIO

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

ELOOP

A loop exists in symbolic links encountered during resolution of the pathname in the socket address, or more than SYMLOOP_MAX symbolic links were encountered during the resolution of the pathname in the socket address.

ENAMETOOLONG

A component of a pathname exceeded NAME_MAX characters, or an entire pathname exceeded PATH_MAX characters, or pathname resolution of a symbolic link produced an intermediate result whose length exceeds PATH_MAX.

ENOENT

A component of the pathname doesn't 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 isn't a directory.

Classification:

POSIX 1003.1

See also:

getsockopt(), ioctl(), recv(), select(), send(), sendto(), socket(), write()