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 bitwise OR of zero or more of the following:

MSG_DONTWAIT — if no data is available, then instead of blocking, return immediately with the error EAGAIN.
MSG_NOTIFICATION — if the flag is not set, then the function returns data. If a notification has arrived, then the flag is set and it returns a notification in the msg_iov field.
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.
MSG_WAITFORONE — wait for one packet or an error. This flag causes the operation to block until at least one packet is available. With this setting, you can receive as many as vlen packets, but if only one is available, then the function returns one. However, the call may return less data than available under the following scenarios:
- a signal is caught
- an error or disconnect occurs
- the incoming packet is a different data type from the one previously received
This flag turns on MSG_DONTWAIT after the first message has been received.

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 poll() to determine when it's possible to send more data. This behavior applies if the socket was opened as SOCK_STREAM.

You typically use this function on datagram (SOCK_DGRAM) sockets, which use the send buffer only as a length check and place the message in the interface transmit queue. If the interface transmit queue is full, this fails, causing sendmsg() to return -1 and set errno to ENOBUFS.

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.
EOVERFLOW: An attempt was made to send an amount of data that when added to the sizes of the socket send message and socket address length structures exceeds the allowable limit.
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

Safety:
Cancellation point	Yes
Interrupt handler	No
Signal handler	No
Thread	Yes