sendmsg

Writes data to a socket using a message structure.

Library:LibC
Standard:BSD
Service:Networking

Syntax

  #include <sys/socket.h>
  
  ssize_t sendmsg (
    int                   s,
    const struct msghdr  *msg,
    int                   flags);

Parameters

s

(IN) Specifies the socket file handle of a socket to which the data is to be written.

msg

(IN) Points to the message to be sent.

flags

(IN) Specifies the type of message transmission. See Message Types.

Return Values

If successful, returns the number of characters sent. Otherwise, returns -1 and sets errno to one of the following:

Decimal

Constant

Description

4

EBADF

An invalid handle was specified.

35

EWOULDBLOCK

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

38

ENOTSOCK

The s parameter specifies a file, not a socket handle.

40

EMSGSIZE

The socket requires that a message be sent atomically. The size of the message to be sent made it impossible.

55

ENOBUFS

The system could not allocate an internal buffer. The operation might succeed if retried when buffers become available.

Remarks

The sendmsg function can send messages to both connection and connectionless sockets.

The function can be used on SOCK_STREAM sockets. When called on a blocking SOCK_STREAM socket, the function blocks until all of the client’s data can be sent or buffered by the socket. When called on a nonblocking socket, the function sends or buffers the maximum amount of data that can be handled without blocking and returns the amount that was taken. If no data is taken, it returns -1, indicating an EWOULDBLOCK error.

Only TCP sockets support out-of-band transmission (accomplished by placing a new urgent boundary just before the last byte of the sent message).

If the message is too long to pass atomically through the underlying protocol, EMSGSIZE is returned in errno, and the message is not transmitted.

A successful completion of a call to sendmsg does not guarantee delivery of the message. A return value of -1 only indicates locally detected errors.

See Also