Send multiple messages to a socket
Synopsis:
#include <sys/types.h>
#include <sys/socket.h>
int sendmmsg( int s,
struct mmsghdr * mmsg,
unsigned int vlen,
unsigned int flags );
Arguments:
- s
- The descriptor for the socket; see
socket().
- mmsg
- A pointer to the array of messages that you want to send.
- vlen
- The length of the mmsg array; limited to 1024 elements.
- 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 sendmmsg() function transmits multiple messages to another socket.
You can use
send()
only when the socket is in a connected state; you can use sendmmsg() at any time.
You can use this function to send multiple messages in the same call using an array of
mmsghdr elements with the following form, as defined in <sys/socket.h>:
struct mmsghdr {
struct msghdr msg_hdr; /* the message to be sent */
unsigned int msg_len; /* number of bytes transmitted */
};
The msg_len member contains the number of bytes sent for each msg_hdr member.
The array has vlen elements, but if there's an error, a number fewer than vlen
may be returned.
For a description of the msghdr structure, see
recvmsg().
No indication of failure to deliver is implicit in a sendmmsg().
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 sendmmsg() 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.
Returns:
The number of messages 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.
- EAFNOSUPPORT
- Addresses in the specified address family can't be used with this socket.
- EAGAIN
- The socket's file descriptor is marked O_NONBLOCK,
and the requested operation would block.
- 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 sendmmsg() before any data was transmitted.
- EINVAL
- The sum of the iov_len values overflows an ssize_t.
- 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.
Classification:
Unix
Safety: |
|
Cancellation point |
Yes |
Interrupt handler |
No |
Signal handler |
No |
Thread |
Yes |