получить сообщение от сокета (receive a message from a socket)
Пролог (Prolog)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
Имя (Name)
recvmsg — receive a message from a socket
Синопсис (Synopsis)
#include <sys/socket.h>
ssize_t recvmsg(int socket, struct msghdr *message, int flags);
Описание (Description)
The recvmsg() function shall receive a message from a connection-
mode or connectionless-mode socket. It is normally used with
connectionless-mode sockets because it permits the application to
retrieve the source address of received data.
The recvmsg() function takes the following arguments:
socket Specifies the socket file descriptor.
message Points to a msghdr structure, containing both the
buffer to store the source address and the buffers
for the incoming message. The length and format of
the address depend on the address family of the
socket. The msg_flags member is ignored on input, but
may contain meaningful values on output.
flags Specifies the type of message reception. Values of
this argument are formed by logically OR'ing zero or
more of the following values:
MSG_OOB Requests out-of-band data. The
significance and semantics of out-of-band
data are protocol-specific.
MSG_PEEK Peeks at the incoming message.
MSG_WAITALL On SOCK_STREAM sockets this requests that
the function block until the full amount
of data can be returned. The function may
return the smaller amount of data if the
socket is a message-based socket, if a
signal is caught, if the connection is
terminated, if MSG_PEEK was specified, or
if an error is pending for the socket.
The recvmsg() function shall receive messages from unconnected or
connected sockets and shall return the length of the message.
The recvmsg() function shall return the total length of the
message. For message-based sockets, such as SOCK_DGRAM and
SOCK_SEQPACKET, the entire message shall be read in a single
operation. If a message is too long to fit in the supplied
buffers, and MSG_PEEK is not set in the flags argument, the
excess bytes shall be discarded, and MSG_TRUNC shall be set in
the msg_flags member of the msghdr structure. For stream-based
sockets, such as SOCK_STREAM, message boundaries shall be
ignored. In this case, data shall be returned to the user as soon
as it becomes available, and no data shall be discarded.
If the MSG_WAITALL flag is not set, data shall be returned only
up to the end of the first message.
If no messages are available at the socket and O_NONBLOCK is not
set on the socket's file descriptor, recvmsg() shall block until
a message arrives. If no messages are available at the socket and
O_NONBLOCK is set on the socket's file descriptor, the recvmsg()
function shall fail and set errno to [EAGAIN] or [EWOULDBLOCK].
In the msghdr structure, the msg_name member may be a null
pointer if the source address is not required. Otherwise, if the
socket is unconnected, the msg_name member points to a sockaddr
structure in which the source address is to be stored, and the
msg_namelen member on input specifies the length of the supplied
sockaddr structure and on output specifies the length of the
stored address. If the actual length of the address is greater
than the length of the supplied sockaddr structure, the stored
address shall be truncated. If the socket is connected, the
msg_name and msg_namelen members shall be ignored. The msg_iov
and msg_iovlen fields are used to specify where the received data
shall be stored. The msg_iov member points to an array of iovec
structures; the msg_iovlen member shall be set to the dimension
of this array. In each iovec structure, the iov_base field
specifies a storage area and the iov_len field gives its size in
bytes. Each storage area indicated by msg_iov is filled with
received data in turn until all of the received data is stored or
all of the areas have been filled.
Upon successful completion, the msg_flags member of the message
header shall be the bitwise-inclusive OR of all of the following
flags that indicate conditions detected for the received message:
MSG_EOR End-of-record was received (if supported by the
protocol).
MSG_OOB Out-of-band data was received.
MSG_TRUNC Normal data was truncated.
MSG_CTRUNC Control data was truncated.
Возвращаемое значение (Return value)
Upon successful completion, recvmsg() shall return the length of
the message in bytes. If no messages are available to be received
and the peer has performed an orderly shutdown, recvmsg() shall
return 0. Otherwise, -1 shall be returned and errno set to
indicate the error.
Ошибки (Error)
The recvmsg() function shall fail if:
EAGAIN or EWOULDBLOCK
The socket's file descriptor is marked O_NONBLOCK and no
data is waiting to be received; or MSG_OOB is set and no
out-of-band data is available and either the socket's file
descriptor is marked O_NONBLOCK or the socket does not
support blocking to await out-of-band data.
EBADF The socket argument is not a valid open file descriptor.
ECONNRESET
A connection was forcibly closed by a peer.
EINTR This function was interrupted by a signal before any data
was available.
EINVAL The sum of the iov_len values overflows a ssize_t, or the
MSG_OOB flag is set and no out-of-band data is available.
EMSGSIZE
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}.
ENOTCONN
A receive is attempted on a connection-mode socket that is
not connected.
ENOTSOCK
The socket argument does not refer to a socket.
EOPNOTSUPP
The specified flags are not supported for this socket
type.
ETIMEDOUT
The connection timed out during connection establishment,
or due to a transmission timeout on active connection.
The recvmsg() function may fail if:
EIO An I/O error occurred while reading from or writing to the
file system.
ENOBUFS
Insufficient resources were available in the system to
perform the operation.
ENOMEM Insufficient memory was available to fulfill the request.
The following sections are informative.
Примеры (Examples)
None.
Использование в приложениях (Application usage)
The select() and poll() functions can be used to determine when
data is available to be received.
Обоснование (Rationale)
None.
Будущие направления (Future directions)
None.
Смотри также (See also)
poll(3p), pselect(3p), recv(3p), recvfrom(3p), send(3p),
sendmsg(3p), sendto(3p), shutdown(3p), socket(3p)
The Base Definitions volume of POSIX.1‐2017, sys_socket.h(0p)
