WSARecv
The Windows
Sockets WSARecv function receives data from a socket.
int WSARecv (
SOCKET s, |
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LPWSABUF lpBuffers, |
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DWORD dwBufferCount, |
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LPDWORD lpNumberOfBytesRecvd, |
|
LPDWORD lpFlags, |
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LPWSAOVERLAPPED lpOverlapped, |
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LPWSAOVERLAPPED_COMPLETION_ROUTINE lpCompletionROUTINE |
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); |
|
Parameters
s
[in] A
descriptor identifying a connected socket.
lpBuffers
[in/out] A
pointer to an array of WSABUF structures. Each WSABUF structure
contains a pointer to a buffer and the length of the buffer.
dwBufferCount
[in] The
number of WSABUF structures in the lpBuffers array.
lpNumberOfBytesRecvd
[out] A
pointer to the number of bytes received by this call if the receive operation
completes immediately.
lpFlags
[in/out] A
pointer to flags.
lpOverlapped
[in] A
pointer to a WSAOVERLAPPED structure (ignored for nonoverlapped sockets).
lpCompletionRoutine
[in] A
pointer to the completion routine called when the receive operation has been completed
(ignored for nonoverlapped sockets).
Remarks
This function
provides functionality over and above the standard recv function in
three important areas:
1. It can be used in conjunction with overlapped
sockets to perform overlapped receive operations.
2. It allows multiple receive buffers to be specified
making it applicable to the scatter/gather type of I/O.
3. The lpFlags parameter is both an INPUT
and an OUTPUT parameter, allowing applications to sense the output state of the
MSG_PARTIAL flag bit. Note however, that the MSG_PARTIAL flag bit is not
supported by all protocols.
WSARecv is used on connected sockets or bound connectionless
sockets specified by the s parameter and is used to read incoming data.
For
overlapped sockets WSARecv is used to post one or more buffers
into which incoming data will be placed as it becomes available, after which
the application-specified completion indication (invocation of the completion
routine or setting of an event object) occurs. If the operation does not
complete immediately, the final completion status is retrieved through the
completion routine or WSAGetOverlappedResult.
If both lpOverlapped
and lpCompletionRoutine are NULL, the socket in this function will be
treated as a nonoverlapped socket.
For
nonoverlapped sockets, the blocking semantics are identical to that of the
standard recv function and the lpOverlapped and lpCompletionRoutine
parameters are ignored. Any data which has already been received and
buffered by the transport will be copied into the supplied user buffers. For
the case of a blocking socket with no data currently having been received and
buffered by the transport, the call will block until data is received.
The supplied
buffers are filled in the order in which they appear in the array pointed to by
lpBuffers, and the buffers are packed so that no holes are created.
The array of WSABUF
structures pointed to by the lpBuffers parameter is transient. If this
operation completes in an overlapped manner, it is the service provider s responsibility to capture these WSABUF
structures before returning from this call. This enables applications to build
stack-based WSABUF arrays.
For byte
stream style sockets (for example, type SOCK_STREAM), incoming data is placed
into the buffers until the buffers are filled, the connection is closed, or
internally buffered data is exhausted. Regardless of whether or not the
incoming data fills all the buffers, the completion indication occurs for
overlapped sockets. For message-oriented sockets (for example, type
SOCK_DGRAM), an incoming message is placed into the supplied buffers, up to the
total size of the buffers supplied, and the completion indication occurs for
overlapped sockets. If the message is larger than the buffers supplied, the
buffers are filled with the first part of the message. If the MSG_PARTIAL
feature is supported by the underlying service provider, the MSG_PARTIAL flag is
set in lpFlags and subsequent receive operations will retrieve the rest
of the message. If MSG_PARTIAL is not supported but the protocol is reliable, WSARecv
generates the error WSAEMSGSIZE and a subsequent receive operation with a
larger buffer can be used to retrieve the entire message. Otherwise, (that is,
the protocol is unreliable and does not support MSG_PARTIAL), the excess data
is lost, and WSARecv generates the error WSAEMSGSIZE.
For
connection-oriented sockets, WSARecv can indicate the graceful
termination of the virtual circuit in one of two ways, depending on whether the
socket is a byte stream or message oriented. For byte streams, zero bytes
having been read indicates graceful closure and that no more bytes will ever be
read. For message-oriented sockets, where a zero byte message is often
allowable, a return error code of WSAEDISCON is used to indicate graceful
closure. In any case a return error code of WSAECONNRESET indicates an abortive
close has occurred.
lpFlags may be used to influence the behavior of the function
invocation beyond the options specified for the associated socket. That is, the
semantics of this function are determined by the socket options and the lpFlags
parameter. The latter is constructed by or-ing any of the following values:
Value |
Meaning |
MSG_PEEK |
Peek at the
incoming data. The data is copied into the buffer but is not removed from the
input queue. This flag is valid only for nonoverlapped sockets. |
MSG_OOB |
Process
out-of-band data. (See section Out-Of-Band dataVFXYGI for a discussion of this topic.) |
MSG_PARTIAL |
This flag
is for message-oriented sockets only. On output, indicates that the data
supplied is a portion of the message transmitted by the sender. Remaining
portions of the message will be supplied in subsequent receive operations. A
subsequent receive operation with MSG_PARTIAL flag cleared indicates end of
sender s message. As an input
parameter, this flag indicates that the receive operation should complete
even if only part of a message has been received by the service provider. |
For message-oriented
sockets, the MSG_PARTIAL bit is set in the lpFlags parameter if a
partial message is received. If a complete message is received, MSG_PARTIAL is
cleared in lpFlags. In the case of delayed completion, the value pointed
to by lpFlags is not updated. When completion has been indicated the
application should call WSAGetOverlappedResult and examine the flags
pointed to by the lpdwFlags parameter.
Overlapped
socket I/O:
If an
overlapped operation completes immediately, WSARecv returns a value of
zero and the lpNumberOfBytesRecvd parameter is updated with the number
of bytes received and the flag bits pointed by the lpFlags parameter are
also updated. If the overlapped operation is successfully initiated and will
complete later, WSARecv returns SOCKET_ERROR and indicates error code
WSA_IO_PENDING. In this case, lpNumberOfBytesRecvd and lpFlags
are not updated. When the overlapped operation completes the amount of data
transferred is indicated either through the cbTransferred parameter in
the completion routine (if specified), or through the lpcbTransfer
parameter in WSAGetOverlappedResult. Flag values are obtained by
examining the lpdwFlags parameter of WSAGetOverlappedResult.
This function
may be called from within the completion routine of a previous WSARecv, WSARecvFrom,
WSASend or WSASendTo function. For a given socket, I/O completion
routines will not be nested. This permits time-sensitive data transmissions to
occur entirely within a preemptive context.
The lpOverlapped
parameter must be valid for the duration of the overlapped operation. If
multiple I/O operations are simultaneously outstanding, each must reference a
separate overlapped structure. The WSAOVERLAPPED structure has the following
form:
typedef struct _WSAOVERLAPPED {
DWORD Internal; // reserved
DWORD InternalHigh; // reserved
DWORD Offset; // reserved
DWORD OffsetHigh; // reserved
WSAEVENT hEvent;
} WSAOVERLAPPED, FAR * LPWSAOVERLAPPED;
If the lpCompletionRoutine
parameter is NULL, the hEvent field of lpOverlapped is signaled
when the overlapped operation completes if it contains a valid event object
handle. An application can use WSAWaitForMultipleEvents or WSAGetOverlappedResult
to wait or poll on the event object.
If lpCompletionRoutine
is not NULL, the hEvent field is ignored and can be used by the
application to pass context information to the completion routine.
The
completion routine follows the same rules as stipulated for Win32 file I/O
completion routines. The completion routine will not be invoked until the
thread is in an alertable wait state such as can occur when the function WSAWaitForMultipleEvents
with the fAlertable parameter set to TRUE is invoked.
The transport
providers allow an application to invoke send and receive operations from
within the context of the socket I/O completion routine, and guarantee that,
for a given socket, I/O completion routines will not be nested. This permits
time-sensitive data transmissions to occur entirely within a preemptive
context.
The prototype
of the completion routine is as follows:
void
CALLBACK CompletionROUTINE(
IN DWORD dwError,
IN DWORD cbTransferred,
IN LPWSAOVERLAPPEDlpOverlapped,
IN DWORD dwFlags
);
CompletionRoutine is a placeholder for an application-defined or
library-defined function name. dwError specifies the completion status
for the overlapped operation as indicated by lpOverlapped. cbTransferred
specifies the number of bytes received. dwFlags contains information
that would have appeared in lpFlags if the receive operation had
completed immediately. This function does not return a value.
Returning
from this function allows invocation of another pending completion routine for
this socket. When using WSAWaitForMultipleEvents, all waiting completion
routines are called before the alertable thread s wait is satisfied with a return code of WSA_IO_COMPLETION. The
completion routines may be called in any order, not necessarily in the same
order the overlapped operations are completed. However, the posted buffers are
guaranteed to be filled in the same order they are supplied.
Return Values
If no error
occurs and the receive operation has completed immediately, WSARecv
returns zero. Note that in this case, the completion routine will have already
been scheduled, and to be called once the calling thread is in the alertable
state. Otherwise, a value of SOCKET_ERROR is returned, and a specific error
code may be retrieved by calling WSAGetLastError. The error code
WSA_IO_PENDING indicates that the overlapped operation has been successfully
initiated and that completion will be indicated at a later time. Any other
error code indicates that the overlapped operation was not successfully
initiated and no completion indication will occur.
Error Codes
WSANOTINITIALISED |
A
successful WSAStartup must occur before using this function. |
WSAENETDOWN |
The network
subsystem has failed. |
WSAENOTCONN |
The socket
is not connected. |
WSAEINTR |
The
(blocking) call was canceled through WSACancelBlockingCall. |
WSAEINPROGRESS |
A blocking
Windows Sockets 1.1 call is in progress, or the service provider is still
processing a callback function. |
WSAENETRESET |
The connection
has been broken due to the remote host resetting. |
WSAENOTSOCK |
The
descriptor is not a socket. |
WSAEFAULT |
The lpBuffers
argument is not totally contained in a valid part of the user address
space. |
WSAEOPNOTSUPP |
MSG_OOB was
specified, but the socket is not stream style such as type SOCK_STREAM,
out-of-band data is not supported in the communication domain associated with
this socket, or the socket is unidirectional and supports only send
operations. |
WSAESHUTDOWN |
The socket
has been shut down; it is not possible to WSARecv on a socket after shutdown
has been invoked with how set to SD_RECEIVE or SD_BOTH. |
WSAEWOULDBLOCK |
Overlapped
sockets: There are too many outstanding overlapped I/O requests.
Nonoverlapped sockets: The socket is marked as nonblocking and the receive
operation cannot be completed immediately. |
WSAEMSGSIZE |
The message
was too large to fit into the specified buffer and (for unreliable protocols
only) any trailing portion of the message that did not fit into the buffer
has been discarded. |
WSAEINVAL |
The socket
has not been bound with bind, or the socket is not created with the
overlapped flag. |
WSAECONNABORTED |
The virtual
circuit was terminated due to a time-out or other failure. |
WSAECONNRESET |
The virtual
circuit was reset by the remote side. |
WSAEDISCON |
Socket s
is message oriented and the virtual circuit was gracefully closed by the
remote side. |
WSA_IO_PENDING |
An
overlapped operation was successfully initiated and completion will be
indicated at a later time. |
WSA_OPERATION_ABORTED |
The overlapped
operation has been canceled due to the closure of the socket. |
See Also