2010年12月06号之-----Win32 socket编程学习_C/C++_编程开发_程序员俱乐部

几个基本函数的解释：

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fd_set Structure:

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The fd_set structure is used by various Windows Sockets functions and service providers, such as the select function, to place sockets into a "set" for various purposes, such as testing a given socket for readability using the readfds parameter of the select function.

fd_set结构简单的来说就是用来存储各种不同类型的Windows Socket的一个数据结构。

原型是：

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typedef struct fd_set {
  u_int fd_count;
  SOCKET fd_array[FD_SETSIZE];
} fd_set;

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select Function:

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The select function determines the status of one ore more sockets,waiting if necessary, to perform synchronous I/O.

select 函数用来判断一个或更多个socket的状态，来决定是否有必要来执行同步I/O操作。

函数原型是：

int select(
  __in          int nfds,
  __in_out      fd_set* readfds,
  __in_out      fd_set* writefds,
  __in_out      fd_set* exceptfds,
  __in          const struct timeval* timeout
);

Return Value:

The select function returns the total number of socket handles that are ready and contained in the fd_set structures, zero if the time limit expired, or SOCKET_ERROR if an error occurred. If the return value is SOCKET_ERROR, WSAGetLastError can be used to retrive a specific error code.

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Remarks

The select function is used to determine the status of one or more sockets. For each socket, the caller can request information on read, write, or error status. The set of sockets for which a given status is requested is indicated by an fd_set structure. The sockets contained within the fd_set structures must be associated with a single service provider. For the purpose of this restriction, sockets are considered to be from the same service provider if the WSAPROTOCOL_INFO structures describing their protocols have the same providerId value. Upon return, the structures are updated to reflect the subset of these sockets that meet the specified condition. The select function returns the number of sockets meeting the conditions. A set of macros is provided for manipulating an fd_set structure. These macros are compatible with those used in the Berkeley software, but the underlying representation is completely different.

The parameter readfds identifies the sockets that are to be checked for readability. If the socket is currently in the listen state, it will be marked as readable if an incoming connection request has been received such that an accept is guaranteed to complete without blocking. For other sockets, readability means that queued data is available for reading such that a call to recv, WSARecv, WSARecvFrom, or recvfrom is guaranteed not to block.

For connection-oriented sockets, readability can also indicate that a request to close the socket has been received from the peer. If the virtual circuit was closed gracefully, and all data was received, then a recv will return immediately with zero bytes read. If the virtual circuit was reset, then a recv will complete immediately with an error code such as WSAECONNRESET. The presence of OOB data will be checked if the socket option SO_OOBINLINE has been enabled (see setsockopt).

The parameter writefds identifies the sockets that are to be checked for writability. If a socket is processing a connect call (nonblocking), a socket is writeable if the connection establishment successfully completes. If the socket is not processing a connect call, writability means a send, sendto, or WSASendto are guaranteed to succeed. However, they can block on a blocking socket if the len parameter exceeds the amount of outgoing system buffer space available. It is not specified how long these guarantees can be assumed to be valid, particularly in a multithreaded environment.

The parameter exceptfds identifies the sockets that are to be checked for the presence of OOB data or any exceptional error conditions.

Note??Out-of-band data will only be reported in this way if the option SO_OOBINLINE is FALSE. If a socket is processing a connect call (nonblocking), failure of the connect attempt is indicated in exceptfds (application must then call getsockopt SO_ERROR to determine the error value to describe why the failure occurred). This document does not define which other errors will be included.

Any two of the parameters, readfds, writefds, or exceptfds, can be given as null. At least one must be non-null, and any non-null descriptor set must contain at least one handle to a socket.

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timeval Structure:

The timeval structure is used to specify time values. It is associated with the Berkeley Software Distribution (BSD) Time.h header file.

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typedef struct timeval {
  long tv_sec;
  long tv_usec;
} timeval;

这个也比较常用，第一个类型tv_sec秒值，第二个usec毫秒值。

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Send Function :

The send function sends data on a connected socket.

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int send(
  __in          SOCKET s,
  __in          const char* buf,
  __in          int len,
  __in          int flags
);

Return Value

If no error occurs, send returns the total number of bytes sent, which can be less than the number requested to be sent in the len parameter. Otherwise, a value of SOCKET_ERROR is returned, and a specific error code can be retrieved by calling WSAGetLastError.

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如果没有错误发生，就返回被传递的总字节数。

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