Shulou(Shulou.com)06/01 Report--

The select of Icano multiplex transfer (only responsible for, etc.)

The system provides select function to realize the multiplexing input / output model.

The parameters passed to select tell the kernel:

1) the file descriptor that we care about.

The parameter nfds is the largest file descriptor value to monitor + 1

2) for each descriptor, we are concerned about the state.

Rdset,wrset,exset corresponds to the set of readable file descriptors to be detected, the set of writable file descriptors and the set of exception file descriptors.

The monitored file descriptors are divided into three types of set, each corresponding to waiting for a different event. The file descriptors listed in readfds are monitored for data to read (not blocked if the read operation is complete). The file descriptors listed in writefds are monitored for write completion without blocking, and the file descriptors listed in exceptfds are monitored for exceptions or the availability of uncontrolled data (these states apply only to sockets). These three types of set can be NULL, in which case select () does not monitor such events.

The following macros provide ways to deal with these three descriptive phrases:

FD_ZERO removes all file descriptors in the specified set. You should call select () before each call.

FD_CLR removes a file descriptor from the specified set

FD_SET adds a file descriptor to the specified set

FD_ISSET tests whether a file descriptor specifies part of the set. If the file descriptor is in set, it returns a non-zero integer, and if it is not, the zero _ zero _ FDdescripISSET is used after the return of the call select () to test whether the specified file descriptor is ready for the relevant action.

3) how long we have to wait. (we can wait an infinitely long time, wait a fixed period of time, or not wait at all)

The timeout parameter is a pointer to the timeval structure, and timeval is defined as follows:

The struct timeval structure is used to describe the length of time during which the function returns if no event occurs for the descriptor to be monitored, with a return value of 0.

After returning from the select function, the kernel tells us the following information:

1) the number of descriptors that are ready.

2) which descriptors are ready for the three conditions. (read, write, exception)

If the execution is successful, the number of descriptors whose status has changed is returned. If 0 is returned, the timeout time has elapsed before the state of the descriptor has changed, and no one has been returned.

When an error is sent,-1 is returned. The cause of the error is stored in errno, and the values of the parameters readfds,writefds,exceptfds and timeout become unpredictable. The error value may be: the EBADF description word is valid or the piece is closed, the EINVAL parameter n interrupted by the EINTR signal is negative, and the ENOMEM core memory is not.

# include#include#include#include#include#include#include#includeint fds [64]; const fds_nums=sizeof (fds) / sizeof (fds [0]); static int startup (const char* _ ip,int _ port) {int sock=socket (AF_INET,SOCK_STREAM,0); if (sock)

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