Blocking, non-blocking concept and blocking function of Select function __ function

(1) Blocking block
Blocking mode block, as the name suggests, is when processes or threads execute to these functions must wait for an event to occur, and if the event does not occur, the process or thread is blocked and the function cannot be returned immediately.
For example, in the socket programming connect, accept, recv, recvfrom such blocking program.
As with most function calls, statement execution, strictly speaking, they are all executed in a blocking manner.

(2) Non-blocking Non-block
So-called non-blocking mode Non-block, is that the process or thread executes this function without having to wait for the event to occur, once the execution is returned, to reflect the function's execution in a different return value, if the event occurs in the same way as blocking, and if the event does not occur, return a code to tell that the event did not occur. The process or thread continues to execute, so it is more efficient.
For example, a program statement: int Len=read (fd,buffer,bufsize); the function read reads only once, and returns the result regardless of whether you read the data or read the data. Also like while (1) {len=read (fd,buffer,bufsize); if (...) break;}, although you can iterate through the desired data, it is non-blocking and can be a huge waste of system resources.
Note: Use in socket programming: FCNTL (Sockfd,f_setfl,o_nonblock), will sockfd set as non-blocking mode, then connect, accept, recv, recvfrom and other functions will lose the blocking function, becomes a non-blocking function.

(3) Select function

int select (int maxfdp,fd_set *readfds,fd_set *writefds,fd_set *errorfds,struct timeval *timeout);

The above non-blocking while loop is obviously undesirable, and functions such as connect without blocking function also need to be improved, and in both cases the Select function can be useful.
There are two articles that are very clear about the use of the SELECT function in this regard:

Http://blog.chinaunix.net/u/11557/showart_104967.html
Http://blog.ednchina.com/thinkker/151601/message.aspx

There are a few things to note about the use of the Select function:
MAXFDP for the maximum value of all FD plus 1.
Readfds and timeout are reinitialized each time the select is executed. For Readfds, the collection is emptied for Each loop, and the descriptor changes cannot be detected, and for timeout, the value is initialized each time, or the timeout is initialized to 0 by default.

//A typical example of a proper use of the Select function (program segment): int Read (int fd, char *readbuf, int bufsize) {int Len1,len2,nfds,sel
  Ect_ret;
  struct Timeval timeout;
  Fd_set Readfds;
  Fd_zero (&readfds); 
  Fd_set (FD1, &readfds);
  Fd_set (FD2, &readfds); Nfds=fd1>fd2?
  (fd1+1):(fd2+1);
  Timeout.tv_sec = 1;
  Timeout.tv_usec = 500000; while (Select_ret = select (Nfds, &readfds, NULL, NULL, &timeout)) > 0 {len1 + = read (fd1, Readbuf1 + le
    n, Bufsize1-len);
    Len2 + = Read (FD2, readbuf2 + len, Bufsize2-len);
    Fd_zero (&readfds); 
    Fd_set (FD1, &readfds);
    Fd_set (FD2, &readfds); Nfds=fd1>fd2?
    (fd1+1):(fd2+1);
    timeout.tv_sec = 0;
  Timeout.tv_usec = 500000;
  } readbuf1[bufsize1-1]= ';
  Readbuf2[bufsize2-1]= ' ";
return len1+len2; }