C language select function usage in Linux

Select (I/O multi-work mechanism)
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Define functions	Int select (int n, fd_set * readfds, fd_set * writefds, fd_set * contains TFDs, struct timeval * timeout );
Function Description	Select () is used to wait for the state change of the description word of the file. Parameter n represents the largest file descriptive word plus 1. The readfds, writefds, and limit TFDs parameters are called descriptive phrases and are used to return the reading, writing, or exception of the descriptive word. The macro below provides a way to deal with these three descriptive phrases: Fd_clr (inr fd, fd_set * Set); used to clear the related FD bits in the description phrase set. Fd_isset (int fd, fd_set * Set); used to test whether the bits of the related FD in the phrase set are true. Fd_set (int fd, fd_set * Set); used to set the FD bit in the description phrase set. Fd_zero (fd_set * Set); used to clear all bits of the phrase set.
Parameters	Timeout is the timeval structure used to set the select () wait time. Its structure is defined as follows: Struct timeval { Time_t TV _sec; Time_t TV _usec; };
Return Value	If the timeout parameter is set to null, select () does not have timeout.
Error Code	If execution is successful, the number of changes in the state of the description word in the file is returned. If 0 is returned, it indicates that the time before the State of the description word changes has exceeded the timeout time. If an error occurs,-1 is returned, the error cause is stored in errno. The value of readfds, writefds, limit TFDs, and timeout parameters becomes unpredictable. The description of the ebadf file is invalid or the file is closed. Eintr this call is interrupted by the signal The value of N is negative. Insufficient enomem core memory
Example	Common Program snippets: fs_set readset; Fd_zero (& readset ); Fd_set (FD, & readset ); Select (FD + 1, & readset, null ); If (fd_isset (FD, readset ){......}