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This article will explain in detail about the detailed explanation of select functions and example analysis, the content of the article is of high quality, so the editor will share it with you for reference. I hope you will have a certain understanding of the relevant knowledge after reading this article.

Select functions are still quite important in Socket programming, but for beginners of Socket, they are not very fond of using Select to write programs, they are just used to writing blocking procedures such as connect, accept, recv or recvfrom (the so-called blocking method block, as the name implies, is that the process or thread must wait for an event to occur when it is executed to these functions, and if the event does not occur, the process or thread is blocked and the function cannot be returned immediately). However, non-blocking can be accomplished by using Select (the so-called non-blocking non-block, that is, when a process or thread executes this function, it does not have to wait for the event to occur. Once it is sure to return, it reflects the execution of the function with a different return value. If the event occurs, it is the same as blocking. If the event does not occur, a code is returned to inform the process or thread that the event has not occurred, and the process or thread continues to execute. So a program that works efficiently) can monitor changes in the file descriptors we need to monitor-read, write or exception. The following is a detailed introduction!

Let's start with two structures:

1. A data structure struct fd_set is provided in the select mechanism, which can be understood as a collection, which is actually a bitmap, each of which is specific to mark the corresponding size file descriptor. This collection stores a file descriptor (file descriptor), that is, a file handle (that is, each bit on the bitmap can establish a connection with an open file handle (file descriptor), which is done by the programmer). This can be what we call a file in the general sense, of course, any device, pipe, FIFO, etc. under Unix is a file, all included, so there is no doubt that a socket is a file and a socket handle is a file descriptor. The fd_set collection can be manipulated artificially through some macros, and programmers perform the most fd_set operations by manipulating four types of macros:

(1) FD_ZERO (fd_set *) clears a collection of file descriptors

(2), FD_SET (int, fd_set *) adds a file descriptor to a specified file descriptor set

(3), FD_CLR (int, fd_set*) removes a given file descriptor from the collection

(4) FD_ISSET (int, fd_set*) checks whether the file descriptor specified in the collection can be read and written.

The key point of deeply understanding select model is to understand fd_set. For convenience of illustration, we take the length of fd_set as 1 byte, and each bit in fd_set can correspond to a file descriptor FD. Then a 1-byte fd_set can correspond to a maximum of 8 fd.

(1) execute fd_set set;FD_ZERO (& set); then the set is expressed as 0000j0000 in bits.

(2) if fd = 5, after executing FD_SET (fd,&set), set becomes 0001j0000 (position 5 is 1)

(3) if fd=2 and fd=1 are added, the set becomes 0001.0011

(4) execute select (6 minutes set0 0) blocking wait

(5) if all readable events occur on the fd=1,fd=2, the select returns, and the set becomes 00000 and 0011. Fd = 5 is emptied when no readable event occurs.

2. Struct timeval, a commonly used structure to represent the time value, has two members, one is the number of seconds, the other is the number of milliseconds.

Struct timeval {long tv_sec; / / second long tv_usec; / / microsecond}

The accuracy of this structure can be as accurate as 1 part per million.

Next, we introduce the select function, which is in the following format:

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

Explain the parameters of select:

(1) int maxfdp is an integer value, which refers to the range of all file descriptors in the set, that is, the maximum value of all file descriptors plus 1, can not be wrong.

Explanation: for an explanation of this principle, you can see the detailed explanation of fd_set above. Fd_set stores these file descriptors in the form of bitmaps. Maxfdp defines the number of valid bits in a bitmap.

(2) fd_set

* readfds is a pointer to the fd_set structure, and this collection should include file descriptors. We want to monitor the read changes of these file descriptors, that is, we are concerned about whether we can read data from these files. If there is a file in this collection that can be read, select will return a value greater than 0, indicating that there are files to read. If there is no readable file, then determine whether the timeout is based on the timeout parameter. If the timeout time is exceeded, select returns 0, and a negative value if an error occurs. You can pass in a null value to indicate that you don't care about any read changes in the file.

(3) fd_set * writefds is a pointer to the fd_set structure. File descriptors should be included in this collection. We need to monitor the write changes of these file descriptors, that is, we are concerned about whether data can be written to these files. If there is a file in this set to write, select will return a value greater than 0, indicating that there is a file to write. If there is no file to write to, select will return a value greater than 0. Then determine whether the timeout is based on the timeout parameter. If the time of timeout is exceeded, select returns 0, and negative value if an error occurs. You can pass in a null value to indicate that you don't care about any write changes in the file.

(4) fd_set * errorfds is used to monitor file error exception files with the intention of the above two parameters.

(5) struct

Timeval* timeout is the timeout of select, this parameter is very important, it can make select in three states. First, if you pass NULL as a formal parameter, that is, if you do not pass in the time structure, you will put select in the blocking state until a file descriptor in the collection of monitoring file descriptors changes. Second, if the time value is set to 0 seconds and 0 milliseconds, it becomes a pure non-blocking function, regardless of whether the file descriptor has changed or not, it will immediately return to continue execution, the file will return 0 without change, and a positive value will be returned if there is any change. Third, the value of timeout is greater than 0, which is the timeout for waiting, that is, select is blocked within the timeout time, and an event is returned within the timeout. Otherwise, the return value is the same as above no matter how it must be returned after the timeout.

Description:

The function returns:

(1) when the condition is satisfied in the corresponding file descriptor set monitored, for example, when there is data in the read file descriptor set, the kernel (Iwhite O) modifies the file descriptor set according to the status and returns a number greater than 0.

(2) when there is no file descriptor that meets the condition, and the set timeval monitoring time times out, the select function will return a value of 0.

(3) an error occurs when select returns a negative value.

Some reference parsing of select function: http://www.groad.net/bbs/read.php?tid-1064.html

The difference between select function and pselect function reference: http://hi.baidu.com/_jiangming/item/56d5c43fe2cadb4981f1a789

Pselect function is an enhanced select function to prevent signal interference.

Example analysis of select () function:

(1) after having select, you can write decent network programs! A simple example is to accept data from the network and write it to a file.

Int main () {int sock; FILE * fp; struct fd_set fds; struct timeval timeout= {3jue 0}; / select wait 3 seconds, poll for 3 seconds, set 0 char buffer [256] = {0} for non-blocking / / 256byte receive buffer / * it is assumed that the UDP connection has been established, and the specific process is simple. Of course, the same is true for TCP. The host ip and port have been given, and the file to be written has been opened with sock=socket (...); bind (...); fp=fopen (...); * / while (1) {FD_ZERO (& fds). / / clear the collection for each loop, otherwise you cannot detect the descriptor change FD_SET (sock,&fds); / / add the descriptor FD_SET (fp,&fds); / / ditto maxfdp=sock > fp?sock+1:fp+1 / / descriptor maximum plus 1 switch (select (maxfdp,&fds,&fds,NULL,&timeout)) / / select uses {case-1: exit (- 1); break; / / select error, exit program case 0:break / / poll default again: if (FD_ISSET (sock,&fds)) / / Test whether sock is readable, that is, whether there is data {recvfrom (sock,buffer,256,.) on the network. / / accept network data if (FD_ISSET (fp,&fds)) / / Test whether the file is writable fwrite (fp,buffer...); / / write the file / / buffer empty;} / / end if break;} / / end switch} / / end while} / / end main

(2) is there any data coming on the monitoring keyboard under Linux?

# include # include int main () {int keyboard; int ret,i; char c; fd_set readfd; struct timeval timeout; keyboard = open ("/ dev/tty", O_RDONLY | O_NONBLOCK); assert (keyboard > 0); while (1) {timeout.tv_sec=1; timeout.tv_usec=0; FD_ZERO (& readfd); FD_SET (keyboard,&readfd) / / Monitoring function ret=select (keyboard+1,&readfd,NULL,NULL,&timeout); if (ret= =-1) / / error condition cout

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