Application of select in socket programming

 

Select is very important in socket programming, but for beginners of socket, they do not like to use select to write programs. They are just used to writing such as connect,
A blocking program such as accept, Recv, or recvfrom (the so-called blocking method block, as the name suggests, means that when a process or thread executes these functions, it must wait for the sending of an event
If the event does not occur, the process or thread is blocked and the function cannot return immediately ). However, you can use select to perform non-blocking (the so-called non-block method is process or
When the thread executes this function, it does not have to wait for the event to occur. Once the execution is successful, the return value is different to reflect the execution of the function. If the event occurs, it is blocked in the same way, if the event does not occur, return
A code to inform the program that the event has not occurred, but the process or thread continues to execute, so the efficiency is high, it can monitor the changes in file descriptors that we need to monitor-read/write or exceptions. Below
For more information, see!

Select Function Format (I am talking about Berkeley socket programming in UNIX, which is different from that in windows. I will explain it later ):

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

Two struct types are described first:

The
1. struct fd_set can be understood as a collection in which file descriptors (
Descriptor), that is, the file handle. This can be what we call a common file. Of course, any device, pipeline, or FIFO in UNIX is in the file format, all of which are included.
There is no doubt that a socket is a file, and a socket handle is a file descriptor. The fd_set set can be operated manually by some macros, such as clearing the fd_zero collection.
(Fd_set *) to add a given file descriptor to the set fd_set (INT, fd_set
*) To delete a given file descriptor from the collection fd_clr (int
, Fd_set *), check whether the specified file descriptor in the set can read and write fd_isset (INT, fd_set *). For example.

Second, struct timeval is a common structure used to represent the time value. It has two members: one is the number of seconds and the other is the number of milliseconds.

Specific interpretation of select parameters:

Int maxfdp is an integer that indicates the range of all file descriptors in the collection. That is, the maximum value of all file descriptors is 1. It cannot be wrong! In Windows, the value of this parameter does not matter. You can set it incorrectly.

Fd_set
* Readfds is a pointer to the fd_set structure. This collection should contain file descriptors. We want to monitor the read changes of these file descriptors, that is, whether they can be retrieved from these files.
When the data is read, if there is a file in this collection that is readable, select returns a value greater than 0, indicating that the file is readable. If there is no readable file, it is determined based on the timeout parameter.
Whether to time out. If the time exceeds the timeout, select returns 0. If an error occurs, a negative value is returned. You can pass in a null value, indicating that you do not care about any file read changes.

Fd_set
* Writefds is a pointer to the fd_set structure. This collection should contain file descriptors. We want to monitor the write changes of these file descriptors, that is, whether or not we can send data to these files.
If the set contains a file that can be written, select returns a value greater than 0, indicating that there is a file that can be written. If there is no file that can be written, it is determined based on the timeout parameter.
Whether the disconnection times out. If the time exceeds the timeout, select returns 0. If an error occurs, a negative value is returned. You can pass in a null value, indicating that you do not care about any file write changes.

Fd_set * errorfds indicates the intention of the two parameters above, which is used to monitor file error exceptions.

Struct
Timeval *
Timeout is the select timeout time. This parameter is crucial. It can make the SELECT statement in three States. First, if null is input as an input parameter, that is, the time structure is not input.
When the SELECT statement is placed in a blocking state, it must wait until a file descriptor in the collection of monitored file descriptors changes. Second, if the time value is set to 0 s and 0 ms, it becomes a pure non-blocking function,
No matter whether the file descriptor changes or not, the system immediately returns the code to continue execution. If the file does not change, 0 is returned, and a positive value is returned. Third, the value of timeout is greater than 0, which is the waiting timeout time, that is
The SELECT statement is blocked within the time-out period. If an event arrives within the time-out period, it is returned. Otherwise, no matter how long the SELECT statement is returned, the return value is the same as the preceding one.

Return Value:

Negative value: Select error positive value: some files can be read or written, or errors 0: wait for timeout, there is no readable or wrong file

With select, you can write a decent network program! A simple example is to accept data from the network and write it into a file.

Example:

Main ()

{

Int sock;

File * FP;

Struct fd_set FDS;

Struct timeval timeout = {3, 0}; // select wait 3 seconds, 3 seconds round-robin, set to 0 if it is not blocked

Char buffer [256] = {0}; // receiving buffer of 256 bytes

/* Assume that a UDP connection has been established, and the specific process is not written or simple. Of course, the same applies to TCP. The host IP address and port are already specified, and the file to be written has already been opened.

Sock = socket (...);

BIND (...);

Fp = fopen (...);*/

While (1)

{

Fd_zero (& FDs); // clear the set for each loop. Otherwise, the descriptor changes cannot be detected.

Fd_set (sock, & FDs); // Add Descriptor

Fd_set (FP, & FDs); // same as above

Maxfdp = sock> FP? Sock + 1: FP + 1; // Add 1 to the maximum value of the descriptor

Switch (select (maxfdp, & FDS, & FDS, null, & timeout) // select

{

Case-1: exit (-1); break; // select error, exit the program

Case 0: break; // Round Robin again

Default:

If (fd_isset (sock, & FDs) // test whether sock is readable, that is, whether there is data on the network.

{

Recvfrom (sock, buffer, 256,...); // accept Network Data

If (fd_isset (FP, & FDs) // test whether the file can be written

Fwrite (FP, buffer...); // write a file

Buffer clearing;

} // End if break;

} // End Switch

} // End while

} // End main




The Select () mechanism provides an fd_set data structure, which is actually a long array,
Each array element can be associated with an open file handle (whether it is a socket handle or other
File, named pipe, or device handle) to establish a contact. The programmer completes the process of establishing a contact,
When select () is called, the content of fd_set is modified by the kernel according to the IO status.
Which socket or file is readable for the select () process:

# Include <sys/types. h>
# Include <sys/times. h>
# Include <sys/select. h>

Int select (NFDs, readfds, writefds, limit TFDs, timeout)
Int NFDs;
Fd_set * readfds, * writefds, * required TFDs;
Struct timeval * timeout;

NDfS: the number of file handles monitored by the SELECT statement, depending on the number of files opened in the process.
The maximum file number plus one.
Readfds: a collection of readable file handles monitored by select.
Writefds: a set of writable file handles monitored by select.
Except TFDs: a collection of abnormal file handles monitored by select.
Timeout: the timeout end time of this select () operation. (See/usr/sys/select. H,
Accurate to one second per million !)

When the image file in readfds or writefds is readable, writable, or times out, this select ()
Returns the result. Programmers can use the macros provided by a set of systems to determine when select () ends.
Which file is readable or writable. Readfds is particularly useful for socket programming.
Several related macros are explained as follows:

Fd_zero (fd_set * fdset): clears the contact between fdset and all file handles.
Fd_set (int fd, fd_set * fdset): Establishes the connection between the file handle FD and fdset.
Fd_clr (int fd, fd_set * fdset): clears the contact between the file handle FD and fdset.
Fd_isset (int fd, fdset * fdset): Check whether the file handle FD associated with fdset is
It can be read and written.> 0 indicates that it can be read and written.
(For definition of fd_set and related macros, see/usr/include/sys/types. h)

In this way, your socket only needs to be read when there is something to read, roughly as follows:

...
Int sockfd;
Fd_set FDR;
Struct timeval timeout = ..;
...
For (;;){
Fd_zero (& FDR );
Fd_set (sockfd, & FDR );
Switch (select (sockfd + 1, & FDR, null, & timeout )){
Case-1:
Error handled by u;
Case 0:
Timeout hanled by u;
Default:
If (fd_isset (sockfd )){
Now u read or Recv something;
/* If sockfd is father and
Server socket, u can now
Accept ()*/
}
}
}

Therefore, an fd_isset (sockfd) is equivalent to a FD readable notification.
For the struct timeval function, use man select. Different timeval settings
Enable Select () to display three features: timeout termination, no timeout blocking, and round robin. Because
Timeval can be accurate to one second per million, so windows settimer () is not
What. You can use select () to make a super clock.

What is the implementation of fd_accept? Still above, because the client will send
The Connection Request message. At this time, select () will certainly end. fd_isset (sockfd) is of course large.
At zero, because there is a message readable! As for this application, it mainly lies in the parent of the service provider.
Socket, if you do not like the active accept (), you can change it to the above mechanism to accept ().

As for the implementation and processing of fd_close, it takes a lot of CPU processing time and is not finished yet.

--
This article discusses how to use select () to detect socket shutdown by the other Party:

The local socket is still readable, because when the socket of the other party is closed, a closed connection will be sent.
The notification message will be immediately detected by select. TCP connection (three-way handshake) and off
For more information about the closed (secondary handshake) mechanism, see related books on TCP/IP.

For some reason, Unix does not seem to have provided a notification to the process about socket or pipe being disabled by the other party.
Signal, or the CPU is limited. In short, when the other party closes, one executes Recv () or read (),
Return-1 immediately. The value of the global variable errno is 115, and the corresponding sys_errlist [errno]
Connect refused (see/usr/include/sys/errno. h ). Therefore
In the for (;)... select () block, when something is readable, check Recv () or
When the return value of read () is-1, the local socket is shut down; otherwise, select () will
I always think that there is something to read, and the result is that the CPU has been heartbroken to cut the pin. If you don't believe it, try: No Check
Query Recv () to return the result, and write the received result (not actually received) to the standard output...
Similar problems also occur in programming of famous pipelines. For details, refer to release as: release a useful
The source code of the Socket Client.

As for the processing that the other party suddenly closes when writing the socket, it can simply capture the signal sigpipe and perform
The corresponding process of shutting down the local socket and so on. Sigpipe is interpreted as writing a non-reader pipeline.
We will not repeat it here. Please refer to man signal.

The above is the experience accumulated by the CPU in the TCP/IP data transmission experiment. If there is any error or omission, please kill it.

Alas, yesterday, a bunch of grandsons in the hacker area had almost no short circuits. Ren CPU (Pentium Core) Z80

In addition, the application of select in asynchronous (non-blocking) connect was just started with socket programming.
I have always used blocking-type connect. The problem with non-blocking connect is that proxy scan was used at the time.
However
Through online communication with netizens and related FAQs, I finally learned how to solve this problem.
Using select can solve this problem well. The general process is as follows:

1. Set the opened socket to non-blocking. You can use fcntl (socket, f_setfl, o_ndelay)
(Some systems can also use fnedlay ).

2. Send the Connect call. At this time,-1 is returned, but errno is set to einprogress, meaning that connect is still
The process is not completed yet.

3. Set the opened socket to the monitored writable (note that it is not readable) file set and use select for monitoring,
If it can be written, use
Getsockopt (socket, sol_socket, so_error, & error, sizeof (INT ));
To get the error value. If it is zero, connect is successful.

In many UNIX versions of proxyscan programs, you can see similar processes, in addition to the essence of Solaris
Area-> there is a general connect module with timeout parameters in programming skills.