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This article introduces the relevant knowledge of "detailed explanation of Linux network programming wait () and waitpid ()". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

After the client disconnects, there are a large number of zombie processes on the server side. This is because after the server child process terminates, it sends a SIGCHLD signal to the parent process, which the parent process ignores by default. To avoid zombie processes, whenever we create a child process, the main process needs to wait for the child process to return in order to clean up the child process. To do this, we add SIGCHLD signal handling functions to the server program.

The code is as follows:

# include

# include

# include

# include

# include

# include

# include

# include

# include

# define SERV_PORT 1113

# define LISTENQ 32

# define MAXLINE 1024

/ * connection handler function * /

Void str_echo (int fd)

Void

Sig_chld (int signo)

{

Pid_t pid

Int stat

Pid = wait (& stat); / / get the child process number

Printf ("child% d terminated\ n", pid)

Return

}

Int

Main (int argc, char * argv []) {

Int listenfd,connfd

Pid_t childpid

Socklen_t clilen

Struct sockaddr_in servaddr

Struct sockaddr_in cliaddr

/ / struct sockaddr_in servaddr

/ / struct sockaddr_in cliaddr

If ((listenfd = socket (AF_INET, SOCK_STREAM,0)) =-1) {

Fprintf (stderr, "Socket error:%s\ n\ a", strerror (errno))

Exit (1)

}

/ * the server side is populated with sockaddr structure * /

Bzero (& servaddr, sizeof (servaddr))

Servaddr.sin_family = AF_INET

Servaddr.sin_addr.s_addr = htonl (INADDR_ANY)

Servaddr.sin_port = htons (SERV_PORT)

Signal (SIGCHLD,sig_chld); / / processing SIGCHLD signals

/ * bind listenfd descriptor * /

If (bind (listenfd, (struct sockaddr*) (& servaddr), sizeof (struct sockaddr)) =-1) {

Fprintf (stderr, "Bind error:%s\ n\ a", strerror (errno))

Exit (1)

}

/ * listen for listenfd descriptor * /

If (listen (listenfd,5) =-1) {

Fprintf (stderr, "Listen error:%s\ n\ a", strerror (errno))

Exit (1)

}

For (;;) {

Clilen = sizeof (cliaddr)

/ * the server blocks until the client establishes a connection * /

If ((connfd=accept (listenfd, (struct sockaddr*) (& cliaddr), & clilen)) 0)

Write (sockfd, buf, n)

If (n < 0 & & errno = = EINTR) / / interrupted, reentrant

Goto again

Else if (n < 0) {/ / error

Fprintf (stderr, "read error:%s\ n\ a", strerror (errno))

Exit (1)

}

}

After modifying the code, when the client disconnects, the parent process of the server receives the SIGCHLD signal of the child process and executes the sig_chld function to clean up the child process, so that there will be no more zombie processes. At this point, when a client actively disconnects, the server outputs information similar to the following:

Child 12306 terminated

Wait and waitpid

In the sig_chld function in the above program, we use wait () to clear the terminated child processes. There is also a similar function wait_pid. Let's first look at these two function prototypes:

Pid_t wait (int * status)

Pid_t waitpid (pid_t pid, int * status, int options)

Official description: All of these system calls are used to wait for state changes in a child of the calling process, and obtain information about the child whose state has changed. A state change is considered to be: the child ter minated; the child was stopped by a signal; or the child was resumed by a signal. In the case of a terminated child, performing a wait allows the system to release the resources associated with the child; if a wait is not performed, then the terminated child remains ina "zombie" state (see NOTES below).

About the differences and relations between wait and waitpid:

The wait () system call suspends execution of the calling process until one of its children terminates. The call wait (& status) is equivalent to:

Waitpid (- 1, & status, 0)

The waitpid () system call suspends execution of the calling process until a child specified by pid argument has changed state. By default, waitpid () waits only for terminated children, but this behavior is modifiable via the options argument, as described below.

That is, the wait () system call suspends the calling process until any of its child processes terminate. Calling wait (& status) has the same effect as calling waitpid (- 1, & status, 0).

Waitpid () suspends the calling process until the process state specified by the parameter pid changes. By default, waitpid () only waits for the termination state of the child process. If desired, you can handle the non-terminating state by setting the value of options. For example:

The value of options is an OR of zero or more of the following constants:

WNOHANG return immediately if no child has exited.

WUNTRACED also return if a child has stopped (but not traced via ptrace (2)) Status for traced children which have stopped is provided even if this option is not specified.

WCONTINUED (since Linux 2.6.10) also return if a stopped child has been resumed by delivery of SIGCONT.

Wait a minute. Non-terminating state.

Now let's look at the difference between wait () and waitpid () through an example.

By modifying the client program, set up 5 sockets in the client program at one time to connect to the server, as shown in the following figure (with code):

The code is as follows:

# include

# include

# include

# include

# include

# include

# include

# include

# include

# define SERV_PORT 1113

# define MAXLINE 1024

Void str_cli (FILE * fp, int sockfd)

Int

Main (int argc, char * * argv)

{

Int i,sockfd [5]

Struct sockaddr_in servaddr

If (argc! = 2) {

Fprintf (stderr, "usage: tcpcli\ n\ a")

Exit (0)

}

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