Concurrent Servers

Idea Behind Concurrent Servers Server Client 1 Server 1 X

Idea Behind Concurrent Servers Server Client 1 Server 1

Idea Behind Concurrent Servers Server Client 1 Server 1 Client 2

Idea Behind Concurrent Servers Server Client 1 Server 1 Client 2Server 2

Idea Behind Concurrent Servers Server Client 1 Server 1 Client 2Server 2

Idea Behind Concurrent Servers Server Client 1 Server 1 Client 2Server 2 X

Idea Behind Concurrent Servers Server Client 1 Server 1 Client 2Server 2

Creating a New Server - fork() listenfd = Socket( … ) Initialize server address Bind( listenfd, … ) for ( ;; ) { /* wait for client connection */ connfd = Accept(listenfd,…); if( (pid = Fork() ) = = 0) { /*I am the child */ Close(listenfd); service_client(connfd); Close(connfd); exit(0); } else /* I am the parent */ Close(connfd); }

Points to Note fork() is called once … …but it returns twice!! –Once in the parent server and –Once in the child server How to distinguish parent and child??

Points to Note fork() is called once … …but it returns twice!! –Once in the parent server and –Once in the child server How to distinguish parent and child?? –Return value in child = 0 –Return value in parent = process id of child

Points to Note fork() is called once … …but it returns twice!! –Once in the parent server and –Once in the child server How to distinguish parent and child?? –Return value in child = 0 –Return value in parent = process id of child Child server exits after servicing the client.

Running another program in child – exec()

listenfd = Socket(…) Connfd = Accept(…) Inetd daemon

Running another program in child – exec() listenfd = Socket(…) Connfd = Accept(…) Inetd daemon Close(listenfd) Fork(…) Inetd child

Running another program in child – exec() listenfd = Socket(…) Connfd = Accept(…) Inetd daemon Close(listenfd) Fork(…) Exec(…) Service telnet client Close(connfd) Inetd child Telnet server

Different Types of exec() int execl(char * pathname, char * arg0, …, (char *)0); int execv(char * pathname, char * argv[]); int execle(char * pathname, char * arg0, …, (char *)0, char envp[]); int execve(char * pathname, char * argv[], char envp[]); int execlp(char * filename, char * arg0, …, (char *)0); int execvp(char * filename, char * argv[]);

Properties of exec() Replaces current process image with new program image. –E.g. inetd image replaced by telnet image All descriptors open before exec remain open after exec.

Getting IP address/port from socket int getsockname(int sockfd, struct sockaddr *localaddr, socklen_t *addrlen) –Get the local IP/port bound to socket int getpeername(int sockfd, struct sockaddr *remoteaddr, socklen_t *addrlen) –Get the IP/port of remote endpoint Why do we need these?

Two other useful functions struct hostent *gethostbyaddr (void *addr, size_t len, int type); –Converts from IP addr to domain name struct hostent *gethostbyname (char *name); –Converts from domain name to IP address struct hostent { char *h_name;/* official name of host */ char **h_aliases; /* alias list */ int h_addrtype; /* address type */ int h_length; /* address length*/ char **h_addr_list; /* address list */ }

Signals Signal is a notification to process (from OS or from another process) that an event has occurred. Type of event determined by type of signal Try listing all signal types using % kill –l Some interesting signals –SIGCHLD, SIGTERM, SIGKILL, SIGSTOP

Handling Signals Signals can be caught – i.e. an action associated with them –SIGKILL and SIGSTOP cannot be caught. Actions can be customized using sigaction(…) which associates a signal handler with the signal. Details in page 120 of Steven’s book Default action for most signals is to terminate the process –SIGCHLD and SIGURG are ignored by default. Unwanted signals can be ignored –except SIGKILL or SIGSTOP

Zombie Processes When a child server dies, a SIGCHLD is sent to the parent server. If parent doesn’t wait() on the child, child becomes a zombie (status “Z” seen with ps). Zombies hang around forever.

How to avoid zombies? Parent should install a signal handler for SIGCHLD Call wait(…)/waitpid(…) inside the signal handler void handle_sigchld(int signo) { pid_tpid; int stat; pid = wait(&stat); printf(“child %d terminated\n”, pid); }