Lecture 10 Overview!

Transcription

1 Lecture 10 Overview! Last Lecture! Wireless Sensor Networks! This Lecture! Daemon processes and advanced I/O functions! Source: Chapters 13 &14 of Stevens book! Next Lecture! Unix domain protocols and non-blocking I/O! Source: Chapters 15 & 16 of Stevens book! TELE Lecture 10 Daemon processes and advanced I/O functions 1

2 daemon! A daemon is a process that runs in the background and is independent of control from all terminals! Reasons for daemons independence of terminals! Prevent daemons error message from appearing on a user s terminal! Signals generated from terminal keys must not affect any daemons that were started from that terminal earlier! Ways to start a daemon! Started by the system initialization scripts! Started by inetd superserver! Performed by cron daemon on a regular basis! Started from user terminals (foreground or background)! TELE Lecture 10 Daemon processes and advanced I/O functions 2

3 syslogd daemon! How it works?! Read the configuration file /etc/syslog.conf! A Unix domain socket is created and bound to the pathname /var/run/log! A UDP socket is created and bound to port 514! The pathname /dev/klog is opened to read kernel error messages! Runs in an infinite loop that calls select, waiting for any one of the above descriptors to be readable, reads the log message, and does what the configuration file says to do with that message.! If the daemon receives the SIGHUP signal, it rereads the configuration file! TELE Lecture 10 Daemon processes and advanced I/O functions 3

4 syslog function! How to send log messages?! create a Unix domain datagram socket and send our messages to the pathname the daemon has bound, or send them to port 514 by a UDP socket! syslog function! An easy interface to the syslogd daemon! void syslog(int priority, const char *message, );! priority is a combination of a level and a facility shown later! message is like a format string to printf, with the addition of a %m specification, which is replaced with the error message corresponding to the current value of errno.! TELE Lecture 10 Daemon processes and advanced I/O functions 4

5 level! Log messages have a level between 0 and 7! If no level is specified, LOG_NOTICE is the default! TELE Lecture 10 Daemon processes and advanced I/O functions 5

6 facility! Identify the type of process sending the message! If no facility is specified, LOG_USER is the default! TELE Lecture 10 Daemon processes and advanced I/O functions 6

7 Example for syslog! The following call could be issued by a mail daemon when a call to open unexpectedly fails:! syslog(log_info LOG_MAIL, open(%s ): %m, file);! When an application calls syslog for the first time, it creates a Unix domain datagram socket and then calls connect to the well-known pathname of the socket created by the syslogd daemon. This socket remains open until the process terminates.! Alternatively, the process can call openlog and closelog! void openlog(const char *ident, int options, int facility);! ident is a string that will be inserted in front of each log message! options is formed as the logical OR of one or more of the constants in Figure 12.3! facility specifies a default facility! void closelog(void);! TELE Lecture 10 Daemon processes and advanced I/O functions 7

8 Options for openlog! TELE Lecture 10 Daemon processes and advanced I/O functions 8

9 daemon_init function! Control flow of daemon_init (refer to attachment 1: lib/ daemon_init.c)! fork: change the process into a child process! setsid: create a new session and the process becomes the session leader and group leader! Ignore SIGHUP and fork again, so that the daemon cannot automatically acquire a controlling terminal should it open a terminal device in the future. We must ignore SIGHUP because when the session leader terminates, all processes in the session are sent the SIGHUP signal! Set flag for error functions (err_xxx) so that they send error messages to syslogd. We cannot use printf to print any error message from a daemon (because of no controlling terminal)! Change working directory and clear file mode creation mask! Close any open descriptors! openlog for errors! TELE Lecture 10 Daemon processes and advanced I/O functions 9

10 inetd daemon! With systems before 4.3BSD, each service, such as FTP and Telnet, had a process associated with it, which is started at boot time.! All these daemons contained nearly identical startup code, first with respect to socket creation, and also with respect to becoming a daemon process (similar to the daemon_init function).! Each daemon took a slot in the process table, but each daemon was asleep more of the time.! Since 4.3BSD release, it was simplified by providing an Internet superserver: the inetd daemon! It simplifies writing daemon process since most of the startup details are handled by inetd. This obviates the need for each server to the daemon_init function.! It allows a single process (inetd) to be waiting for incoming client requests for multiple services, instead of one process for each service. This reduces the total number of processes in the system.! TELE Lecture 10 Daemon processes and advanced I/O functions 10

11 inetd daemon (cont.)! The inetd process establishes itself as a daemon using the techniques described in the daemon_init function. It then reads and processes its configuration file, typically /etc/inetd.conf. This file specifies the services that the superserver is to handle, and what to do when a service request arrives.! Some sample lines are:!! ftp stream tcp nowait root /usr/bin/ftpd ftpd -1 telnet stream tcp nowait root /usr/bin/telnetd telnetd login stream tcp nowait root /usr/bin/rlogind rlogind -s TELE Lecture 10 Daemon processes and advanced I/O functions 11

12 inetd daemon! TELE Lecture 10 Daemon processes and advanced I/O functions 12

13 Service handled by inetd! TELE Lecture 10 Daemon processes and advanced I/O functions 13

14 Service handled by inetd (cont.)! TELE Lecture 10 Daemon processes and advanced I/O functions 14

15 Daemon invoked by inetd! Control flow (refer to inetd/daytimetcpsrv3.c)! Set daemon flag so that err_xxx can print error messages to syslogd! openlog! Use getpeername to find out the peer address! Receive requests from the client by reading from the descriptor 0! Send response to the client! Close connection.! TELE Lecture 10 Daemon processes and advanced I/O functions 15

16 tcpd! Control flow! Check the client IP address using getpeername! Check the service (port number) using getsockname! Compare the above with the corresponding entries in hosts.allow and hosts.deny files and then decide if the connection should be closed or not! If the connection is allowed, call exec to start the server (using argv)! TELE Lecture 10 Daemon processes and advanced I/O functions 16

17 Socket timeouts! Three ways to place a timeout on an I/O operation involving a socket! Call alarm, which generates the SIGALRM signal when the specified time has expired (refer to Appendix 2: lib/ connect_timeo.c)! Block waiting for I/O in select, which has a time limit as an argument (refer to Appendix 3: lib/readable_timeo.c)! Use the newer SO_RCVTIMEO and SO_SNDTIMEO socket options (refer to Appendix 4: advio/dgclitimeo2.c)! The important point for the programmer is to find out how to test the timeout condition.! TELE Lecture 10 Daemon processes and advanced I/O functions 17

18 recv and send! Prototype! ssize_t recv(int sockfd, void *buff, size_t nbytes, int flags);! ssize_t send(int sockfd, const void *buff, size_t nbytes, int flags);! Both return: number of bytes read or written if OK, -1 on error! The first three arguments are the same as the first three arguments to read and write! flags is either 0, or is formed by logically OR ing one or more of the constants shown in Figure 13.6! TELE Lecture 10 Daemon processes and advanced I/O functions 18

19 Constants for flags! TELE Lecture 10 Daemon processes and advanced I/O functions 19

20 readv and writev! Prototype! ssize_t readv(int filedes, const struct iovec *iov, int iovcnt);! Called scatter read since the input data is scattered into multiple application buffers! ssize_t writev(int filedes, const struct iovec *iov, int iovcnt);! Called gather write since multiple buffers are gathered for a single output operation.! Both return: number of bytes read or written if OK, -1 on error! struct iovec { void *iov_base;! size_t iov_len;};! iovcnt is the number of elements in the array of iovec structures! TELE Lecture 10 Daemon processes and advanced I/O functions 20

21 recvmsg and sendmsg! Prototype! ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);! ssize_t sendmsg(int sockfd, struct msghdr *msg, int flags);! Both return: number of bytes read or written if OK, -1 on error! struct msghdr { void *msg_name;! socklen_t msg_namelen;! struct iovec *msg_iov;! size_t msg_iovlen;! void *msg_control;! socklen_t msg_controllen;! int msg_flags;}! msg_name and msg_namelen are used for unconnected UDP sockets to store a socket address and its length! msg_control and msg_controllen are used for optional ancillary data! TELE Lecture 10 Daemon processes and advanced I/O functions 21

22 Summary of flags!! msg_flags is used only by recvmsg. When recvmsg is called, the flags is copied into the msg_flags member and this value is used by the kernel to drive its receiving processing. This value is then updated based on the result of recvmsg! msg_flags is ignored by sendmsg since this function uses its argument flags! TELE Lecture 10 Daemon processes and advanced I/O functions 22

23 recvmsg! TELE Lecture 10 Daemon processes and advanced I/O functions 23

24 recvmsg (cont.)! TELE Lecture 10 Daemon processes and advanced I/O functions 24

25 Comparison of I/O functions! TELE Lecture 10 Daemon processes and advanced I/O functions 25

26 Ancillary data! struct cmsghdr {socklen_t cmsg_len; int cmsg_level; int cmsg_type; /* followed by char array */ }! TELE Lecture 10 Daemon processes and advanced I/O functions 26

27 Ancillary data (cont.)! TELE Lecture 10 Daemon processes and advanced I/O functions 27

28 Macros for ancillary data! TELE Lecture 10 Daemon processes and advanced I/O functions 28

29 Example for macros! TELE Lecture 10 Daemon processes and advanced I/O functions 29

30 Miscellaneous! Ways to find out how much data is queued! Use MSG_PEEK flag (MSG_DONTWAIT for nonblocking)! Use the FIONREAD command of ioctl! Handle sockets with standard I/O library! Use fdopen to convert a socket (descriptor) into a FILE pointer! Standard I/O uses three types of buffering! Fully buffered: I/O takes place only when the buffer is full, or the process calls fflush or exit! Line buffered: I/O takes place only when a new line is encountered, or the process calls fflush or exit! Unbuffered: I/O takes place each time a standard I/O output function is called! Most Unix implements standard I/O based on the following rules! Standard error is always unbuffered; standard input and output, and other streams are fully buffered, unless they refer to a terminal device, in which case they are line buffered.! TELE Lecture 10 Daemon processes and advanced I/O functions 30