Daemon Processes

1
Daemon Processes

• processes that run in the background
• have no controlling terminal
• typically, system services that need to be
  continuously available are run as daemons
• typically started at boot, and run continuously
• can be any process that we wish to disassociate
  from a session

2
Daemon Characteristics

• background process having no controlling terminal
• typically, daemons are process and group leaders,
  with no other processes in their group
• system daemons are typically started by, or
  inherited by init
• conventionally, programs intended to run as
  daemons have the character d appended to their
  names
• syslogd, initd, ftpd, etc.

3
Daemon Coding Rules

• fork and exit
• guarantees not process group leader
• setsid()
• new session leader, process group leader, no
  controlling terminal
• CWD to local filesystem
• prevents system hang on attempted unmount
• set file mode mask to 0
• avoid inherited mask limitations
• close unnecessary descriptors
• prevents holding open inherited file descriptors

4
Error Logging

• daemons cant write to stderr
• no controlling terminal
• shouldnt write to console unless message urgent
• console may be running windowing server
• can make text console unusable
• can write to specific log file, but makes system
  daemons difficult to monitor
• typically write to system log facility

5
Streams log Driver

• SVR4 uses streams log system
• log messages can go to any of three loggers
• error (for unremarkable errors)
• trace (for debugging)
• console (for urgent messages or those intended
  for a syslog daemon)

6
BSD Syslog system

• used by the vast majority of system daemons
• syslogd handles logging requests via syslog
  functions
• syslogd is itself a daemon
• messages can be generated in three ways
• kernel
• local process
• remote process or local process using network
  port
• syslogd configured from a file on startup
• controls destination file of log messages

7
syslog functions

• three functions for interfacing to syslogd
• openlog()
• if not specifically called, will be called
  automatically when syslog() called for first time
• syslog()
• takes log message and priority
• closelog()
• closes the file descriptor associated with the
  log message (optional)

8
Client-Server Model

• server processes wait for client processes to
  make requests
• one way communication with client
• syslogd, etc.
• two way communication with client
• ftpd, httpd, etc.
• typically, servers run as daemons

9
Interprocess Communication

• methodologies for passing information between
  processes
• implementations and support vary widely across
  systems
• half-duplex pipes are universal
• LINUX and most modern Unices support SVR4 IPC
• messages
• semaphores
• shared memory

10
Pipes

• oldest common form of UNIX IPC
• half-duplex
• data flows in only one direction
• can only be used by processes with a common
  ancestor
• usually used between parent and child
• created using the pipe() function
• pipe() returns a two element array of file
  descriptors
• FD in element 0 used for reading
• FD in element 1 used for writing

11
using pipes

• pipes can be identified using the fstat function
  against the FD
• since pipes are half-duplex, two pipes must be
  opened for two-way communication between
  processes
• if piping from parent to child, parent closes
  read FD and child closes write FD
• if piping from child to parent, parent closes
  write FD and child closes read FD

12
rules for pipes

• when write end of a pipe is closed
• after reading data, read returns 0 to indicate
  end of file
• when read end of a pipe is closed
• write generates SIGPIPE signal
• if signal ignored or caught, write returns error

13
popen() and pclose() Functions

• used for common task of creating a pipe to
  another program
• creates a pipe
• forks a child
• execs a program from the child
• pipe connects to STDIN of output command if type
  is w, STDOUT of output command if type is r

14
Coprocesses

• a coprocess is a filter used by a program which
  both supplies input and accepts output
• filters read from STDIN and write to STDOUT
• typically used to perform data transformations
• grep, sort, etc.
• a process invokes a filter as a coprocess by
  opening two pipes and forking
• the child dups one pipe to STDIN, the other to
  STDOUT then execs the filter

15
FIFOs

• sometimes referred to as named pipes
• used to pipe between unrelated processes
• FIFOs have pathnames, and behave as ordinary
  files
• mkfifo() is used to create a FIFO
• FIFOs can be accessed by multiple processes

16
rules for FIFOs

• when opened for reading or writing, a FIFO will
  block until some process opens it for writing or
  reading respectively, unless opened non-blocking
• if opened non-blocking, an open for reading will
  return immediately, an open for writing will
  return an error if no process has the FIFO open
  for reading
• writing to a FIFO that no process has open for
  reading generates a SIGPIPE signal
• multiple processes may read or write to a FIFO so
  the rules for atomic I/O apply