System Programming Project 2

1
System ProgrammingProject 2

• Due 4/19 ...?

2
Foreground Background

• Foreground job
• Only one at moment
• Having users focus
• Background job
• Many jobs runningin background
• They are totallyinpendent
• No focus

BG
BG
FG
BG
3
FG BG Stopped in Unix
BG
FG
fg
cmd
cmd
fg
bg
CtrlZ
Stopped
4
FG BG Stopped Example

• gtparrot hi
• 01-hi
• 02-hi
• gtjobs
• 1 Stopped parrot hi
• gtbg 1
• gt03-hi
• 04-hi
• gtls
• hello.c abc.txt shell
• gt05-hi
• 06-hi

gtparrot hi gt01-hi 02-hi gtjobs 1 Running
parrot hi gt03-hi 04-hi gtfg 1 05-hi ... 10-hi gt
CtrlZ
Ex.1
Ex.2
5
Signal Handling in shell

• Ex. Terminating FG job

user push CtrlC
Shell receives SIGINT signal
Shell sends SIGINT to FG job
FG job receives SIGINTand terminates
6
Important Unix Signals

• SIGINT (CtrlC by user)
• terminate process
• SIGTSTP (CtrlZ by user)
• stop process
• SIGCONT (from shell to child)
• continue stopped process
• SIGCHLD (from child to shell)
• notify state change of child process
• Run -gt Complete
• Run -gt Terminate or Stop
• Stop -gt Run
• gt kill l show all signals

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signal(), sigaction()

• include ltsignal.hgt
• int flag
• void myHandler (int sig)
• flag1 // this can stop while loop
• main()
• flag0 while (!flag) printf(1)
• flag0 while (!flag) printf(2)
• flag0 while (!flag) printf(3)

// After myHandler is executed // SIGINT handler
changes to default signal(SIGINT, myHandler)
// SIGINT handler is myHandler forever struct
sigaction action, old_action action.sa_handler
myHandler sigemptyset(action.sa_mask) action.sa
_flags SA_RESTART sigaction(SIGINT, action,
old_action
This is exampleof busy-waiting
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Specification 1/2

• CtrlC terminates FG job
• CtrlC shouldnt terminate shell
• Do nothing if there is no FG job
• CtrlZ stops FG job
• CtrlZ shouldnt stop shell
• Do nothing if there is no FG job
• Running in background
• Putting in the end of command
• Terminate all jobs when shell exit

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Specification 2/2

• jobs command
• Shows list of job state
• job_id state command
• State is BG running or Stopped
• fg job_id command
• Change job_id job to FG job
• Stopped or BG ? FG
• bg job_id command change a job to BG
• Change job_id job to BG job
• Stopped ? BG
• - Dont use exec().
• - If job_id is not specified, its up to you.

Make it work like bash
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SIGINT, SIGTSTP Handler

• sigint_handler
• search FG job
• if there is FG job
• send SIGINT to FG job to terminate
• sigtstp_handler
• search FG job
• if there is FG job
• change the jobs state to Stopped
• send SIGTSTP to FG job to stop

Use kill (pid, sig) to send signal!!
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SIGCHLD Handler

• sigchld_handler
• get pid whose state changed to
  completed / terminated / stopped / continue ...
• if it is terminated(SIGINT) or completed
• delete the job from job list
• shell comes back
• else if it is stopped(SIGTSTP)
• shell comes back

See Hints page!!
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Hints

• waitpid(pid, status, option)
• If pid-1, wait for any child to change state
• If option WUNTRACEDWNOHANG
• Return value is pid of process when process is
  terminated or stopped. 0 or -1 for other state
  change
• WNOHANG means doesnt wait
• Status can be checked with macros. - If
  WIFEXITED(status) gt 0, completed - If
  WIFSIGNALED(status) gt 0, terminated - If
  WIFSTOPPED(status) gt 0, stopped
• from http//database.sarang.net/study/glibc/24.htm
  
• and my try and error...T.T

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Hints

• Signal from group A goes to all process in group
  A
• Shells group and child group is same as default.
• But only shell should receive users signal.
• setpgid(pid, pgid)
• Set process group id
• If pid0, it is same as getpid()
• If pgid0, it is same as pid.(pid becomes a
  group leader)
• getpgrp(), getpgid(pid)
• get process group id

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Hints

• sigprocmask(mode, set, oldset)
• Block signals
• Example code
• sigset_t sigset, oldsetsigemptyset(sigset)
  sigaddset(sigset, SIGCHLD) sigprocmask(SIG_BLOC
  K, sigset, oldset)
• / run the job // add job to job list
  /sigprocmask(SIG_UNBLOCK, sigset, oldset)
• ? Short jobs are terminated(sending SIGCHLD)
  before shell adds them to job list. Mask prevents
  this by blocking SIGCHLD.