How to run a shell program

1
Using Processes
2
The fork System Call Revisited

• While fork is called one, it returns twice
• If successful, it will return a value of 0 to the
  child process
• If failed, it will return a 1 and set the global
  variable, errno
• After the fork system cal, both the parent and
  child processes are running and continue their
  execution at the next statement after the fork

3
The fork System Call Revisited (continue)

• To view how to generate a child process, click
  here
• The output sequence is dependent upon the
  scheduling algorithm used by the kernel
• To view Multiple activities parent/child
  processes, click here

a.out PARENT CHILD CHILD PARENT PARENT CHILD CHI
LD PARENT PARENT CHILD
4
execs Minions

• It is important to remember that when a process
  issues any exec call, if the call is successful,
  the existing process is overlaid with a new set
  of program code
• The text, data and stack segment of the process
  are replaced and only the u (user) area of the
  process remains the same
• The new program code begins its execution at the
  function main
• Some things, by necessity, must change
• Signals that were specified as being caught by
  the process are reset to their default action, as
  the address for the signal catching routines are
  no longer valid

5
execs Minions (continue.)

• If the process was profiling (determining how
  much time is spent in individual routines), the
  profiling will be turned off in the overlaid
  process
• If the new program has its SUID bit set, the
  effective EUID and EGID are set accordingly
• If successful, the exec calls do not return as
  the calling image is lost
• In a programming environment, the exec system
  calls can be used to execute another program

6
execs Minions (continue.)

• The prototypes for the exec system calls, listed
  in their entirely, are

7
exec system call functionality
8
exclp system call
9
execs Minions (continue.)

• For the execlp call to be successful, the file
  referenced must be found and be marked as
  executable
• If the number of arguments for the program to be
  executed is dynamic, then the system call execvp
• To view how to use execlp system call, click here

a.out test.txt This is a sample text file for a
program to display!
10
excvp system call
11
execs Minions (continue.)

• To view how to use execvp with argv values, click
  here

a.out cat test.txt This is a sample text file
for a program to display! a.out cat -n
test.txt 1 This is a sample text file for a
program to display!
12
execs Minions (continue.)

• If argv values of the current program are not
  used with execvp, then the programmer must
  construct a new argv to be passed.
• An example of how this can be done is shown in
  the following
• To view how to use execvp with a
  programmer-generated argument, click here

13
Using fork and execs together

• The parent process generates a child process
  which it then overlays by a call to exec
• To view how to use fork with execlp, click here

a.out Fie Foh Fum
14
Using fork and execs together (continue)

• Shell program that restricts the user to a few
  basic commands (ls, ps and df)
• To view the huh? Shell, click here

15
Ending a Process (continue.)

• A process normally terminates in one of three
  ways
• In order of preferences, this are
• It issues (at any point in its code) a call to
  either exit or _exit
• It issues a return in the function main
• It falls off the end of the function main ending
  implicitly

16
exit system call
17
Ending a Process (continue.)

• The exit function accepts a single parameter, and
  integer status value, that will be returned to
  the parent process
• The header file ltstdlib.hgt contains two defined
  constants, EXIT_SUCCESS and EXIT_FAILURE, which
  can be used to indicate program success and
  failure respectively

18
Ending a Process (continue.)

• Upon invocation, the exit function performs three
  actions

exit( ) atexit Function Registered? _cleanup(
) _exit( )
Execute the Function
yes
no
19
Ending a Process (continue.)

• The definition of atexit indicates that functions
  to be called (when the process terminates
  normally) are registered by passing the atexit
  function the address of the function
• The registered function should not have any
  parameters
• If the atexit is successful in registering the
  function, atexit will return a 0, otherwise, it
  returns a non-zero value

20
atexit system call
21
Ending a Process (continue.)

• To view how to use atexit library function, click
  here
• When run, the output of the programs shows that
  the registered functions are called in inverse
  order

a.out Getting ready to exit Doing F3 Doing
F2 Doing F1
22
Ending a Process (continue.)

• Programmers may call _exit directly, if they wish
  to circumvent the invocation of atexit registered
  functions and _cleanup

23
_exit system call
24
Ending a Process (continue.)

• When terminating a process, the system performs a
  number of housekeeping operations
• All open file descriptors are flushed and closed
• The parent of the process is notified (via a
  SIGCHILD signal) that the process is terminating)
• Status information is returned to the parent
  process (if it is waiting for it). If the parent
  process is not waiting, the system stores the
  status information until a wait process is
  affected
• All child processes of the terminating process
  have their parent ID (PPID) set to 1 the process
  ID of init

25
Ending a Process (continue.)

• Shared memory segments and semaphore references
  are readjusted
• If the process was running accounting, the
  accounting record is written out to the
  accounting file

26
Waiting on Processes

• A parent process needs to synchronize its actions
  by waiting until a child process has either
  stopped or terminated its actions

27
wait system call
28
Waiting on Processes (continue.)
wait( ) Child process present? Wait for
Child Process Terminated? Return Status Value
and Child PID
Returns 1 and Set errno
no
yes
no
Block
yes
29
Waiting on Processes (continue.)

• When a waited-for child process terminates, the
  status information for the child and its process
  ID (PID) are returned to the parent
• The status information is stored as an integer
  value at the location referenced by the pointer,
  stat_loc
• The low-order 16 bits of the location contain the
  actual information, and the high-order bits are
  set to zero
• The low-order bit information can be further
  subdivided into low- and high-order byte. This
  information is interpreted in one of two ways

30
Waiting on Processes (continue.)

• If the child process terminated normally, the
  low-order byte will be ) and the high_order byte
  will contain the exit code (0-255)
• If the child process terminated due to an
  uncaught signal, the low-order byte will contain
  the signal number and the high-order byte will be
  0

31
Waiting on Processes (continue.)

• If a core file has been produced the leftmost bit
  of byte 0 will be a 1
• If a NULL argument is specified for wait, the
  child status information is not returned to the
  parent process, the parent is only notified of
  the childs termination
• To view how to use wait, click here for parent
  process and here for child process

32
Waiting on Processes (continue.)

• a.out
• Forked child 3556
• Forked child 3557
• Forked child 3559
• Wait on PID 3559 returns status of 0000
• Forked child 3558
• Forked child 3560
• Wait on PID 3557 returns status of 0000
• Wait on PID 3560 returns status of 0000
• Forked child 3562
• Wait on PID 3562 returns status of 0000
• Forked child 3561
• Wait on PID 3558 returns status of 0000
• Wait on PID 3556 returns status of 0000
• Wait on PID 3561 returns status of 0000

33
Waiting on Processes (continue.)

• a.out
• Forked child 3564
• Forked child 3565
• Forked child 3566
• Forked child 3567
• Wait on PID 3567 returns status of 0000
• Forked child 3568
• Wait on PID 3565 returns status of 0000
• Wait on PID 3568 returns status of 0000
• Forked child 3570
• Wait on PID 3570 returns status of 0000
• Forked child 3569
• Wait on PID 3566 returns status of 0000
• Wait on PID 3564 returns status of 0000
• Wait on PID 3569 returns status of 0000

34
References

• INTERPROCES COMMUNICATIONS IN UNIX BY J. GRAY
• UNIX SYSTEM PROGRAMMING BY K.HAVILAND, D. GRAY
  AND B. SALAMA