Today

1
Lecture 25

• Today
• exec() in Unix
• CreateProcess in Windows
• Announcements

2
Creating new processes

• How do you replace a running process with a new
  process (new set of instructions)
• Use the exec function
• First, create a second version of your process
• via fork( )
• Next, replace the contents of this new process
  with another program
• via exec( )

3
Exec function

• Exec comes in many flavors (see man exec)
• We will use execl
• Specify location of new process (1st arg)
• Specify name of new process (2nd arg)
• Specify any arguments passed to new process
• Specify NULL to end parameter list (3rd arg here)

• include ltiostream.hgt
• include ltstdlib.hgt
• include ltunistd.hgt
• int main( )
• cout ltlt "hello world" ltlt endl
• int pid fork()
• coutltlt"PID is "ltltpidltltendl
• if (pid 0)
• execl("/usr/bin/ls","ls",NULL)
• else
• coutltlt"original proc"ltltendl

4
What happens with exec ?

• Before fork( )
• After fork( )

• After exec( )
• One process still contains the original code we
  started with
• Other process now has completely different code

int main() int pid fork( ) if (pid
0) exec( )
int main() int pid fork( ) if (pid
0) exec( )
int main() int pid fork( ) if (pid
0) exec( )
int main() // another program // starts
at beginning
int main() int pid fork( ) if (pid
0) exec( )
5
Another example with exec

• Consider the print routine shown below
• Prints a number N times
• include ltiostreamgt
• include ltstdlib.hgt
• int main(int argc, char argv )
• // argv0 is the program name
• int num atoi(argv1)
• int loops atoi(argv2)
• for (int a0 altloops a)
• cout ltlt num ltlt " "

• Use print in this code
• include ltiostream.hgt
• include ltstdlib.hgt
• include ltunistd.hgt
• int main() cout ltlt "hello world" ltlt endlint
  pid fork()cout ltlt PID is " ltlt pid ltlt
  endlif (pid 0)
• execl("./print", "print", "1", "100",
  NULL)else
• execl("./print", "print", "2", "100",
  NULL)

6
Class Exercises

• Run the example shown on the previous page
• Compile print.C (from previous slide) into an
  executable print
• Compile the main program and run it
• Try it printing each number
• 100 times
• 1000 times
• 10,000 times
• 100,000 times

7
Creating new processes

• Lots of applications where the ability to
  generate a new process comes in handy
• Simple example command shell in Unix
• The shell is a C program
• Basic algorithm
• Get a command from user
• Interpret the command
• Invoke a process to execute this command

8
Implementing a simple shell

• Overview
• Prompt user
• Get command
• If not time-to-exit
• Fork new process
• Replace new process with either who, ls or uptime
• Read next command

• int main() int cmd, numcout ltlt "Enter 1who,
  2ls, 3uptime -gt "cin gtgt cmdwhile (cmd ! 0)
  int pid fork() if (pid 0) if
  (cmd1) execl("/usr/bin/who", "who",
  NULL) if (cmd2) execl("/usr/bin/ls", "l
  s", NULL) if (cmd3) execl("/usr/bin/uptime
  ", "uptime",NULL) exit(1) cin gtgt
  cmd

9
Class Exercises

• Implement the program shown on the previous
  slide. Run it, and watch its execution.
• What happens when you give an invalid number?
• Modify the program (add two more commands to the
  shell we are writing), consider
• date
• hostname

10
Programs Processes

• We have seen how to write a program that
• When executed, could invoke additional instances
  of itself (fork)
• When executed, could then replace these
  additional instances with other programs
  replace the code in the executing process (exec)
• Unix and Windows handle these basic tasks
  differently

11
Creating processes in Windows

• No exact Windows equivalent of fork and exec
• Windows has CreateProcess method
• Creates a new process and loads the specified
  program into that process (one step)
• Requires include ltwindows.hgt
• More parameters than fork exec
• Most can be NULL
• Just need to specify program to run and a few
  flags

12
Example using CreateProcess

• Need two variables to handle basic info must
  initialize correctly
• Specify the location of the new process (our
  print process from last time)
• Wait for it to finish

• include ltwindows.hgt
• include ltiostream.hgt
• void main( ) STARTUPINFO siPROCESS_INFORMATION
  piZeroMemory( si, sizeof(si) )si.cb
  sizeof(si)if ( ! CreateProcess( NULL,
  ..\\print\\debug\\print.exe 5 10", NULL,
  NULL, TRUE, 0, NULL, NULL, si, pi) ) cerr ltlt
  "CreateProcess failed." ltlt endlWaitForSingleObje
  ct( pi.hProcess, INFINITE )CloseHandle(
  pi.hProcess )CloseHandle( pi.hThread )

13
CreateProcess syntax

• BOOL CreateProcess (
• LPCTSTR lpApplicationName, // pointer to
  executable module
• LPTSTR lpCommandLine, // pointer to command line
  string
• LPSECURITY_ATTRIBUTES lpProcessAttrib, // process
  security
• LPSECURITY_ATTRIBUTES lpThreadAttrib, // thread
  security
• BOOL bInheritHandles, // handle inheritance flag
• DWORD dwCreationFlags, // creation flags
• LPVOID lpEnvironment, // pointer to new
  environment block
• LPCTSTR lpCurrentDirectory, // pointer to current
  dir name
• LPSTARTUPINFO lpStartupInfo, // pointer to
  STARTUPINFO
• LPPROCESS_INFORMATION lpProcessInformation //
  pointer to PROCESS_INFORMATION
• )

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Comments on CreateProcess

• Can specify program in either 1st or 2nd args
• If in first, give location of program to run
• If in second, give the command line to execute
• Creation flags
• If 0, runs in existing window
• Also have other flags (combine with )
• CREATE_NEW_CONSOLE probably most useful
• Specify priority, linkage to parent, etc.
• Structures pi and si used for process
  communication (how to start, basic info)

15
Class Exercises

• Write a program (in the Windows VC environment)
  that starts up a copy of notepad as part of its
  execution
• Notepad is located in c\winnt\ directory
• Dont forget to double the \ in string literal