This is the html version of the file http://cs.ua.edu/325/Fall2002/lectures/lec24thruFinal/cs325Fall02lecture-25.ppt.
G o o g l e automatically generates html versions of documents as we crawl the web.
To link to or bookmark this page, use the following url: http://www.google.com/search?q=cache:JCR9v1XUCeEC:cs.ua.edu/325/Fall2002/lectures/lec24thruFinal/cs325Fall02lecture-25.ppt+CreateProcess+simple+example&hl=en&ie=UTF-8

Google is not affiliated with the authors of this page nor responsible for its content.
These search terms have been highlighted:  createprocess  simple  example 
 

Page 1 

Lecture 25 

Today

      exec() in Unix

      CreateProcess in Windows

Announcements 
 
 

Page 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( ?)

 

Page 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 <iostream.h>

#include <stdlib.h>

#include <unistd.h> 

int main( ) {

    cout << "hello world" << endl;

    int pid = fork();

    cout<<"PID is "<<pid<<endl;

    if (pid == 0)

        execl("/usr/bin/ls","ls",NULL);

    else

        cout<<"original proc"<<endl;

}
 

Page 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() {

  ?

  int pid = fork( );

  if (pid == 0) exec( ? ) 

int main() {

   // another program

   // starts at beginning

}
 

Page 5 

Another example with ?exec? 

Consider the ?print? routine shown below

      Prints a number N times

#include <iostream>

#include <stdlib.h>

int main(int argc, char *argv[ ]) {

        // argv[0] is the program name

        int num = atoi(argv[1]);

        int loops = atoi(argv[2]);

        for (int a=0; a<loops; a++)

                cout << num << " ";

} 

Use ?print? in this code

#include <iostream.h>

#include <stdlib.h>

#include <unistd.h>

int main() { 
cout << "hello world" << endl; 
int pid = fork(); 
cout << ?PID is " << pid << endl; 
if (pid == 0)

        execl("./print", "print", 
  "1", "100", NULL); 
else

        execl("./print", "print", 
  "2", "100", NULL);

}
 

Page 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

 

Page 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

 

Page 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, num; 
cout << "Enter 1=who, 2=ls, 3=uptime -> "; 
cin >> cmd; 
while (cmd != 0) { 
 int pid = fork(); 
 if (pid == 0) { 
     if (cmd==1) execl("/usr/bin/who",     "who", NULL); 
     if (cmd==2) execl("/usr/bin/ls", 
   "ls", NULL); 
     if (cmd==3) execl("/usr/bin/uptime",    "uptime",NULL); 
     exit(1); 
 } 
 cin >> cmd; 
}

}
 

Page 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 

 

Page 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
 

Page 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 <windows.h>

      More parameters than fork & exec

      Most can be NULL

      Just need to specify program to run and a few flags 

 

Page 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 <windows.h>

#include <iostream.h>

void main( ) { 
STARTUPINFO si; 
PROCESS_INFORMATION pi; 
ZeroMemory( &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 << "CreateProcess failed." << endl; 
WaitForSingleObject( pi.hProcess, INFINITE ); 
CloseHandle( pi.hProcess ); 
CloseHandle( pi.hThread );

}
 

Page 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

);
 

Page 14 

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)
 

Page 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

      Don?t forget to double the ?\? in string literal