Fall 2002 CS 325 Class Notes Page 1

Lecture 25• Today

– exec() in Unix– CreateProcess in Windows

• Announcements

Fall 2002 CS 325 Class Notes 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( …)

Fall 2002 CS 325 Class Notes 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;

}

Fall 2002 CS 325 Class Notes 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

}

Fall 2002 CS 325 Class Notes 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);

}

Fall 2002 CS 325 Class Notes 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

Fall 2002 CS 325 Class Notes 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

Fall 2002 CS 325 Class Notes 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;

}}

Fall 2002 CS 325 Class Notes 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

Fall 2002 CS 325 Class Notes 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

Fall 2002 CS 325 Class Notes 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

Fall 2002 CS 325 Class Notes 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 );

}

Fall 2002 CS 325 Class Notes Page 13

CreateProcess syntaxBOOL CreateProcess (

LPCTSTR lpApplicationName, // pointer to executable moduleLPTSTR lpCommandLine, // pointer to command line string LPSECURITY_ATTRIBUTES lpProcessAttrib, // process securityLPSECURITY_ATTRIBUTES lpThreadAttrib, // thread securityBOOL 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

);

Fall 2002 CS 325 Class Notes 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)

Fall 2002 CS 325 Class Notes 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