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 (1 st arg) –Specify name of new process (2 nd arg) –Specify any arguments passed to new process –Specify NULL to end parameter list (3 rd arg here) #include 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 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 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 "; 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 –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 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 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 );

Fall 2002 CS 325 Class Notes Page 14 Comments on CreateProcess Can specify program in either 1 st or 2 nd 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