4.1 Operating Systems Lecture 9 Fork and Exec Read Ch

4.2 Process Creation A parent process creates child process(es). Parent Child

4.3 The fork( ) command In UNIX, a child process is created with the fork( ) command. cpid = fork( ); This creates a copy of the process that executed the fork( ) command.  Address spaces for the two processes are (almost) identical.  Both processes have identical open files  Both processes have the program counter (PC) pointing to the statement after the fork( ) command.  The ID of the child process is returned to the parent by fork( ).  Zero is returned to the child process.

4.4 Using fork( ) Example: int main( void) { int cpid; cpid = fork( ); cout << "Hello there." << endl; } Question: What is the output? Demo 1

4.5 Diagram of fork( ) We will draw this in class. Question: What is the difference between the two processes?

4.6 Distinguishing Parent and Child Typical code to distinguish parent and child: cpid = fork( ); if (cpid == 0) { //child code } else { //parent code } Demo 2

4.7 Multiple forks What is the output of the following code? int main (void) { int cpid, cpid2; cpid = fork( ); cpid2 = fork( ); if (cpid == 0) { cout << "I am the first child." << endl; } else if (cpid2 == 0 ) { cout << "I am the second child." << endl; } else { cout << "I am the parent." << endl; } return 0; } Demo 3

4.8 Diagram of two process code We will diagram this in class.

4.9 Distinguishing the grandchild from the child Question: How do you distinguish the grandchild from the child? We will write this code in class.

4.10 Multiple forks from the parent only Question: How do you get the first child not to fork( )? We will write this code in class.

4.11 The exec( ) function There are many versions of the exec( ) function: execv( ), execl( ), execlp( ), execv( ), execvp( ), etc. We will use execlp( ) and execvp( ) execlp( "cmd", arg0, arg1, arg2..., NULL); execvp( "cmd", argv);

4.12 Properties of the exec( ) call The process that calls exec( ) is demolished.  exec( ) overlays (most of) the address space of the caller with the executable file, cmd.  exec( ) overlays the memory image of the process.  The parameters are passed appropriately. Note: Open files are inherited. If you want both child1 and child2 to read/write to the same file, the parent should open it.

4.13 Example with execlp( ) int main(void) { int cpid; cpid = fork( ); if (cpid == 0) {//child execlp( "ls", "ls", "-l", NULL); } else { cout << "I am the parent." << endl; } return 0; } Demo 4

4.14 Process synchronization The parent can wait for a child to terminate using the wait( ) function. int cpid_done, status; cpid_done = wait(&status); The caller sleeps until any child terminates. cpid_done gets the pid of the child that terminated. status gets the exit status of the terminated process. int cpid_done, status, options = 0; cpid_done = waitpid(cpid, &status, options); caller sleeps until child with cpid terminates. returns the pid of the terminating child stores the status of the terminating child in status.

4.15 Process Termination A process terminates with the exit (status) call This terminates the process. It closes all open files associated with the process. Destroys the image of the memory image.

4.16 Typical code using fork, exec and wait int main( void ) int cpid, cpid_done, status; cpid = fork( ); if (cpid == 0) {//child // call exec function with desired cmd } else { cpid = wait(&status); //rest of parent code } return 0; } Demo 5

4.17 Basic Shell code Pseudo-code fpr basic shell (not real C++ code): while (not EOF) { parse_command_line; (get command, args, redirect, etc.) p = fork( ); if (p == 0) { //redirect I/O (takes a few steps to do) // exec (cmd, args) } else { if (command doesn't end with &) wait( ); }