Zombie and orphan processes

1. Zombie and orphan processes

3. Zombie process (from wikipedia) • When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes.

4. Zombie process (from wikipedia) • However, the process's entry in the process table remains. • The parent is sent a SIGCHLD signal indicating that a child has died; the handler for this signal will typically execute the wait system call, which reads the exit status and removes the zombie. • The zombie's process ID and entry in the process table can then be reused. • However, if a parent ignores the SIGCHLD, the zombie will be left in the process table.

5. Orphan process (from wikipedia) • A zombie process is not the same as an orphan process. • Orphan processes don't become zombie processes; instead, they are adopted by init (process ID 1), which waits on its children.

7. Example child process code printf( "child: %s running with %d arg(s). \n", argv[0], argc ); puts( "child: sleeping" ); sleep( 15 ); puts( "child: bye" ); return 0;

8. Parent that creates an orphan. puts( "parent: forking" ); pid_t ret = fork(); //now there’s two of us! switch (ret) { //which one? case -1: //here when fork failed puts( "parent: error: fork failed!" ); break; case 0: //here when executing in child puts( "child: here (before execl)!" ); if (execl( "./childTest.exe", "./childTest.exe", 0 )==-1) perror( "child: execl failed:" ); puts( "child: here (after execl)!" ); //should never get here break; default: //here when executing in parent printf( "parent: child has pid=%d \n", ret ); puts( "parent: bye" ); break; } return 0;

9. Parent that creates a zombie (but only for 2 minutes). puts( "parent: forking" ); pid_t ret = fork(); switch (ret) { case -1: //here when fork failed puts( "parent: error: fork failed!" ); break; case 0: //here when executing in child puts( "child: here (before execl)!" ); if (execl( "./childTest.exe", "./childTest.exe", 0 )==-1) perror( "child: execl failed:" ); puts( "child: here (after execl)!" ); //should never get here break; default: //here when executing in parent printf( "parent: child has pid=%d \n", ret ); puts( "parent: sleeping" ); sleep( 120 ); puts( "parent: bye" ); break; } return 0;

10. What does a zombie look like? • normal (no zombie) $ ps PID TTY TIME CMD 1074 pts/2 00:00:00 bash 1280 pts/2 00:00:00 parentTest.exe 1281 pts/2 00:00:00 childTest.exe 1283 pts/2 00:00:00 ps

11. What does a zombie look like? • abnormal (zombie) $ ps PID TTY TIME CMD 1074 pts/2 00:00:00 bash 1280 pts/2 00:00:00 parentTest.exe 1281 pts/2 00:00:00 childTest.exe <defunct> 1288 pts/2 00:00:00 ps

12. What does a zombie look like? $ ps -l Warning: /boot/System.map has an incorrect kernel version. F S UID PID PPID C PRI NI ADDR SZ WCHAN TTY TIME CMD 000 S 561 1074 1073 0 76 0 - 628 11a418 pts/2 00:00:00 bash 000 S 561 1301 1074 0 70 0 - 436 11f22b pts/2 00:00:00 parentTes 004 Z 561 1302 1301 0 70 0 - 0 119ffb pts/2 00:00:00 childTest 000 R 561 1320 1074 0 77 0 - 646 - pts/2 00:00:00 ps

13. Zombie process (from wikipedia) • When a process ends, all of the memory and resources associated with it are deallocated so they can be used by other processes. However, the process's entry in the process table remains. • The parent is sent a SIGCHLD signal indicating that a child has died; the handler for this signal will typically execute the wait system call, which reads the exit status and removes the zombie. • The zombie's process ID and entry in the process table can then be reused. However, if a parent ignores the SIGCHLD, the zombie will be left in the process table.

14. Avoiding zombies • To avoid creating zombie processes, the parent must: • Handle the SIGCHLD signal. • See man signal. • In this signal handling function, the parent must wait for the child process. • See man 2 wait (linux) or man –s 3C wait (solaris). • We will explore signals as an interprocess communications (IPC) mechanism shortly.