Notes on Wait & Waitpid

Zombies

• Zombie = a process that has terminated, but whose parent has not yet waited for it
• One way to see zombies is to press Ctrl-Z (suspend) in the midst of execution and then enter a ps command. Zombies appear as <defunct> processes.

Wait / Waitpid

Either of wait or waitpid can be used to remove zombies.

wait (and waitpid in it's blocking form) temporarily suspends the execution of a parent process while a child process is running. Once the child has finished, the waiting parent is restarted.

Declarations:

    #include <sys/types.h>
    #include <sys/wait.h>

    pid_t wait(int *statloc);
    /* returns process ID if OK, or -1 on error */

    pid_t waitpid(pid_t pid, int *statloc, int options);
    /* returns process ID : if OK,
     *          0         : if non-blocking option && no zombies around
     *          -1        : on error
     */

The statloc argument can be one of two values:

• NULL pointer: the argument is simply ignored
• pointer to an integer: when wait returns, the integer this describes will contain status information of the terminated process (see Stevens p.198 for macros that examine the termination status)

wait()	waitpid()
wait blocks the caller until a child process terminates	waitpid can be either blocking or non-blocking: If options is 0, then it is blocking If options is WNOHANG, then is it non-blocking
if more than one child is running then wait() returns the first time one of the parent's offspring exits	waitpid is more flexible: If pid == -1, it waits for any child process. In this respect, waitpid is equivalent to wait If pid > 0, it waits for the child whose process ID equals pid If pid == 0, it waits for any child whose process group ID equals that of the calling process If pid < -1, it waits for any child whose process group ID equals that absolute value of pid

References

• "UNIX System Programming", by Richard Stevens
• "Advanced Programming in the UNIX Environment", by Richard Stevens