CS 3733 Operating Systems Notes: USP Chapter 3
(change semester)

  Files are available by executing ~classque/usp-03 on the CS network.

Read USP Chapter 3
Read USP Sections 3.1-3.5 carefully.

Chapter 3 Highlights


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

   pid_t fork(void);
return -1 if error
return 0 to child
return child's pid to parent

Example 3.5, page 65 (simplefork.c): What is the value of x for each process
Example 3.6, Page 65 (twoprocs.c): Parent and child print different messages
Program 3.1, Page 67 (simplechain.c): a chain of processes, parent always breaks
Program 3.2, Page 68 (simplefan.c): a fan of processes, the child breaks

Problem from Spring 2001 Final Exam:
Trace the following program segment and determine how many processes are created. Assume that no errors occur. Draw a graph that shows how the processes are related. In this graph each process will be represented by a small circle containing a number that represents which fork created the process. The original process will contain 0 and the process created by the first fork will contain 1. There will be arrows from each parent to all of its children. Each arrow should point in a downward direction.

   c2 = 0;
   c1 = fork();      /* fork number 1 */
   if (c1 == 0)
      c2 = fork();   /* fork number 2 */
   fork();           /* fork number 3 */
   if (c2 > 0)
      fork();        /* fork number 4 */


   #include <sys/wait.h>

   pid_t wait(int *stat_loc);
   pid_t waitpid(pid_t pid, int *stat_loc, int options);
If a child terminated, return its pid
Otherwise return -1 and set errno

Allows you to wait for a particular process, or all processes if pid is -1. Important option is NOHANG which will return 0 if there is a specified child to wait for but it has not yet terminated.

Important values of errno:
ECHILD no unwaited for children
EINTR a signal was caught

The status value is 0 if and only if the process terminated normally and returned 0. In all other cases the status should be examined using the provided macros defined in sys/wait.h.

Program 3.3 (r_wait.c) shows how to restart a wait call when it is interrupted by a signal.
Exercise 3.20 (fanwaitmsg.c), the parent waits for all of its children
Exercise 3.21 (chainwaitmsg.c) , each process waits for its child.

The following code segments waits for all children that have finished but does not block if there are no children whose status is available:

   pid_t childpid;

   while (childpid = waitpid(-1, NULL, WNOHANG))
      if ((childpid == -1) && (errno != EINTR))

Status values
The following macros are available for checking the return status of a child:

   #include <sys/wait.h>

   WIFEXITED(int stat_val)
   WEXITSTATUS(int stat_val)
   WIFSIGNALED(int stat_val)
   WTERMSIG(int stat_val)
   WIFSTOPPED(int stat_val)
   WSTOPSIG(int stat_val)
They are used in pairs.
If WIFEXITED returns true, the child executed normally and the return status (at most 8 bits) can be gotten with WEXITSTATUS.

The other ones are used when the process exits because of a signal or is stopped.
Example 3.22 on page 77 (showreturnstatus.c) gives an example of using these.

Wait return Value


   #include <unistd.h>

   extern char **environ;
   int execl(const char *path, const char *arg0, ... /*, char *(0) */);
   int execle (const char *path, const char *arg0, ... /*, char *(0),
               char *const envp[] */);
   int execlp (const char *file, const char *arg0, ... /*, char *(0) */);
   int execv(const char *path, char *const argv[]);
   int execve (const char *path, char *const argv[], char *const envp[]);
   int execvp (const char *file, char *const argv[]);
The execl forms take a variable number of parameters with a NULL-terminated list of command line arguments.
The execv forms take an argv array which can be created with makeargv.

The p forms use the PATH environment variable to search for the executable.
The e forms allow you to set the environment of the new process rather than inheriting it from the calling process.

Program 3.4 (execls.c) creates a child to run ls -l.
Program 3.5 (execcmd.c) creates a child process to execute a command given on the command line.
Program 3.6 (execcmdargv.c) creates a child process to execute a command given as a single string on the command line.

Many of the attributes of a program are inherited after an exec.
These include the processID, the userID (owner), current working directory, and open file descriptors.

exec variations

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