Fork, semctl, wait

[spoofG]

I create n processes with fork(). Every child must calculate something and write result in shared memory. For each child i use semaphore and when child is writing something to memory, semaphore is red(value 0), after first child is done, it calls signalize(semaphores value is 1) so next child can start with writing in critical part . Now I dont know how to wait, that all childs end with writing without using wait call? How could i wait for all childs to complete their work, so father can print the results from memory?

Thanks

[Rubberman]

When each child terminates, the parent will get a SIGCHLD, so it can wait until all the child processes are done, at which time it can print the results. It should not need to do anything except create/destroy the semaphore.

[spoofG]

Ok.. i do like this..
for(i=0; i<10; i++){
if(fork()==0){
W(semid, i) puts semaphore on 0
/*here is something written to memory
*/
S(semid,i) signalizes so next child can use semaphore
exit(0);
}

after this i have to check if all childs are terminated.. so how does this SIGCHLD checking looks in code..

thanks for help
and when

[Rubberman]

The man page for wait():

WAIT(2)                    Linux Programmer’s Manual                   WAIT(2)

NAME
       wait, waitpid - wait for process to change state

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

       pid_t wait(int *status);
       pid_t waitpid(pid_t pid, int *status, int options);
       int waitid(idtype_t idtype, id_t id, siginfo_t *infop, int options);

DESCRIPTION
       All  of  these  system  calls are used to wait for state changes in a
       child of the calling process, and obtain information about the  child
       whose  state  has  changed.   A state change is considered to be: the
       child terminated; the child was stopped by a signal; or the child was
       resumed by a signal.  In the case of a terminated child, performing a
       wait allows the system to release the resources associated  with  the
       child;  if a wait is not performed, then terminated the child remains
       in a "zombie" state (see NOTES below).

       If a child has already changed state, then these calls return immedi-
       ately.   Otherwise they block until either a child changes state or a
       signal handler interrupts the call (assuming that  system  calls  are
       not  automatically  restarted  using  the  SA_RESTART  flag of sigac-
       tion(2)).  In the remainder of this page, a  child  whose  state  has
       changed and which has not yet been waited upon by one of these system
       calls is termed waitable.

   wait() and waitpid()
       The wait() system call suspends  execution  of  the  current  process
       until  one  of  its  children  terminates.  The call wait(&status) is
       equivalent to:

           waitpid(-1, &status, 0);

       The waitpid() system call suspends execution of the  current  process
       until  a  child  specified  by  pid  argument  has changed state.  By
       default, waitpid() waits  only  for  terminated  children,  but  this
       behaviour is modifiable via the options argument, as described below.

       The value of pid can be:

       < -1   meaning wait for any child process whose process group  ID  is
              equal to the absolute value of pid.

       -1     meaning wait for any child process.

       0      meaning  wait  for any child process whose process group ID is
              equal to that of the calling process.

       > 0    meaning wait for the child whose process ID is  equal  to  the
              value of pid.

       The  value  of options is an OR of zero or more of the following con-
       stants:

       WNOHANG
              return immediately if no child has exited.

       WUNTRACED
              also return if  a  child  has  stopped  (but  not  traced  via
              ptrace(2)).   Status for traced children which have stopped is
              provided even if this option is not specified.

       WCONTINUED
              (Since Linux 2.6.10) also return if a stopped child  has  been
              resumed by delivery of SIGCONT.

       (For Linux-only options, see below.)

       The  WUNTRACED  and  WCONTINUED  options  are  only  effective if the
       SA_NOCLDSTOP flag has not been set for the SIGCHLD signal (see sigac-
       tion(2)).

       If  status is not NULL, wait() and waitpid() store status information
       in the int to which it points.  This integer can  be  inspected  with
       the  following  macros (which take the integer itself as an argument,
       not a pointer to it, as is done in wait() and waitpid()!):

       WIFEXITED(status)
              returns true if the child terminated  normally,  that  is,  by
              calling exit(3) or _exit(2), or by returning from main().

       WEXITSTATUS(status)
              returns  the  exit  status of the child.  This consists of the
              least significant 16-8 bits of the status  argument  that  the
              child specified in a call to exit() or _exit() or as the argu-
              ment for a return statement in main().  This macro should only
              be employed if WIFEXITED returned true.

       WIFSIGNALED(status)
              returns  true if the child process was terminated by a signal.

       WTERMSIG(status)
              returns the number of the signal that caused the child process
              to  terminate.  This  macro should only be employed if WIFSIG-
              NALED returned true.

       WCOREDUMP(status)
              returns true if the child produced a core  dump.   This  macro
              should  only  be  employed if WIFSIGNALED returned true.  This
              macro is not specified in POSIX.1-2001 and is not available on
              some  Unix  implementations (e.g., AIX, SunOS).  Only use this
              enclosed in #ifdef WCOREDUMP ... #endif.

       WIFSTOPPED(status)
              returns true if the child process was stopped by delivery of a
              signal;  this is only possible if the call was done using WUN-
              TRACED or when the child is being traced (see ptrace(2)).

       WSTOPSIG(status)
              returns the number of the signal which  caused  the  child  to
              stop.   This  macro  should  only  be  employed  if WIFSTOPPED
              returned true.

       WIFCONTINUED(status)
              (Since Linux 2.6.10) returns true if  the  child  process  was
              resumed by delivery of SIGCONT.

   waitid()
       The  waitid() system call (available since Linux 2.6.9) provides more
       precise control over which child state changes to wait for.

       The idtype and id arguments select the child(ren)  to  wait  for,  as
       follows:

       idtype == P_PID
              Wait for the child whose process ID matches id.

       idtype == P_PGID
              Wait for any child whose process group ID matches id.

       idtype == P_ALL
              Wait for any child; id is ignored.

       The  child  state  changes  to wait for are specified by ORing one or
       more of the following flags in options:

       WEXITED
              Wait for children that have terminated.

       WSTOPPED
              Wait for children that have been stopped by delivery of a sig-
              nal.

       WCONTINUED
              Wait  for (previously stopped) children that have been resumed
              by delivery of SIGCONT.

       The following flags may additionally be ORed in options:

       WNOHANG
              As for waitpid().

       WNOWAIT
              Leave the child in a waitable state; a later wait call can  be
              used to again retrieve the child status information.

       Upon successful return, waitid() fills in the following fields of the
       siginfo_t structure pointed to by infop:

       si_pid The process ID of the child.

       si_uid The real user ID of the child.  (This field is not set on most
              other implementations.)

       si_signo
              Always set to SIGCHLD.

       si_status
              Either  the exit status of the child, as given to _exit(2) (or
              exit(3)), or the signal that caused the  child  to  terminate,
              stop, or continue.  The si_code field can be used to determine
              how to interpret this field.

       si_code
              Set to one of: CLD_EXITED (child called _exit(2));  CLD_KILLED
              (child  killed  by signal); CLD_STOPPED (child stopped by sig-
              nal); or CLD_CONTINUED (child continued by SIGCONT).

       If WNOHANG was specified in options and there were no children  in  a
       waitable  state, then waitid() returns 0 immediately and the state of
       the siginfo_t structure pointed to by infop is unspecified.  To  dis-
       tinguish  this  case from that where a child was in a waitable state,
       zero out the si_pid field before the call and check  for  a  non-zero
       value in this field after the call returns.

RETURN VALUE
       wait():  on  success, returns the process ID of the terminated child;
       on error, -1 is returned.

       waitpid(): on success, returns the process  ID  of  the  child  whose
       state has changed; on error, -1 is returned; if WNOHANG was specified
       and no child(ren) specified by pid has yet changed state, then  0  is
       returned.

       waitid():  returns  0  on  success or if WNOHANG was specified and no
       child(ren) specified by id has yet changed state;  on  error,  -1  is
       returned.

       Each of these calls sets errno to an appropriate value in the case of
       an error.

ERRORS
       ECHILD (for wait()) The calling process does not have  any  unwaited-
              for children.

       ECHILD (for  waitpid()  or  waitid())  The  process  specified by pid
              (waitpid()) or idtype and id (waitid()) does not exist  or  is
              not  a  child  of  the  calling process.  (This can happen for
              one’s own child if the action for SIGCHLD is set  to  SIG_IGN.
              See also the LINUX NOTES section about threads.)

       EINTR  WNOHANG  was  not set and an unblocked signal or a SIGCHLD was
              caught.

       EINVAL The options argument was invalid.

NOTES
       A child that terminates, but has not been waited for becomes a  "zom-
       bie".   The  kernel  maintains a minimal set of information about the
       zombie process (PID, termination status, resource usage  information)
       in order to allow the parent to later perform a wait to obtain infor-
       mation about the child.  As long as a zombie is not removed from  the
       system  via  a wait, it will consume a slot in the kernel process ta-
       ble, and if this table fills, it will not be possible to create  fur-
       ther  processes.   If  a parent process terminates, then its "zombie"
       children (if any) are adopted by init(8),  which  automatically  per-
       forms a wait to remove the zombies.

       POSIX.1-2001  specifies  that if the disposition of SIGCHLD is set to
       SIG_IGN or the SA_NOCLDWAIT flag  is  set  for  SIGCHLD  (see  sigac-
       tion(2)),  then  children  that terminate do not become zombies and a
       call to wait() or waitpid() will block until all children have termi-
       nated,  and  then fail with errno set to ECHILD.  (The original POSIX
       standard left the behaviour of setting SIGCHLD  to  SIG_IGN  unspeci-
       fied.)  Linux 2.6 conforms to this specification.  However, Linux 2.4
       (and earlier) does not: if a wait() or waitpid() call is  made  while
       SIGCHLD  is  being  ignored,  the call behaves just as though SIGCHLD
       were not being ignored, that is, the call blocks until the next child
       terminates  and then returns the process ID and status of that child.

LINUX NOTES
       In the Linux kernel, a kernel-scheduled thread is not a distinct con-
       struct  from a process. Instead, a thread is simply a process that is
       created using the Linux-unique clone(2) system call;  other  routines
       such  as  the  portable  pthread_create(3) call are implemented using
       clone(2).  Before Linux 2.4, a thread was just a special  case  of  a
       process,  and as a consequence one thread could not wait on the chil-
       dren of another thread, even when the  latter  belongs  to  the  same
       thread  group.   However,  POSIX  prescribes  such functionality, and
       since Linux 2.4 a thread can, and by default will, wait  on  children
       of other threads in the same thread group.

       The  following  Linux-specific options are for use with children cre-
       ated using clone(2); they cannot be used with waitid():

       __WCLONE
              Wait for "clone" children only.   If  omitted  then  wait  for
              "non-clone"  children  only.   (A  "clone"  child is one which
              delivers no signal, or a signal other than SIGCHLD to its par-
              ent  upon  termination.)   This option is ignored if __WALL is
              also specified.

       __WALL (Since Linux 2.4) Wait for all children,  regardless  of  type
              ("clone" or "non-clone").

       __WNOTHREAD
              (Since Linux 2.4) Do not wait for children of other threads in
              the same thread group. This was the default before Linux  2.4.

EXAMPLE
       The following program demonstrates the use of fork(2) and waitpid(2).
       The program creates a child process.  If no command-line argument  is
       supplied  to the program, then the child suspends its execution using
       pause(2), to allow the user to send signals to the child.  Otherwise,
       if  a command-line argument is supplied, then the child exits immedi-
       ately, using the integer supplied on the command  line  as  the  exit
       status.   The  parent process executes a loop that monitors the child
       using waitpid(2), and uses the W*() macros described above to analyse
       the wait status value.

       The following shell session demonstrates the use of the program:

       $ ./a.out &
       Child PID is 32360
       [1] 32359
       $ kill -STOP 32360
       stopped by signal 19
       $ kill -CONT 32360
       continued
       $ kill -TERM 32360
       killed by signal 15
       [1]+  Done                    ./a.out
       $

       #include <sys/wait.h>
       #include <stdlib.h>
       #include <unistd.h>
       #include <stdio.h>

       int
       main(int argc, char *argv[])
       {
           pid_t cpid, w;
           int status;

           cpid = fork();
           if (cpid == -1) { perror("fork"); exit(EXIT_FAILURE); }

           if (cpid == 0) {            /* Code executed by child */
               printf("Child PID is %ld\n", (long) getpid());
               if (argc == 1)
                   pause();                    /* Wait for signals */
               _exit(atoi(argv[1]));

           } else {                    /* Code executed by parent */
               do {
                   w = waitpid(cpid, &status, WUNTRACED | WCONTINUED);
                   if (w == -1) { perror("waitpid"); exit(EXIT_FAILURE); }

                   if (WIFEXITED(status)) {
                       printf("exited, status=%d\n", WEXITSTATUS(status));
                   } else if (WIFSIGNALED(status)) {
                       printf("killed by signal %d\n", WTERMSIG(status));
                   } else if (WIFSTOPPED(status)) {
                       printf("stopped by signal %d\n", WSTOPSIG(status));
                   } else if (WIFCONTINUED(status)) {
                       printf("continued\n");
                   }
               } while (!WIFEXITED(status) && !WIFSIGNALED(status));
               exit(EXIT_SUCCESS);
           }
       }

CONFORMING TO
       SVr4, 4.3BSD, POSIX.1-2001.

SEE ALSO
       _exit(2),  clone(2),  fork(2), kill(2), ptrace(2), sigaction(2), sig-
       nal(2), wait4(2), pthread_create(3), signal(7)

Linux                             2004-11-11                           WAIT(2)

[spoofG]

I can't use wait()? This is what i am asking you..what to use so it would work same as wait()?

[Rubberman]

Why can't you use wait? An alternative would be to set up a signal handler to process SIGCHLD events, but that is not optimal. The wait() functions were designed for exactly the purpose you mention.

[spoofG]

This is our task. I know how to use wait..but we can't use wait in this case. Is there any chance i could check if all child processes finished. Or if all semaphors are set to 1 or smt?

[Rubberman]

An explanation of why you can't use wait() would be helpful. As I said, you can set up the father/parent process with a signal handler and know when each child is finished. A simple counter would be fine. One of the reasons to use wait() is to keep zombie processes from cluttering the system. They will stay around until either the parent finishes them with wait(), or the parent dies.

[trutheality]

This is our task. I know how to use wait..but we can't use wait in this case. Is there any chance i could check if all child processes finished. Or if all semaphors are set to 1 or smt?

Is this some sort of homework assignment?

Anyway, the obvious way to do it without using wait() is to set up a while loop that keeps going while the semaphores aren't all 1.

That's the ugly solution. A nicer one is a signal handler and the nicest is wait()

[MikeTbob]

Is this some sort of homework assignment?

Anyway, the obvious way to do it without using wait() is to set up a while loop that keeps going while the semaphores aren't all 1.

That's the ugly solution. A nicer one is a signal handler and the nicest is wait()

I agree. Homework and school assignments are forbidden here on these forums...we'll be keeping and eye on this thread.