semctl(2)                                               semctl(2)



     NAME
          semctl - semaphore control operations

     SYNOPSIS
          #include <sys/types.h>
          #include <sys/ipc.h>
          #include <sys/sem.h>

          int semctl (semid, semnum, cmd, arg)
          int semid, cmd;
          int semnum;
          union semun {
               int val;
               struct semidds *buf;
               ushort *array;
          } arg;

     DESCRIPTION
          semctl provides a variety of semaphore control operations as
          specified by cmd.

          The following cmds are executed with respect to the
          semaphore specified by semid and semnum:

               GETVAL     Return the value of semval (see intro(2)).
                          {READ}

               SETVAL     Set the value of semval to arg.val.  {ALTER}
                          When this command is successfully executed,
                          the semadj value corresponding to the
                          specified semaphore in all processes is
                          cleared.

               GETPID     Return the value of sempid.  {READ}

               GETNCNT    Return the value of semncnt.  {READ}

               GETZCNT    Return the value of semzcnt.  {READ}

          The following cmds return and set, respectively, every
          semval in the set of semaphores.

               GETALL     Place semvals into array pointed to by
                          arg.array.  {READ}

               SETALL     Set semvals according to the array pointed
                          to by arg.array.  {ALTER} When this command
                          is successfully executed, the semadj values
                          corresponding to each specified semaphore in
                          all processes are cleared.

          The following cmds are also available:



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     semctl(2)                                               semctl(2)



               IPCSTAT   Place the current value of each member of
                          the data structure associated with semid
                          into the structure pointed to by arg.buf.
                          The contents of this structure are defined
                          in intro(2).  {READ}

               IPCSET    Set the value of the following members of
                          the data structure associated with semid to
                          the corresponding value found in the
                          structure pointed to by arg.buf:
                          semperm.uid
                          semperm.gid
                          semperm.mode /*only low 9 bits*/

                          This command can only be executed by a
                          process that has an effective user ID equal
                          to either that of super-user or to the value
                          of semperm.uid in the data structure
                          associated with semid.

               IPCRMID   Remove the semaphore identifier specified by
                          semid from the system and destroy the set of
                          semaphores and data structure associated
                          with it.  This command can only be executed
                          by a process that has an effective user ID
                          equal to either that of super-user or to the
                          value of semperm.uid in the data structure
                          associated with semid.

               semctl will fail if one or more of the following are
               true:

               [EINVAL]       semid is not a valid semaphore
                              identifier.

               [EINVAL]       semnum is less than zero or greater than
                              semnsems.

               [EINVAL]       cmd is not a valid command.

               [EACCES]       Operation permission is denied to the
                              calling process (see intro(2)).

               [ERANGE]       cmd is SETVAL or SETALL and the value to
                              which semval is to be set is greater
                              than the system imposed maximum.

               [EPERM]        cmd is equal to IPCRMID or IPCSET and
                              the effective user ID of the calling
                              process is not equal to that of super-
                              user and it is not equal to the value of
                              semperm.uid in the data structure



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     semctl(2)                                               semctl(2)



                              associated with semid.

               [EFAULT]       arg.buf points to an illegal address.

     RETURN VALUE
          Upon successful completion, the value returned depends on
          cmd as follows:
               GETVAL         The value of semval.
               GETPID         The value of sempid.
               GETNCNT        The value of semncnt.
               GETZCNT        The value of semzcnt.
               All others     A value of 0.
          Otherwise, a value of -1 is returned and errno is set to
          indicate the error.

     SEE ALSO
          intro(2), semget(2), semop(2).






































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