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 semid_ds *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 (see intro(2) for
     required permissions and structure declarations):

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

     SETVAL
          Set the value of semval to arg.val.  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.

     GETNCNT
          Return the value of semncnt.

     GETZCNT
          Return the value of semzcnt.

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

     GETALL
          Place semvals into array pointed to by arg.array.

     SETALL
          Set semvals according to the array pointed to by
          arg.array.  When this command is successfully executed,



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semctl(2)                                               semctl(2)



          the semadj values corresponding to each specified
          semaphore in all processes are cleared.

     The following cmds are also available:

     IPC_STAT
          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).

     IPC_SET
          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:
          sem_perm.uid
          sem_perm.gid
          sem_perm.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 superuser
          or to the value of sem_perm.uid in the data structure
          associated with semid.

     IPC_RMID
          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 superuser or to the value of
          sem_perm.uid in the data structure associated with
          semid.  The identifier and its associated data
          structure are not actually removed until there are no
          more referencing processes.  See ipcrm(1), and ipcs(1).

STATUS MESSAGES AND VALUES
     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



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semctl(2)                                               semctl(2)



               A value of 0.

     Otherwise, a value of -1 is returned and errno is set to
     indicate the error.

     The semctl command will fail if one or more of the following
     is true:

     EINVAL
          semid is not a valid semaphore identifier.

     EINVAL
          semnum is less than zero or greater than sem_nsems.

     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 IPC_RMID or IPC_SET and the effective
          user ID of the calling process is not equal to that of
          superuser and it is not equal to the value of
          sem_perm.uid in the data structure associated with
          semid.

     EFAULT
          arg.buf points to an illegal address.

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

















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