SEMCTL(2-SysV)      RISC/os Reference Manual       SEMCTL(2-SysV)



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 sema-
     phore 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 cmd 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 sem-
     val 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 cmd 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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          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 struc-
                     ture pointed to by arg.buf:
                     semperm.uid
                     semperm.gid
                     semperm.mode /* only low 9 bits */

                     This cmd 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.cuid or 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 cmd 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.cuid or semperm.uid in
                     the data structure associated with semid.

ERRORS
     semctl fails 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, or to the value



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                              of semperm.cuid or semperm.uid in
                              the data structure associated with
                              semid.

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

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

DIAGNOSTICS
     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.



































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