Manual Pages for UNIX Operating System command usage for man semctl

System Calls                                            semctl(2)



NAME
     semctl - semaphore control operations

SYNOPSIS
     #include 
     #include 
     #include 

     int semctl(int semid, int semnum, int cmd...);


DESCRIPTION
     The semctl() function provides a variety of  semaphore  con-
     trol  operations as specified by cmd. The fourth argument is
     optional,  depending  upon  the  operation   requested.   If
     required,  it  is of type  union semun, which must be expli-
     citly declared by the application program.

       union semun {
               int             val;
               struct semid_ds *buf;
               ushort_t        *array;
       } arg ;



     The permission required for a semaphore operation  is  given
     as  {token},  where  token is the type of permission needed.
     The types of permission are interpreted as follows:

       00400    READ by user
       00200    ALTER by user
       00040    READ by group
       00020    ALTER by group
       00004    READ by others
       00002    ALTER by others



     See the Semaphore Operation Permissions  subsection  of  the
     DEFINITIONS  section  of  Intro(2) for more information. The
     following semaphore operations as specified by cmd are  exe-
     cuted  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



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System Calls                                            semctl(2)



                all processes is cleared.


     GETPID     Return the value of (int) sempid. {READ}


     GETNCNT    Return the value of semncnt. {READ}


     GETZCNT    Return the value of semzcnt. {READ}



     The following operations 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 cmd is successfully
               executed, the semadj values corresponding to  each
               specified semaphore in all processes are cleared.



     The following operations 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). {READ}


     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  /* access permission bits only */

                 This command can be executed only by  a  process
                 that  has  either the {PRIV_IPC_OWNER} privilege
                 or an effective user ID equal to  the  value  of
                 msg_perm.cuid or msg_perm.uid in the data struc-
                 ture associated with msqid. Only a process  with
                 the  {PRIV_SYS_IPC_CONFIG}  privilege  can raise
                 the value of msg_qbytes.




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System Calls                                            semctl(2)



     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 be executed only by a pro-
                 cess that has the {PRIV_IPC_OWNER} privilege  or
                 an  effective  user  ID  equal  to  the value of
                 sem_perm.cuid or sem_perm.uid in the data struc-
                 ture associated with semid.


RETURN VALUES
     Upon successful completion, the value  returned  depends  on
     cmd as follows:

     GETVAL     the value of semval


     GETPID     the value of (int) sempid


     GETNCNT    the value of semncnt


     GETZCNT    the value of semzcnt



     All other successful completions return  0; otherwise, -1 is
     returned and errno is set to indicate the error.

ERRORS
     The semctl() function will fail if:

     EACCES       Operation permission is denied to  the  calling
                  process (see Intro(2)).


     EFAULT       The source or target is not a valid address  in
                  the user process.


     EINVAL       The semid argument is  not  a  valid  semaphore
                  identifier;  the semnum argument is less than 0
                  or greater than sem_nsems -1; or the cmd  argu-
                  ment  is  not a valid command or is IPC_SET and
                  sem_perm.uid or sem_perm.gid is not valid.


     EPERM        The  cmd  argument  is  equal  to  IPC_RMID  or
                  IPC_SET,  the  effective user ID of the calling
                  process  is  not  equal   to   the   value   of
                  sem_perm.cuid   or  sem_perm.uid  in  the  data



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System Calls                                            semctl(2)



                  structure   associated    with    semid,    and
                  {PRIV_IPC_OWNER}  is not asserted in the effec-
                  tive set of the calling process.


     EOVERFLOW    The cmd argument is IPC_STAT and uid or gid  is
                  too large to be stored in the structure pointed
                  to by arg.buf.


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


ATTRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:



     ____________________________________________________________
    |       ATTRIBUTE TYPE        |       ATTRIBUTE VALUE       |
    |_____________________________|_____________________________|
    | Interface Stability         | Committed                   |
    |_____________________________|_____________________________|
    | Standard                    | See standards(5).           |
    |_____________________________|_____________________________|


SEE ALSO
     ipcs(1),  Intro(2),  semget(2),   semop(2),   attributes(5),
     privileges(5), standards(5)






















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