LinuxGuruz
  • Last 5 Forum Topics
    Replies
    Views
    Last post


The Web Only This Site
  • BOOKMARK

  • ADD TO FAVORITES

  • REFERENCES


  • MARC

    Mailing list ARChives
    - Search by -
     Subjects
     Authors
     Bodies





    FOLDOC

    Computing Dictionary




  • Text Link Ads






  • LINUX man pages
  • Linux Man Page Viewer


    The following form allows you to view linux man pages.

    Command:

    semctl

    
    
    

    SYNOPSIS

           #include <sys/types.h>
           #include <sys/ipc.h>
           #include <sys/sem.h>
    
           int semctl(int semid, int semnum, int cmd, ...);
    
    
    

    DESCRIPTION

           semctl()  performs the control operation specified by cmd on the System
           V semaphore set identified by semid, or on the semnum-th  semaphore  of
           that set.  (The semaphores in a set are numbered starting at 0.)
    
           This  function  has  three  or  four arguments, depending on cmd.  When
           there are four, the fourth has the type union semun.  The calling  pro-
           gram must define this union as follows:
    
               union semun {
                   int              val;    /* Value for SETVAL */
                   struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */
                   unsigned short  *array;  /* Array for GETALL, SETALL */
                   struct seminfo  *__buf;  /* Buffer for IPC_INFO
                                               (Linux-specific) */
               };
    
           The semid_ds data structure is defined in <sys/sem.h> as follows:
    
               struct semid_ds {
                   struct ipc_perm sem_perm;  /* Ownership and permissions */
                   time_t          sem_otime; /* Last semop time */
                   time_t          sem_ctime; /* Last change time */
                   unsigned long   sem_nsems; /* No. of semaphores in set */
               };
    
           The  ipc_perm  structure  is defined as follows (the highlighted fields
           are settable using IPC_SET):
    
               struct ipc_perm {
                   key_t          __key; /* Key supplied to semget(2) */
                   uid_t          uid;   /* Effective UID of owner */
                   gid_t          gid;   /* Effective GID of owner */
                   uid_t          cuid;  /* Effective UID of creator */
                   gid_t          cgid;  /* Effective GID of creator */
                   unsigned short mode;  /* Permissions */
                   unsigned short __seq; /* Sequence number */
               };
    
           Valid values for cmd are:
    
           IPC_STAT  Copy information from the kernel  data  structure  associated
                     with semid into the semid_ds structure pointed to by arg.buf.
                     The argument semnum is ignored.   The  calling  process  must
                     have read permission on the semaphore set.
                     ing process must match the creator or owner of the  semaphore
                     set,  or  the caller must be privileged.  The argument semnum
                     is ignored.
    
           IPC_INFO (Linux-specific)
                     Returns information about system-wide  semaphore  limits  and
                     parameters  in  the  structure pointed to by arg.__buf.  This
                     structure is of type seminfo, defined in <sys/sem.h>  if  the
                     _GNU_SOURCE feature test macro is defined:
    
                         struct  seminfo {
                             int semmap;  /* Number of entries in semaphore
                                             map; unused within kernel */
                             int semmni;  /* Maximum number of semaphore sets */
                             int semmns;  /* Maximum number of semaphores in all
                                             semaphore sets */
                             int semmnu;  /* System-wide maximum number of undo
                                             structures; unused within kernel */
                             int semmsl;  /* Maximum number of semaphores in a
                                             set */
                             int semopm;  /* Maximum number of operations for
                                             semop(2) */
                             int semume;  /* Maximum number of undo entries per
                                             process; unused within kernel */
                             int semusz;  /* Size of struct sem_undo */
                             int semvmx;  /* Maximum semaphore value */
                             int semaem;  /* Max. value that can be recorded for
                                             semaphore adjustment (SEM_UNDO) */
                         };
    
                     The  semmsl,  semmns,  semopm,  and  semmni  settings  can be
                     changed via /proc/sys/kernel/sem; see proc(5) for details.
    
           SEM_INFO (Linux-specific)
                     Returns a seminfo structure containing the  same  information
                     as  for  IPC_INFO,  except  that  the  following  fields  are
                     returned with information about system resources consumed  by
                     semaphores:  the semusz field returns the number of semaphore
                     sets that currently exist on the system; and the semaem field
                     returns  the total number of semaphores in all semaphore sets
                     on the system.
    
           SEM_STAT (Linux-specific)
                     Returns a semid_ds structure as for IPC_STAT.   However,  the
                     semid  argument is not a semaphore identifier, but instead an
                     index into the kernel's internal array that maintains  infor-
                     mation about all semaphore sets on the system.
    
           GETALL    Return semval (i.e., the current value) for all semaphores of
                     the set into arg.array.  The argument semnum is ignored.  The
                     calling  process  must  have read permission on the semaphore
                     set.
                     semaphore  of  the  set.   The calling process must have read
                     permission on the semaphore set.
    
           GETZCNT   The system call returns the value of  semzcnt  for  the  sem-
                     num-th  semaphore  of  the set (i.e., the number of processes
                     waiting for semval of the semnum-th semaphore of the  set  to
                     become  0).  The calling process must have read permission on
                     the semaphore set.
    
           SETALL    Set semval for all semaphores of  the  set  using  arg.array,
                     updating  also the sem_ctime member of the semid_ds structure
                     associated with the set.  Undo  entries  (see  semop(2))  are
                     cleared  for  altered  semaphores  in  all processes.  If the
                     changes to semaphore values  would  permit  blocked  semop(2)
                     calls in other processes to proceed, then those processes are
                     woken up.  The argument semnum is ignored.  The calling  pro-
                     cess must have alter (write) permission on the semaphore set.
    
           SETVAL    Set  the  value  of  semval  to  arg.val  for  the  semnum-th
                     semaphore  of  the set, updating also the sem_ctime member of
                     the semid_ds structure associated with the set.  Undo entries
                     are  cleared for altered semaphores in all processes.  If the
                     changes to semaphore values  would  permit  blocked  semop(2)
                     calls in other processes to proceed, then those processes are
                     woken up.  The calling process must have alter permission  on
                     the semaphore set.
    
    
    

    RETURN VALUE

           On failure semctl() returns -1 with errno indicating the error.
    
           Otherwise  the system call returns a nonnegative value depending on cmd
           as follows:
    
           GETNCNT     the value of semncnt.
    
           GETPID      the value of sempid.
    
           GETVAL      the value of semval.
    
           GETZCNT     the value of semzcnt.
    
           IPC_INFO    the index of the highest used entry in the kernel's  inter-
                       nal  array  recording information about all semaphore sets.
                       (This information can be used with repeated SEM_STAT opera-
                       tions to obtain information about all semaphore sets on the
                       system.)
    
           SEM_INFO    As for IPC_INFO.
    
           SEM_STAT    the identifier of the semaphore set whose index  was  given
                       in semid.
    
           EINVAL Invalid  value  for cmd or semid.  Or: for a SEM_STAT operation,
                  the index value specified in semid referred  to  an  array  slot
                  that is currently unused.
    
           EPERM  The  argument  cmd  has  the  value  IPC_SET or IPC_RMID but the
                  effective user ID of the calling process is not the creator  (as
                  found  in sem_perm.cuid) or the owner (as found in sem_perm.uid)
                  of the  semaphore  set,  and  the  process  does  not  have  the
                  CAP_SYS_ADMIN capability.
    
           ERANGE The argument cmd has the value SETALL or SETVAL and the value to
                  which semval is to be set (for some semaphore  of  the  set)  is
                  less than 0 or greater than the implementation limit SEMVMX.
    
    
    

    CONFORMING TO

           SVr4, POSIX.1-2001.
    
           POSIX.1-2001 specifies the sem_nsems field of the semid_ds structure as
           having the type unsigned short, and the field is  so  defined  on  most
           other  systems.   It was also so defined on Linux 2.2 and earlier, but,
           since Linux 2.4, the field has the type unsigned long.
    
    
    

    NOTES

           The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on  Linux
           or by any version of POSIX.  However, some old implementations required
           the inclusion of these header files, and the SVID also documented their
           inclusion.   Applications  intended  to be portable to such old systems
           may need to include these header files.
    
           The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the  ipcs(1)
           program  to  provide information on allocated resources.  In the future
           these may modified or moved to a /proc filesystem interface.
    
           Various fields in a struct semid_ds were typed as short under Linux 2.2
           and  have  become  long  under Linux 2.4.  To take advantage of this, a
           recompilation under glibc-2.1.91 or later should suffice.  (The  kernel
           distinguishes old and new calls by an IPC_64 flag in cmd.)
    
           In  some  earlier  versions  of  glibc,  the semun union was defined in
           <sys/sem.h>, but POSIX.1-2001 requires  that  the  caller  define  this
           union.  On versions of glibc where this union is not defined, the macro
           _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.
    
           The following system limit on semaphore sets affects a semctl() call:
    
           SEMVMX Maximum value for semval: implementation dependent (32767).
    
           For greater portability it is best to always call  semctl()  with  four
           arguments.
    
    
    

    SEE ALSO

           ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7), svipc(7)
    
  • MORE RESOURCE


  • Linux

    The Distributions





    Linux

    The Software





    Linux

    The News



  • MARKETING






  • Toll Free

webmaster@linuxguruz.com
Copyright © 1999 - 2016 by LinuxGuruz