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NAME

       semctl - semaphore control operations

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
       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
       program 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 short  sem_nsems; /* No. of semaphores in set */
           };

       The  ipc_perm  structure  is  defined  in  <sys/ipc.h>  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.

       IPC_SET   Write  the  values  of some members of the semid_ds structure
                 pointed to by arg.buf to the kernel data structure associated
                 with  this semaphore set, updating also its sem_ctime member.
                 The  following  members  of  the   structure   are   updated:
                 sem_perm.uid, sem_perm.gid, and (the least significant 9 bits
                 of) sem_perm.mode.  The effective UID of the calling  process
                 must    match    the    owner   (sem_perm.uid)   or   creator
                 (sem_perm.cuid) of the semaphore set, or the caller  must  be
                 privileged.  The argument semnum is ignored.

       IPC_RMID  Immediately remove the semaphore set, awakening all processes
                 blocked in semop(2) calls on the set (with  an  error  return
                 and  errno  set  to  EIDRM).   The  effective  user ID of the
                 calling 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
                 information 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.

       GETNCNT   The  system  call  returns  the  value  of  semncnt  for  the
                 semnum-th semaphore of the set (i.e., the number of processes
                 waiting for an increase of semval for the semnum-th semaphore
                 of  the  set).  The calling process must have read permission
                 on the semaphore set.

       GETPID    The system call returns the value of sempid for the semnum-th
                 semaphore  of  the  set  (i.e.,  the  PID of the process that
                 executed the last semop(2) call for the  semnum-th  semaphore
                 of  the  set).  The calling process must have read permission
                 on the semaphore set.

       GETVAL    The system call returns the value of semval for the semnum-th
                 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
                 semnum-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
                 process  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 non-negative 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
                   internal  array  recording  information about all semaphore
                   sets.  (This information can be used with repeated SEM_STAT
                   operations  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.

       All other cmd values return 0 on success.

ERRORS

       On failure, errno will be set to one of the following:

       EACCES The  argument  cmd has one of the values GETALL, GETPID, GETVAL,
              GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVAL and  the
              calling  process  does  not have the required permissions on the
              semaphore set and does not have the CAP_IPC_OWNER capability.

       EFAULT The address pointed to by arg.buf or arg.array isn’t accessible.

       EIDRM  The semaphore set was removed.

       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.

NOTES

       The  IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(8)
       program to provide information on allocated resources.  In  the  future
       these may modified or moved to a /proc file system 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)

COLOPHON

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