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 sema-
       phore 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 short  sem_nsems; /* No. of semaphores in set */
           };

       The  ipc_perm structure is defined in <sys/ipc.h> as follows (the high-
       lighted 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.

                 and  errno set to EIDRM).  The effective user ID of the call-
                 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.

       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  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
                 process  must  have alter (write) permission on the semaphore
                 set.

       SETVAL    Set the value of semval to arg.val for  the  semnum-th  sema-
                 phore  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.

       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(1)
       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
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://man7.org/linux/man-pages/.



Linux                             2009-09-27                         SEMCTL(2)
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