semctl

       #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 Sys-
       tem 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.

       IPC_SET   Write  the  values  of some members of the semid_ds structure
                 pointed to by arg.buf to the kernel data structure associated
                 set,  or  the caller must be privileged.  The argument semnum
                 is ignored.

       IPC_INFO (Linux-specific)
                 Return 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)
                 Return 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  sema-
                 phores: 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)
                 Return 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.

       GETNCNT   Return the value of semncnt for the  semnum-th  semaphore  of
                 the  set  (i.e.,  the  number  of  processes  waiting  for an
                 semaphore set.

       GETZCNT   Return 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 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 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:
              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
       POSIX.1-2001, POSIX.1-2001, SVr4.

       POSIX.1 specifies the sem_nsems field of the semid_ds structure as hav-
       ing  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  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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