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SEMCTL(2)			    Linux Programmer's Manual				SEMCTL(2)

NAME
       semctl - System V 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 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 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 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 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  sema-
		 phore	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 sema-
		 phore 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 pro-
		 cesses 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:

       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 capa-
	      bility.

       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 implemen-
	      tation 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)

COLOPHON
       This  page  is  part of release 3.55 of the Linux man-pages project.  A description of the
       project,    and	  information	 about	  reporting    bugs,	can    be    found     at
       http://www.kernel.org/doc/man-pages/.

Linux					    2013-06-03					SEMCTL(2)
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