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

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 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.

       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 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.

       GETNCNT	 The system call returns the value of semncnt (i.e., the  num‐
		 ber  of  processes waiting for the value of this semaphore to
		 increase) 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  exe‐
		 cuted	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 (i.e., the num‐
		 ber of processes waiting for the value of this	 semaphore  to
		 become	 zero)	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 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	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.

       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
       This  page  is  part of release 3.22 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				  2008-08-06			     SEMCTL(2)
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