Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
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 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
		 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)
		 Return	information about system-wide semaphore	limits and pa-
		 rameters  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  re-
		 turns 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 in-
		 crease	of semval for the semnum-th  semaphore	of  the	 set).
		 The  calling  process	must have read permission on the sema-
		 phore set.

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

       GETVAL	 Return	the value of semval for	the semnum-th semaphore	of the
		 set.	The  calling  process must have	read permission	on the
		 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, up-
		 dating	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:

       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  ef-
	      fective  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.

       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 re-
       compilation  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 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2013-06-03			     SEMCTL(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=semctl&sektion=2&manpath=Debian+8.1.0>

home | help