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

NAME
       shmctl -	System V shared	memory control

SYNOPSIS
       #include	<sys/ipc.h>
       #include	<sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION
       shmctl()	 performs  the	control	operation specified by cmd on the Sys-
       tem V shared memory segment whose identifier is given in	shmid.

       The buf argument	is a pointer  to  a  shmid_ds  structure,  defined  in
       _sys/shm.h_ as follows:

	   struct shmid_ds {
	       struct ipc_perm shm_perm;    /* Ownership and permissions */
	       size_t	       shm_segsz;   /* Size of segment (bytes) */
	       time_t	       shm_atime;   /* Last attach time	*/
	       time_t	       shm_dtime;   /* Last detach time	*/
	       time_t	       shm_ctime;   /* Last change time	*/
	       pid_t	       shm_cpid;    /* PID of creator */
	       pid_t	       shm_lpid;    /* PID of last shmat(2)/shmdt(2) */
	       shmatt_t	       shm_nattch;  /* No. of current attaches */
	       ...
	   };

       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 shmget(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 + SHM_DEST and
					   SHM_LOCKED flags */
	       unsigned	short __seq;	/* Sequence number */
	   };

       Valid values for	cmd are:

       IPC_STAT	 Copy information from the kernel  data	 structure  associated
		 with  shmid  into  the	 shmid_ds structure pointed to by buf.
		 The caller must have read permission  on  the	shared	memory
		 segment.

       IPC_SET	 Write	the  values  of	some members of	the shmid_ds structure
		 pointed to by buf to the  kernel  data	 structure  associated
		 with  this shared memory segment, updating also its shm_ctime
		 member.  The following	fields can be  changed:	 shm_perm.uid,
		 shm_perm.gid,	 and   (the   least  significant  9  bits  of)
		 shm_perm.mode.	 The effective UID of the calling process must
		 match	the owner (shm_perm.uid) or creator (shm_perm.cuid) of
		 the shared memory segment, or the caller must be privileged.

       IPC_RMID	 Mark the segment to be	destroyed.  The	segment	will only  ac-
		 tually	be destroyed after the last process detaches it	(i.e.,
		 when  the  shm_nattch	member	of  the	 associated  structure
		 shmid_ds  is  zero).  The caller must be the owner or creator
		 of the	segment, or be privileged.  The	buf  argument  is  ig-
		 nored.

		 If  a segment has been	marked for destruction,	then the (non-
		 standard) SHM_DEST flag of the	shm_perm.mode field in the as-
		 sociated data structure retrieved by IPC_STAT will be set.

		 The  caller  must  ensure  that  a  segment is	eventually de-
		 stroyed; otherwise its	pages that were	faulted	in will	remain
		 in memory or swap.

		 See  also the description of /proc/sys/kernel/shm_rmid_forced
		 in proc(5).

       IPC_INFO	(Linux-specific)
		 Return	information about system-wide shared memory limits and
		 parameters  in	 the structure pointed to by buf.  This	struc-
		 ture is of type shminfo (thus,	a cast is  required),  defined
		 in  _sys/shm.h_  if the _GNU_SOURCE feature test macro	is de-
		 fined:

		     struct shminfo {
			 unsigned long shmmax; /* Maximum segment size */
			 unsigned long shmmin; /* Minimum segment size;
						  always 1 */
			 unsigned long shmmni; /* Maximum number of segments */
			 unsigned long shmseg; /* Maximum number of segments
						  that a process can attach;
						  unused within	kernel */
			 unsigned long shmall; /* Maximum number of pages of
						  shared memory, system-wide */
		     };

		 The shmmni, shmmax, and shmall	settings can  be  changed  via
		 /proc files of	the same name; see proc(5) for details.

       SHM_INFO	(Linux-specific)
		 Return	 a shm_info structure whose fields contain information
		 about system  resources  consumed  by	shared	memory.	  This
		 structure  is	defined	in _sys/shm.h_ if the _GNU_SOURCE fea-
		 ture test macro is defined:

		     struct shm_info {
			 int	       used_ids; /* # of currently existing
						    segments */
			 unsigned long shm_tot;	 /* Total number of shared
						    memory pages */
			 unsigned long shm_rss;	 /* # of resident shared
						    memory pages */
			 unsigned long shm_swp;	 /* # of swapped shared
						    memory pages */
			 unsigned long swap_attempts;
						 /* Unused since Linux 2.4 */
			 unsigned long swap_successes;
						 /* Unused since Linux 2.4 */
		     };

       SHM_STAT	(Linux-specific)
		 Return	a shmid_ds structure as	for  IPC_STAT.	 However,  the
		 shmid	argument  is  not a segment identifier,	but instead an
		 index into the	kernel's internal array	that maintains	infor-
		 mation	about all shared memory	segments on the	system.

       The  caller  can	 prevent  or allow swapping of a shared	memory segment
       with the	following cmd values:

       SHM_LOCK	(Linux-specific)
		 Prevent swapping of the shared	memory	segment.   The	caller
		 must fault in any pages that are required to be present after
		 locking is enabled.  If a segment has been locked,  then  the
		 (nonstandard)	SHM_LOCKED  flag of the	shm_perm.mode field in
		 the associated	data structure retrieved by IPC_STAT  will  be
		 set.

       SHM_UNLOCK (Linux-specific)
		 Unlock	the segment, allowing it to be swapped out.

       In  kernels  before  2.6.10,  only  a  privileged  process could	employ
       SHM_LOCK	and SHM_UNLOCK.	 Since kernel 2.6.10, an unprivileged  process
       can  employ  these operations if	its effective UID matches the owner or
       creator UID of the segment, and (for SHM_LOCK) the amount of memory  to
       be  locked  falls  within  the RLIMIT_MEMLOCK resource limit (see setr-
       limit(2)).

RETURN VALUE
       A successful IPC_INFO or	SHM_INFO operation returns the	index  of  the
       highest used entry in the kernel's internal array recording information
       about all shared	memory segments.  (This	information can	be  used  with
       repeated	 SHM_STAT  operations  to  obtain information about all	shared
       memory segments on the system.)	A successful  SHM_STAT	operation  re-
       turns the identifier of the shared memory segment whose index was given
       in shmid.  Other	operations return 0 on success.

       On error, -1 is returned, and errno is set appropriately.

ERRORS
       EACCES IPC_STAT or SHM_STAT is requested	and shm_perm.mode does not al-
	      low read access for shmid, and the calling process does not have
	      the CAP_IPC_OWNER	capability.

       EFAULT The argument cmd has value IPC_SET or IPC_STAT but  the  address
	      pointed to by buf	isn't accessible.

       EIDRM  shmid points to a	removed	identifier.

       EINVAL shmid  is	not a valid identifier,	or cmd is not a	valid command.
	      Or: for a	SHM_STAT operation, the	index value specified in shmid
	      referred to an array slot	that is	currently unused.

       ENOMEM (In kernels since	2.6.9),	SHM_LOCK was specified and the size of
	      the to-be-locked segment would mean  that	 the  total  bytes  in
	      locked  shared  memory  segments	would exceed the limit for the
	      real user	ID of the calling process.  This limit is  defined  by
	      the RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).

       EOVERFLOW
	      IPC_STAT	is attempted, and the GID or UID value is too large to
	      be stored	in the structure pointed to by buf.

       EPERM  IPC_SET or IPC_RMID is attempted,	and the	effective user	ID  of
	      the  calling  process  is	 not  that  of	the  creator (found in
	      shm_perm.cuid), or the owner (found in  shm_perm.uid),  and  the
	      process  was not privileged (Linux: did not have the CAP_SYS_AD-
	      MIN capability).

	      Or (in kernels before 2.6.9), SHM_LOCK or	SHM_UNLOCK was	speci-
	      fied,  but  the  process was not privileged (Linux: did not have
	      the CAP_IPC_LOCK capability).  (Since Linux  2.6.9,  this	 error
	      can  also	occur if the RLIMIT_MEMLOCK is 0 and the caller	is not
	      privileged.)

CONFORMING TO
       SVr4, POSIX.1-2001.

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, SHM_STAT and SHM_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.

       Linux permits a process to attach (shmat(2)) a  shared  memory  segment
       that has	already	been marked for	deletion using shmctl(IPC_RMID).  This
       feature is not available	on other UNIX implementations; portable	appli-
       cations should avoid relying on it.

       Various fields in a struct shmid_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.)

SEE ALSO
       mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(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				  2014-02-07			     SHMCTL(2)

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

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