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

       shmctl - System V shared memory control

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

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

       shmctl()	 performs the control operation specified by cmd on the System
       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

       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
		 actually  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

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

		 The   caller	must  ensure  that  a  segment	is  eventually
		 destroyed; 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)
		 Returns information about system-wide	shared	memory	limits
		 and  parameters  in  the  structure  pointed to by buf.  This
		 structure is of type shminfo  (thus,  a  cast	is  required),
		 defined  in <sys/shm.h> if the _GNU_SOURCE feature test macro
		 is defined:

		     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)
		 Returns 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)
		 Returns 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

       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‐

       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
       returns	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.

       EACCES IPC_STAT or SHM_STAT is requested	 and  shm_perm.mode  does  not
	      allow  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)).

	      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_ADMIN 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

       SVr4, POSIX.1-2001.

       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
       recompilation  under glibc-2.1.91 or later should suffice.  (The kernel
       distinguishes old and new calls by an IPC_64 flag in cmd.)

       mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7), svipc(7)

       This page is part of release 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-02-07			     SHMCTL(2)

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