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Linux 2.6 - man page for shmctl (linux section 2)

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

       shmctl - 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 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 in <sys/ipc.h> 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  effec-
		 tive  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  cre-
		 ator,	or be privileged.  If a segment has been marked for destruction, then the
		 (nonstandard) 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.

       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

		     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 feature 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 information about all shared memory segments on the system.

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

       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 mem-
       ory to be locked falls within the RLIMIT_MEMLOCK resource limit (see setrlimit(2)).

       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  iden-
       tifier  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 specified, 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 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
       file system 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 applications 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

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

       This  page  is  part of release 3.27 of the Linux man-pages project.  A description of the
       project,  and  information  about  reporting  bugs,  can  be  found   at   http://www.ker-

Linux					    2008-08-07					SHMCTL(2)

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