redhat man page for shmctl

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

NAME
       shmctl - shared memory control

SYNOPSIS
       #include <sys/ipc.h>

       #include <sys/shm.h>

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

DESCRIPTION
       shmctl()  allows  the user to receive information on a shared memory segment, set the owner, group, and permissions of a shared memory seg-
       ment, or destroy a segment. The information about the segment identified by shmid is returned in a shmid_ds structure:

	   struct shmid_ds {
	       struct ipc_perm shm_perm;  /* operation perms */
	       int 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 */
	       unsigned short shm_cpid;   /* pid of creator */
	       unsigned short shm_lpid;   /* pid of last operator */
	       short shm_nattch;	  /* no. of current attaches */
	       ...
	   };

       The highlighted fields in the member shm_perm can be set:

	   struct ipc_perm {
	       key_t  key;
	       ushort uid;   /* owner euid and egid */
	       ushort gid;
	       ushort cuid;  /* creator euid and egid */
	       ushort cgid;
	       ushort mode;  /* lower 9 bits of access modes */
	       ushort seq;   /* sequence number */
	   };

       The following cmds are available:

       IPC_STAT    is used to copy the information about the shared memory segment into the buffer buf. The user must  have  read  access  to  the
		   shared memory segment.

       IPC_SET	   is used to apply the changes the user has made to the uid, gid, or mode members of the shm_perms field.  Only the lowest 9 bits
		   of mode are used.  The shm_ctime member is also updated.  The user must be the owner, creator, or the super-user.

       IPC_RMID    is used to mark the segment as destroyed. It will actually be destroyed after the last detach.  (I.e., when the shm_nattch mem-
		   ber of the associated structure shmid_ds is zero.)  The user must be the owner, creator, or the super-user.

       The user must ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap.

       In addition, the super-user can prevent or allow swapping of a shared memory segment with the following cmds: (Linux only)

       SHM_LOCK    prevents swapping of a shared memory segment. The user must fault in any pages that are required to be present after locking is
		   enabled.

       SHM_UNLOCK  allows the shared memory segment to be swapped out.

       The IPC_INFO, SHM_STAT and SHM_INFO control calls are used by the ipcs(8) program to provide information on allocated  resources.   In  the
       future, these may be modified as needed or moved to a proc file system interface.

RETURN VALUE
       0 is returned on success, -1 on error.

ERRORS
       On error, errno will be set to one of the following:

       EACCES	   is returned if IPC_STAT is requested and shm_perm.modes does not allow read access for shmid.

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

       EINVAL	   is returned if shmid is not a valid identifier, or cmd is not a valid command.

       EIDRM	   is returned if shmid points to a removed identifier.

       EPERM	   is  returned if IPC_SET or IPC_RMID is attempted, and the effective user ID of the calling process is not the creator (as found
		   in shm_perm.cuid), the owner (as found in shm_perm.uid), or the super-user.

       EOVERFLOW   is returned if IPC_STAT is attempted, and the gid or uid value is too large to be stored in the structure pointed to by buf.

NOTE
       Various fields in a struct shmid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a  recom-
       pilation under glibc-2.1.91 or later should suffice.  (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)

CONFORMING TO
       SVr4,  SVID.   SVr4 documents additional error conditions EINVAL, ENOENT, ENOSPC, ENOMEM, EEXIST.  Neither SVr4 nor SVID documents an EIDRM
       error condition.

SEE ALSO
       shmget(2), shmop(2)

Linux 2.4.1							    2002-01-05								 SHMCTL(2)