shmctl(2) [freebsd man page]

SHMCTL(2)						      BSD System Calls Manual							 SHMCTL(2)

NAME

     shmctl -- shared memory control

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

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

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

DESCRIPTION

     Performs the action specified by cmd on the shared memory segment identified by shmid:

     IPC_STAT	  Fetch the segment's struct shmid_ds, storing it in the memory pointed to by buf.

     IPC_SET	  Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the segment's struct shmid_ds to match those of the struct
		  pointed to by buf.  The calling process's effective uid must match either shm_perm.uid or shm_perm.cuid, or it must have supe-
		  ruser privileges.

     IPC_RMID	  Removes the segment from the system.	The removal will not take effect until all processes having attached the segment have
		  exited; however, once the IPC_RMID operation has taken place, no further processes will be allowed to attach the segment.  For
		  the operation to succeed, the calling process's effective uid must match shm_perm.uid or shm_perm.cuid, or the process must have
		  superuser privileges.

     The shmid_ds structure is defined as follows:

     struct shmid_ds {
	 struct ipc_perm shm_perm;   /* operation permission structure */
	 size_t 	 shm_segsz;  /* size of segment in bytes */
	 pid_t		 shm_lpid;   /* process ID of last shared memory op */
	 pid_t		 shm_cpid;   /* process ID of creator */
	 int		 shm_nattch; /* number of current attaches */
	 time_t 	 shm_atime;  /* time of last shmat() */
	 time_t 	 shm_dtime;  /* time of last shmdt() */
	 time_t 	 shm_ctime;  /* time of last change by shmctl() */
     };

RETURN VALUES

     The shmctl() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate
     the error.

ERRORS

     The shmctl() system call will fail if:

     [EINVAL]		Invalid operation, or no shared memory segment was found corresponding to shmid.

     [EPERM]		The calling process's effective uid does not match the uid of the shared memory segment's owner or creator.

     [EACCES]		Permission denied due to mismatch between operation and mode of shared memory segment.

SEE ALSO

     shmat(2), shmdt(2), shmget(2), ftok(3)

BSD
								   July 17, 1995							       BSD

SHMCTL(2)						      BSD System Calls Manual							 SHMCTL(2)

NAME

     shmctl -- shared memory control operations

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <sys/shm.h>

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

DESCRIPTION

     The shmctl() system call performs control operations on the shared memory segment specified by shmid.

     Each shared memory segment has a shmid_ds structure associated with it which contains the following members:

	 struct ipc_perm shm_perm;   /* operation permissions */
	 size_t 	 shm_segsz;  /* size of segment in bytes */
	 pid_t		 shm_lpid;   /* pid of last shm op */
	 pid_t		 shm_cpid;   /* pid of creator */
	 shmatt_t	 shm_nattch; /* # of current attaches */
	 time_t 	 shm_atime;  /* last shmat() time */
	 time_t 	 shm_dtime;  /* last shmdt() time */
	 time_t 	 shm_ctime;  /* last change by shmctl() */

     The ipc_perm structure used inside the shmid_ds structure is defined in <sys/ipc.h> and contains the following members:

	 uid_t cuid;  /* creator user id */
	 gid_t cgid;  /* creator group id */
	 uid_t uid;   /* user id */
	 gid_t gid;   /* group id */
	 mode_t mode; /* permission (lower 9 bits) */

     The operation to be performed by shmctl() is specified in cmd and is one of:

     IPC_STAT	Gather information about the shared memory segment and place it in the structure pointed to by buf.

     IPC_SET	Set the value of the shm_perm.uid, shm_perm.gid and shm_perm.mode fields in the structure associated with shmid.  The values are
		taken from the corresponding fields in the structure pointed to by buf.  This operation can only be executed by the super-user, or
		a process that has an effective user id equal to either shm_perm.cuid or shm_perm.uid in the data structure associated with the
		shared memory segment.

     IPC_RMID	Remove the shared memory segment specified by shmid and destroy the data associated with it.  Only the super-user or a process
		with an effective uid equal to the shm_perm.cuid or shm_perm.uid values in the data structure associated with the segment can do
		this.

     SHM_LOCK	Lock the shared memory segment specified by shmid in memory.  This operation can only be executed by the super-user.

     SHM_UNLOCK
		Unlock the shared memory segment specified by shmid.  This operation can only be executed by the super-user.

     The read and write permissions on a shared memory identifier are determined by the shm_perm.mode field in the same way as is done with files
     (see chmod(2)), but the effective uid can match either the shm_perm.cuid field or the shm_perm.uid field, and the effective gid can match
     either shm_perm.cgid or shm_perm.gid.

RETURN VALUES

     Upon successful completion, a value of 0 is returned.  Otherwise, -1 is returned and the global variable errno is set to indicate the error.

ERRORS

     shmctl() will fail if:

     [EACCES]		The command is IPC_STAT and the caller has no read permission for this shared memory segment.

     [EFAULT]		buf specifies an invalid address.

     [EINVAL]		shmid is not a valid shared memory segment identifier.

			cmd is not a valid command.

     [ENOMEM]		The cmd is equal to SHM_LOCK and there is not enough physical memory.

     [EPERM]		cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the
			shm_perm.uid or shm_perm.cuid fields of the data structure associated with the shared memory segment.

			An attempt was made to increase the value of shm_qbytes through IPC_SET but the caller is not the super-user.

			The cmd is equal to SHM_LOCK or SHM_UNLOCK and the caller is not the super-user.

SEE ALSO

     ipcrm(1), ipcs(1), shmat(2), shmget(2)

STANDARDS

     The shmctl system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5'').

HISTORY

     Shared memory segments appeared in the first release of AT&T System V UNIX.

BSD
								 November 25, 2006							       BSD