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OpenDarwin 7.2.1 - man page for shmctl (opendarwin section 2)

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

     shmctl -- shared memory control operations

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

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

     The shmctl() system call performs some control operations on the shared memory area speci-
     fied by shmid.  Each shared memory segment has a data structure associated with it, parts of
     which may be altered by shmctl() and parts of which determine the actions of shmctl().  This
     structure is defined as follows in <sys/shm.h>:

     struct shmid_ds {
	 struct ipc_perm shm_perm;     /* operation permissions */
	 int		 shm_segsz;    /* size of segment in bytes */
	 pid_t		 shm_lpid;     /* pid of last shm op */
	 pid_t		 shm_cpid;     /* pid of creator */
	 short		 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() */
	 void		*shm_internal; /* sysv stupidity */
     The ipc_perm structure used inside the shmid_ds structure is defined in <sys/ipc.h> and
     looks like this:

     struct ipc_perm {
	 ushort cuid; /* creator user id */
	 ushort cgid; /* creator group id */
	 ushort uid;  /* user id */
	 ushort gid;  /* group id */
	 ushort mode; /* r/w permission (see chmod(2)) */
	 ushort seq;  /* sequence # (to generate unique msg/sem/shm id) */
	 key_t key;   /* user specified msg/sem/shm key */
     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 associ-
		ated 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
		queue can do this.
     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.

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

     shmctl() will fail if:

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

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

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

     [EFAULT]		buf specifies an invalid address.

     shmat(2), shmdt(2), shmget(2)

BSD					 August 17, 1995				      BSD

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