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SHMCTL(2) System Calls Manual SHMCTL(2)

shmctl — shared memory control operations

#include <sys/shm.h>

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

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

Each shared memory segment has a data structure associated with it, parts of which may be altered by () 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 {
	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;	/* r/w permission (see chmod(2)) */
	u_short		seq;	/* sequence # (to generate unique msg/sem/shm id) */
	key_t		key;	/* user specified msg/sem/shm key */
};

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

Gather information about the shared memory segment and place it in the structure pointed to by buf.
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 superuser, 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.
Mark the shared memory segment specified by shmid for removal when it is no longer in use by any process. When it is removed, all data associated with it will be destroyed too. Only the superuser 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, the value 0 is returned; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

shmctl() will fail if:

[]
cmd is equal to IPC_SET or IPC_RMID and the caller is not the superuser, 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 superuser.

[]
The command is IPC_STAT and the caller has no read permission for this shared memory segment.
[]
shmid is not a valid shared memory segment identifier.

cmd is not a valid command.

[]
buf specifies an invalid address.

shmat(2), shmget(2)

Segments which are marked for removal (but not yet removed since they are still in use) can be attached to by new callers using shmat(2). This is permitted as an extension beyond the standards.

December 10, 2014 OpenBSD-7.0