Each message queue has a data structure associated with it, parts of
which may be altered by and parts of which determine the actions of The
data structure is defined in and contains (amongst others) the following
members: struct msqid_ds { struct ipc_perm msg_perm; /* msg queue permission
bits */
u_long msg_cbytes; /* # of bytes in use on the queue */
u_long msg_qnum; /* # of msgs in the queue */
u_long msg_qbytes; /* max # of bytes on the queue */
pid_t msg_lspid; /* pid of last msgsnd() */
pid_t msg_lrpid; /* pid of last msgrcv() */
time_t msg_stime; /* time of last msgsnd() */
time_t msg_rtime; /* time of last msgrcv() */
time_t msg_ctime; /* time of last msgctl() */
};
The ipc_perm structure used inside the shmid_ds structure is
defined in 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; /* permission (9 bits, see chmod(2)
) */
ushort seq; /* sequence # (to generate unique id) */
key_t key; /* user specified msg/sem/shm key */
};
The operation to be performed by is specified in and is one of: Gather information about the message queue and place it in the structure pointed to by Set the value of the and fields in the structure associated with The values are taken from the corresponding fields in the structure pointed to by This operation can only be executed by the super-user, or a process that has an effective user id equal to either or in the data structure associated with the message queue. The value of can only be increased by the super-user. Values for that exceed the system limit (MSGMNB from are silently truncated to that limit.
Remove the message queue specified by and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the or values in the data structure associated with the queue can do this.
The permission to read from or write to a message queue (see and is determined by the field in the same way as is done with files (see but the effective uid can match either the field or the field, and the effective gid can match either or
An attempt is made to increase the value of through IPC_SET but the caller is not the super-user. The command is IPC_STAT and the caller has no read permission for this message queue. is not a valid message queue identifier.
is not a valid command. specifies an invalid address.