DESCRIPTION
The msgctl system call performs some control operations on the message queue specified by msqid. Each message queue has a data structure associated with it, parts of which may be altered by msgctl and parts of which determine the actions of msgctl. The data structure is defined in
.In sys/msg.h and contains (amongst others) the following members:
struct msqid_ds {
struct ipc_perm msg_perm; /* msg queue permission bits */
struct msg *msg_first; /* first message in the queue */
struct msg *msg_last; /* last message in the queue */
u_long msg_cbytes;/* number of bytes in use on the queue */
u_long msg_qnum; /* number 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() */
long msg_pad1;
time_t msg_rtime; /* time of last msgrcv() */
long msg_pad2;
time_t msg_ctime; /* time of last msgctl() */
long msg_pad3;
long msg_pad4[4];
};
The
.Vt ipc_perm structure used inside the
.Vt shmid_ds structure is defined in
.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 */
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 msgctl is specified in cmd and is one of:
