MSGCTL

NAME

SYNOPSIS

DESCRIPTION

Each message queue has a data structure associated with it, parts of which may be altered by Fn msgctl and parts of which determine the actions of Fn msgctl . The data structure is defined in Aq Pa sys/msg.h 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 Bf -literal ipc_perm Ef structure used inside the Bf -literal shmid_ds Ef structure is defined in Aq Pa 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; /* 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 Fn msgctl is specified in Fa cmd and is one of:

IPC_STAT

Gather information about the message queue and place it in the structure pointed to by Fa buf .

IPC_SET

Set the value of the msg_perm.uid msg_perm.gid msg_perm.mode and msg_qbytes fields in the structure associated with Fa msqid . The values are taken from the corresponding fields in the structure pointed to by Fa buf . This operation can only be executed by the super-user, or a process that has an effective user id equal to either msg_perm.cuid or msg_perm.uid in the data structure associated with the message queue. The value of msg_qbytes can only be increased by the super-user. Values for msg_qbytes that exceed the system limit (MSGMNB from Aq Pa sys/msg.h ) are silently truncated to that limit.

IPC_RMID

Remove the message queue specified by Fa msqid and destroy the data associated with it. Only the super-user or a process with an effective uid equal to the msg_perm.cuid or msg_perm.uid values in the data structure associated with the queue can do this.

The permission to read from or write to a message queue (see msgsnd(2) and msgrcv(2)) is determined by the msg_perm.mode field in the same way as is done with files (see chmod(2)), but the effective uid can match either the msg_perm.cuid field or the msg_perm.uid field, and the effective gid can match either msg_perm.cgid or msg_perm.gid  

RETURN VALUES

ERRORS

Bq Er EPERM

Fa 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 msg_perm.uid or msg_perm.cuid fields of the data structure associated with the message queue.

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

Bq Er EACCESS

The command is IPC_STAT and the caller has no read permission for this message queue.

Bq Er EINVAL

Fa msqid is not a valid message queue identifier.

cmd is not a valid command.

Bq Er EFAULT

Fa buf specifies an invalid address.

SEE ALSO

HISTORY

Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUES

ERRORS

SEE ALSO

HISTORY