NAME

msgctl − message control operations

SYNOPSIS

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

int msgctl (msqid, cmd, buf)
int msqid, cmd;
struct msqid_ds ∗buf;

DESCRIPTION

msgctl provides a variety of message control operations as specified by cmd. The following cmds are available:

IPC_STAT Place the current value of each member of the data structure associated with msqid into the structure pointed to by buf. The contents of this structure are defined in intro(2).  {READ}

IPC_SET Set the value of the following members of the data structure associated with msqid to the corresponding value found in the structure pointed to by buf:

msg_perm.uid
msg_perm.gid
msg_perm.mode /∗ only low 9 bits ∗/
msg_qbytes

This cmd can only be executed by a process that has an effective user ID equal to either that of super user, or to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqidor if the user has an effective access vector of ACC_IPCCTL. Only super user can raise the value of msg_qbytes.  If the user is super-user, the privilege ID of the message queue is set to the numeric value contained in msg_perm.gid; otherwise, the privilege ID of the message queue is set to the ID of a privilege containing the security label of the message queue’s current privilege ID and the group of the privilege referred to by msg_perm.gid. (B1 only)

All message queue reclassifications are audited. 

IPC_RMID Remove the message queue identifier specified by msqid from the system and destroy the message queue and data structure associated with it.  This cmd can only be executed by a process that has an effective user ID equal to either that of super user, or to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.
Message queues must exist a minimum amount of time before they can be removed. This amount of time is equal to the maximum number of message queues times 1 second. Only super-user can remove a message queue before its time. (B1 only)

msgctl will fail if one or more of the following are true:

­[EINVAL]
Msqid is not a valid message queue identifier. 

­[EINVAL] Cmd is not a valid command. 

­[EACCES] Cmd is equal to IPC_STAT and {READ} operation permission is denied to the calling process [see intro(2)]. 

­[EPERM] Cmd is equal to IPC_RMID or IPC_SET.  The effective user ID of the calling process is not equal to that of super user, or to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqidor the user does not have the ACC_IPCCTL access vector. (If access vectors are configured)

­[EPERM] Cmd is equal to IPC_SET, an attempt is being made to increase to the value of msg_qbytes, and the effective user ID of the calling process is not equal to that of super user. 

­[EPERM] Cmd is equal to IPC_SET, the user’s current security label is not identical to the message queue’s security label, and the user’s effective user ID is not super-user. (B1 only)

­[EPERM] Cmd is equal to IPC_SET, The message queue’s privilege ID cannot be changed because the new group is not defined at the message queue’s current security level, and the user is not super-user. (B1 only)

­[EPERM] Cmd is equal to IPC_RMID, and the message queue has not existed the minimum amount of time. (B1 only)

­[EFAULT] Buf points to an illegal address. 

SEE ALSO

intro(2), msgget(2), msgop(2). 

DIAGNOSTICS

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

CX/UX Programmer’s Reference Manual