MSGCTL

NAME

msgctl - message control operations

SYNOPSIS

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

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

DESCRIPTION

This function performs the control operation specified by cmd on the message queue with identifier msqid. Legal values for cmd are:

IPC_STAT

Copy info from the message queue data structure associated with msqid into the structure pointed to by buf. The caller must have read permission on the message queue.

IPC_SET

Write the values of some members of the msqid_ds structure pointed to by buf to the message queue data structure, updating also its msg_ctime member. The following members of the structure can be updated:


        msg_perm.uid
        msg_perm.gid
        msg_perm.mode   /* only lowest 9-bits */
        msg_qbytes

The calling process must have appropriate (probably, root) privileges or its effective user-ID must be that of the creator (msg_perm.cuid) or owner (msg_perm.uid) of the message queue. Appropriate (probably, root) privileges are required to raise the msg_qbytes value beyond the system parameter MSGMNB.

IPC_RMID

Immediately remove the message queue and its associated data structure, awakening all waiting reader and writer processes (with an error return and errno set to EIDRM). The calling process must have appropriate (probably, root) privileges or its effective user-ID must be either that of the creator or owner of the message queue.

RETURN VALUE

On success, the return value will be 0, otherwise -1 with errno indicating the error.

ERRORS

On failure, errno is set to one of the following:

EACCES: The argument cmd is equal to IPC_STAT but the calling process does not have read permission on the message queue msqid.
EFAULT: The argument cmd has the value IPC_SET or IPC_STAT, but the address pointed to by buf isn't accessible.
EIDRM: The message queue was removed.
EINVAL: Invalid value for cmd or msqid.
EPERM: The argument cmd has the value IPC_SET or IPC_RMID, but the calling process has insufficient privileges to execute the command.

NOTES

The IPC_INFO, MSG_STAT and MSG_INFO control calls are used by the ipcs(8) program to provide information on allocated resources. In the future these may be modified as needed or moved to a proc file system interface.

Various fields in a struct msqid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)

CONFORMING TO

SVr4, SVID. SVID does not document the EIDRM error condition.