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

The 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 into the structure pointed to by buf. The user must have read access privileges 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. Considered members from the user supplied struct msqid_ds pointed to by buf are

msg_perm.uid

msg_perm.gid

msg_perm.mode /* only lowest 9-bits */

msg_qbytes

The calling process effective user-ID must be one among super-user, creator or owner of the message queue. Only the super-user can raise the msg_qbytes value beyond the system parameter MSGMNB.

IPC_RMID

Remove immediately the message queue and its data structures awakening all waiting reader and writer processes (with an error return and errno set to EIDRM). The calling process effective user-ID must be one among super-user, creator or owner of the message queue.

RETURN VALUE

If successful, the return value will be 0, otherwise -1 with errno indicating the error.

ERRORS

For a failing return, errno will be set to one among the following values:

EACCES

The argument cmd is equal to IPC_STAT but the calling process has no read access permissions on the message queue msqid.

EFAULT

The argument cmd has 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 value IPC_SET or IPC_RMID but the calling process effective user-ID has insufficient privileges to execute the command. Note

this is also the case of a non super-user process trying to increase the msg_qbytes value beyond the value specified by the system parameter MSGMNB.

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 can 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.

SEE ALSO

ipc(5), msgget(2), msgsnd(2), msgrcv(2)