The Linux Programming Interface

System V Message Queues 947

46.3 Message Queue Control Operations

The msgctl() system call performs control operations on the message queue identi-

fied by msqid.

The cmd argument specifies the operation to be performed on the queue. It can be

one of the following:

IPC_RMID

Immediately remove the message queue object and its associated msqid_ds

data structure. All messages remaining in the queue are lost, and any

blocked reader or writer processes are immediately awakened, with msgsnd()

or msgrcv() failing with the error EIDRM. The third argument to msgctl() is

ignored for this operation.

IPC_STAT

Place a copy of the msqid_ds data structure associated with this message

queue in the buffer pointed to by buf. We describe the msqid_ds structure

in Section 46.4.

IPC_SET

Update selected fields of the msqid_ds data structure associated with this

message queue using values provided in the buffer pointed to by buf.

Further details about these operations, including the privileges and permissions

required by the calling process, are described in Section 45.3. We describe some

other values for cmd in Section 46.6.

The program in Listing 46-4 demonstrates the use of msgctl() to delete a mes-

sage queue.

Listing 46-4: Deleting System V message queues

––––––––––––––––––––––––––––––––––––––––––––––––––––––––– svmsg/svmsg_rm.c

#include <sys/types.h>

#include <sys/msg.h>

#include "tlpi_hdr.h"

int

main(int argc, char *argv[])

{

int j;

#include <sys/types.h> /* For portability */

#include <sys/msg.h>

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

Returns 0 on success, or –1 on error