Message Queue Control

The ownership and access permissions, established when the message queue was created, can be examined and modified using the msgctl system call (see Table 6.7).

The msgctl system call references the message queue indicated by the msqid argument. The value of the cmd argument is used to indicate the action that msgctl should take. The following defined constants/actions can be specified:

Table 6.7. Summary of the msgget System Call.

• IPC_STAT Return the current values for each member of the msqid_ds data structure (remember that this also contains the permission structure). When using the IPC_STAT flag, the user must provide a location to store the returned information. The address of the storage location for the information is passed as the third argument to the msgctl system call. Of course, the calling process must have read-access privileges for the message queue.
• IPC_SET With this flag, the user (creator, owner, or superuser) can modify a limited number of msqid_ds structure member values. The following members can be modified:

  msg_perm.uid, msg_perm.gid, msg_perm.mode, and msg_qbytes

  Similar to IPC_STAT, the user must first generate a structure of type msqid_ds, modify the appropriate structure members, and then call msgctl with the IPC_SET flag and pass the address of the modified structure. A successful update will also update the msg_ctime member.

• IPC_RMID Immediately removes all associated message queue structures. When specifying IPC_RMID, the third argument to msgctl is not considered and thus may be left out. However, wanting to leave nothing to chance, most programmers enter the third argument as a NULL value cast to be a pointer to an msqid_ds structure.

If the msgctl system call fails, it returns a -1 and sets errno ; otherwise , it returns a 0 indicating success. The value that errno may be assigned when msgctl fails is given in Table 6.8.

Program 6.3 creates a message queue, uses the msgctl system call to obtain the message queue structure information, and displays pertinent data to the screen.

Table 6.8. msgctl Error Messages.

Program 6.3 Using msgctl .

File : p6.3.cxx /* Displaying message queue status information */ #include + #include #include #include #include using namespace std; 10 int main( ){ int mid; key_t key; struct msqid_ds buf; <-- 1 + key = ftok(".", 'z'); if ((mid = msgget(key, IPC_CREAT 0660)) == -1) { <-- 2 perror("Queue create"); return 1; 20 } msgctl(mid, IPC_STAT, &buf); cout << "Message Queue *Permission* Structure Information" << endl; cout << "Owner's user ID " << buf.msg_perm.uid << endl; cout << "Owner's group ID " << buf.msg_perm.gid << endl; + cout << "Creator's user ID " << buf.msg_perm.cuid << endl; cout << "Creator's group ID " << buf.msg_perm.cgid << endl; cout << "Access mode in HEX " << hex << buf.msg_perm.mode << endl; cout << " Additional Selected Message Queue Structure Information "; cout << "Current # of bytes on queue " << dec 30 << buf.__msg_cbytes << endl; cout << "Current # of messages on queue " << buf.msg_qnum << endl; cout << "Maximum # of bytes on queue " << buf.msg_qbytes << endl; msgctl(mid, IPC_RMID, (struct msqid_ds *) 0 ); return 0; + }

(1) The structure buf will store the returned information on the message queue.

(2) Generate the message queue.

Run locally, Program 6.3 produces the output shown in Figure 6.6.

Figure 6.6 Output of Program 6.3.

linux$ p6.3 Message Queue *Permission* Structure Information Owner's user ID 500 Owner's group ID 1000 Creator's user ID 500 Creator's group ID 1000 Access mode in HEX 1b0 Additional Selected Message Queue Structure Information Current # of bytes on queue 0 Current # of messages on queue 0 Maximum # of bytes on queue 16384

As shown, when first generated, the creator of the message queue and the owner are the same. If we convert the displayed hexadecimal access mode value to binary:

1B0 16 110 110 000 2

and examine the lower nine bits of the binary number, we see the access permissions are indeed 0660 as we specified. The value for the maximum number of bytes on the message queue, shown here as 16384, is one of several system-imposed message queue limits. Additional message queue limit information can be found in the header file .