Interprocess communication

Getting message queues

This section describes how to use the msgget system call. The accompanying program illustrates its use.

Using msgget

The synopsis found on the msgget(2) manual page is as follows:

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

int msgget (key_t key, int msgflg);

All of these include files are located in the /usr/include/sys directory of the UNIX operating system.

The following line in the synopsis:

int msgget (key_t key, int msgflg);
informs you that msgget is a function that returns an integer-type value. It also declares the types of the two formal arguments: key is of type key_t, and msgflg is of type int. key_t is defined by a typedef in the sys/types.h header file to be an integral type.

The integer returned from this function upon successful completion is the message queue identifier that was discussed earlier. Upon failure, the external variable errno is set to indicate the reason for failure, and the value -1 (which is not a valid msqid) is returned.

As declared, the process calling the msgget system call must supply two arguments to be passed to the formal key and msgflg arguments.

A new msqid with an associated message queue and data structure is provided if either


The value passed to the msgflg argument must be an integer-type value that will specify the following:

Operation permissions determine the operations that processes are permitted to perform on the associated message queue. ``Read'' permission is necessary for receiving messages or for determining queue status by means of a msgctl IPC_STAT operation. ``Write'' permission is necessary for sending messages.

The following table reflects the numeric values (expressed in octal notation) for the valid operation permissions codes.

Operation permissions codes

Operation permissions Octal value
Read by user 00400
Write by user 00200
Read by group 00040
Write by group 00020
Read by others 00004
Write by others 00002

A specific value is derived by adding or bitwise ORing the octal values for the operation permissions wanted. That is, if read by user and read/write by others is desired, the code value would be 00406 (00400 plus 00006). There are constants located in the sys/msg.h header file which can be used for the user operations permissions. They are as follows:

MSG_W	0200	/* write permissions by owner */

MSG_R 0400 /* read permissions by owner */

Control flags are predefined constants (represented by all upper-case letters). The flags which apply to the msgget system call are IPC_CREAT and IPC_EXCL and are defined in the sys/ipc.h header file.

The value for msgflg is therefore a combination of operation permissions and control commands. After determining the value for the operation permissions as previously described, the desired flag(s) can be specified. This is accomplished by adding or bitwise ORing (|) them with the operation permissions; the bit positions and values for the control commands in relation to those of the operation permissions make this possible.

The msgflg value can easily be set by using the flag names in conjunction with the octal operation permissions value:

msqid = msgget (key, (IPC_CREAT | 0400));

msqid = msgget (key, (IPC_CREAT | IPC_EXCL | 0400));

As specified by the msgget(2) manual page, success or failure of this system call depends upon the argument values for key and msgflg or system-tunable parameters. The system call will attempt to return a new message queue identifier if one of the following conditions is true:

The key argument can be set to IPC_PRIVATE like this:

msqid = msgget (IPC_PRIVATE, msgflg);
The system call will always be attempted. Exceeding the MSGMNI system-tunable parameter always causes a failure. The MSGMNI system-tunable parameter determines the systemwide number of unique message queues that may be in use at any given time.

IPC_EXCL is another control command used in conjunction with IPC_CREAT. It will cause the system call to return an error if a message queue identifier already exists for the specified key. This is necessary to prevent the process from thinking that it has received a new identifier when it has not. In other words, when both IPC_CREAT and IPC_EXCL are specified, a new message queue identifier is returned if the system call is successful.

Refer to the msgget(2) manual page for specific, associated data structure initialization for successful completion. The specific failure conditions and their error names are contained there also.

Example program

``msgget system call example'' is a menu-driven program. It allows all possible combinations of using the msgget system call to be exercised.

From studying this program, you can observe the method of passing arguments and receiving return values. The user-written program requirements are pointed out.

This program begins (lines 4-8) by including the required header files as specified by msgget(2). Note that the sys/errno.h header file is included as opposed to declaring errno as an external variable; either method will work.

Variable names have been chosen to be as close as possible to those in the synopsis for the system call. Their declarations are self explanatory. These names make the programs more readable are perfectly valid since they are local to the program.

The variables declared for this program and what they are used for are as follows:

used to pass the value for the desired key

used to store the desired operation permissions

used to store the desired control commands (flags)

used to store the combination from the logical ORing of the opperm and flags variables; it is then used in the system call to pass the msgflg argument

used for returning the message queue identification number for a successful system call or the error code (-1) for an unsuccessful one.

The program begins by prompting for a hexadecimal key, an octal operation permissions code, and finally for the control command combinations (flags) which are selected from a menu (lines 15-32). All possible combinations are allowed even though they might not be viable. This allows errors to be observed for invalid combinations.

Next, the menu selection for the flags is combined with the operation permissions, and the result is stored in the opperm_flags variable (lines 36-51).

The system call is made next, and the result is stored in the msqid variable (line 53).

Since the msqid variable now contains a valid message queue identifier or the error code (-1), it is tested to see if an error occurred (line 55). If msqid equals -1, a message indicates that an error resulted, and the external errno variable is displayed (line 57).

If no error occurred, the returned message queue identifier is displayed (line 61).

The example program for the msgget system call follows. We suggest you name the program file msgget.c and the executable file msgget.

 1    /*This is a program to illustrate
 2     *the message get, msgget(),
 3     *system call capabilities*/

4 #include <stdio.h> 5 #include <sys/types.h> 6 #include <sys/ipc.h> 7 #include <sys/msg.h> 8 #include <errno.h>

9 /*Start of main C language program */ 10 main() 11 { 12 key_t key; 13 int opperm, flags; 14 int msqid, opperm_flags; 15 /*Enter the desired key*/ 16 printf("Enter the desired key in hex = "); 17 scanf("%x", &key);

18 /*Enter the desired octal operation 19 permissions.*/ 20 printf("\nEnter the operation\n"); 21 printf("permissions in octal = "); 22 scanf("%o", &opperm);

23 /*Set the desired flags.*/ 24 printf("\nEnter corresponding number to\n"); 25 printf("set the desired flags:\n"); 26 printf("No flags = 0\n"); 27 printf("IPC_CREAT = 1\n"); 28 printf("IPC_EXCL = 2\n"); 29 printf("IPC_CREAT and IPC_EXCL = 3\n"); 30 printf(" Flags = ");

31 /*Get the flag(s) to be set.*/ 32 scanf("%d", &flags);

33 /*Check the values.*/ 34 printf ("\nkey =0x%x, opperm = 0%o, flags = 0%o\n", 35 key, opperm, flags);

36 /*Incorporate the control fields (flags) with 37 the operation permissions*/ 38 switch (flags) 39 { 40 case 0: /*No flags are to be set.*/ 41 opperm_flags = (opperm | 0); 42 break; 43 case 1: /*Set the IPC_CREAT flag.*/ 44 opperm_flags = (opperm | IPC_CREAT); 45 break; 46 case 2: /*Set the IPC_EXCL flag.*/ 47 opperm_flags = (opperm | IPC_EXCL); 48 break; 49 case 3: /*Set the IPC_CREAT and IPC_EXCL flags.*/ 50 opperm_flags = (opperm | IPC_CREAT | IPC_EXCL); 51 break; 52 default: /* Invalid Input */ 53 exit(-1); 54 }

55 /*Call the msgget system call.*/ 56 msqid = msgget (key, opperm_flags);

57 /*Perform the following if the call is unsuccessful.*/ 58 if(msqid == -1) 59 { 60 printf ("\nThe msgget call failed, error number = %d\n", errno); 61 } 62 /*Return the msqid upon successful completion.*/ 63 else 64 printf ("\nThe msqid = %d\n", msqid); 65 exit(0); 66 }

msgget system call example

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