15.1 POSIX:XSI Interprocess Communication

The POSIX interprocess communication (IPC) is part of the POSIX:XSI Extension and has its origin in UNIX System V interprocess communication. IPC, which includes message queues, semaphore sets and shared memory, provides mechanisms for sharing information among processes on the same system. These three communication mechanisms have a similar structure, and this chapter emphasizes the common elements of their use. Table 15.1 summarizes the POSIX:XSI interprocess communication functions.

Table 15.1. POSIX:XSI interprocess communication functions.

mechanism	POSIX function	meaning
message queues	msgctl	control
	msgget	create or access
	msgrcv	receive message
	msgsnd	send message
semaphores	semctl	control
	semget	create or access
	semop	execute operation (wait or post)
shared memory	shmat	attach memory to process
	shmctl	control
	shmdt	detach memory from process
	shmget	create and initialize or access

15.1.1 Identifying and accessing IPC objects

POSIX:XSI identifies each IPC object by a unique integer that is greater than or equal to zero and is returned from the get function for the object in much the same way as the open function returns an integer representing a file descriptor. For example, msgget returns an integer identifier for message queue objects. Similarly, semget returns an integer identifier for a specified semaphore set, and shmget returns an integer identifier for a shared memory segment. These identifiers are associated with additional data structures that are defined in sys/msg.h, sys/sem.h or sys/shm.h , respectively. The integer identifiers within each IPC object type are unique, but you might well have an integer identifier 1 for two different types of objects, say, a semaphore set and a message queue.

When creating or accessing an IPC object, you must specify a key to designate the particular object to be created or accessed. Pick a key in one of these three ways.

• Let the system pick a key ( IPC_PRIVATE ).

• Pick a key directly.

• Ask the system to generate a key from a specified path by calling ftok .

The ftok function allows independent processes to derive the same key based on a known pathname. The file corresponding to the pathname must exist and be accessible to the processes that want to access an IPC object. The combination of path and id uniquely identifies the IPC object. The id parameter allows several IPC objects of the same type to be keyed from a single pathname.

  SYNOPSIS  #include <sys/ipc.h>    key_t ftok(const char *path, int id);  POSIX:XSI  

If successful, ftok returns a key. If unsuccessful , ftok returns (key_t)-1 and sets errno . The following table lists the mandatory errors for ftok .

errno	cause
EACCES	search permission on a path component denied
ELOOP	a loop exists in resolution of path
ENAMETOOLONG	length of path exceeds PATH_MAX , or length of a pathname component exceeds NAME_MAX
ENOENT	a component of path is not a file or is empty
ENOTDIR	a component of path 's prefix is not a directory

Example 15.1

The following code segment derives a key from the filename /tmp/trouble.c .

 if ((thekey = ftok("tmp/trouble.c", 1)) == (key_t)-1))    perror("Failed to derive key from /tmp/trouble.c"); 

15.1.2 Accessing POSIX:XSI IPC resources from the shell

The POSIX:XSI Extension for shells and utilities defines shell commands for examining and deleting IPC resources, a convenient feature that is missing for the POSIX:SEM semaphores.

The ipcs command displays information about POSIX:XSI interprocess communication resources. If you forget which ones you created, you can list them from the shell command line.

  SYNOPSIS  ipcs [-qms][-a  -bcopt]  POSIX:XSI,Shell and Utilities  

If no options are given, ipcs outputs, in an abbreviated format, information about message queues, shared memory segments and semaphore sets. You can restrict the display to specific types of IPC resources with the -q, -m and -s options for message queues, shared memory and semaphores, respectively. The -a option displays a long format giving all information available. The -bcopt options specify which components of the available information to print.

Example 15.2

The following command displays all the available information about the semaphores currently allocated on the system.

 ipcs -s -a 

You can remove an individual resource by giving either an ID or a key. Use the ipcrm command to remove POSIX:XSI interprocess communication resources.

  SYNOPSIS  ipcrm [-q msgid  -Q msgkey  -s semid  -S semkey           -m shmid  -M shmkey] ....  POSIX:XSI,Shell and Utilities  

The lower case -q , -s and -m options use the object ID to specify the removal of a message queue, semaphore set or shared memory segment, respectively. The uppercase options use the original creation key.