IPC (Inter Process Communication) - English

IPC Introduction

Introduction

IPC means communication among processes, traditionally it used various types of method to transfer messages, but lately new methods are introduced such as shared memory. So following sections will describe various types of synchronization among processes.

For the past 30 years, transferring message in Unix O/S has been changed as below.

1.       Pipes

were the first widely used form of IPC, available both within programs and from the shell. The problem with pipes is that they are usable only between processes that have a common ancestor.

2.       System V message queues

were added to System V kernels in the early 1980s. These can be used between related or unrelated processes on a given host.

3.       Posix message queues

were added by Posix realtime standard.

4.       Remote Procedure Calls

were introduced in mid 80s. It is being used to call remote function call between heterogeneous systems.

Processes, Threads, and the Sharing of Information

The information among Unix processes can be shared as below diagram.

Different Data Sharing

Threads

For long time, Unix used process method. The concept of thread was introduced lately. Posix. 1 thread standard is accepted in 1995. Since threads are sharing information by global variable, there are several methods to synchronize shared data.

Persistence of IPC Objects

1.       Process-persistent IPC

It exists till the last process, which owns the IPC object, closes the IPC object. 

2.       Kernel-persistent IPC
It exists till kernel is rebooted or IPC object is removed forcefully.

3.       Filesystem-persistent IPC

It exists till it is deleted forcefully.

System V IPC

Introduction

Three types of IPCs are as below.

·         System V message queues

·         System V semaphores

·         System V shared  memory

Following is api lists which can control each IPC object.

	Message queues	Semaphores	Shared memory
Header	sys/msg.h	sys/sem.h	sys/shm.h
Function to create or open	msgget	semget	shmget
Function for control operations	msgctl	semctl	shmctl
Functions for IPC operations	msgsnd msgrcv	semop	shmat shmdt

 Summary of System V IPC functions.

key_t Keys and ftok Function

key_t type might be defined 32bit integer in sys/types.h. ftok function accepts integer type id and pathname and returns key_t type.

#include

key_t ftok(const char *pathname, int id);

                                      returns: IPC key if OK, -1 or error

It will generate IPC key combining low-order 8bit of pathname and id.

ipc_perm Structure

Kernal manages IPC object same like file and the structure is as below.

struct ipc_perm

{

   uid_t  uid;      // owner’s user id

   gid_t  gid;      // owner’s group id

   uid_t  cuid;  // creator’s user id

   gid_t  cgid;  // creator’s group id

   mode_t mode;  // read-write permissions

   ulong_t   seq;      // slot usage sequence number

   key_t  key;      // IPC key

};

Creating and Opening IPC Channels

Three get_xxx functions can be used to create or open IPC object and all functions get integer identifier using IPC key value.
There are two ways to pass function argument.

1. Calls ftok function using pathname and id.

2. Specify Key as IPC_PRIVATE then new IPC object will be uniquely created.

Generating IPC identifiers from IPC keys

All get_xxx functions have read-write permissions.

1. If key is IPC_PRIVATE, it guarantees creating unique IPC object.
2. If oflag is set as IPC_CREATE, it will create new entry to specific key if entry doesn't exist, otherwise it will return existing entry.

3. if oflag is set IPC_CREATE and IPC_EXCL, it only create new entry when there is no entry otherwise it will return EEXIST.

Following is summery as diagram.

Logic for creating or opening an IPC object

Shared Memory

Shared Memory Introduction

Shared memory is most fast among IPC since it doesn't need kernel help. You may understand easily with following diagram.

Flow of file data from server to client

Copying file data from server to client using shared memory

System V Shared Memory

Introduction

All shared memory segment has shmid_ds struct which is defined in sys/shm.h.

struct shmid_ds

{

   struct ipc_perm  shm_perm; // operation permission struct

   size_t        shm_segsz; // segment size

   pid_t         shm_lpid; // pid of last operation

   pid_t         shm_cpid; // creator pid

   shmatt_t         shm_nattch;   // current # attached

   shmat_t          shm_cnattch;  // in-core # attached

   time_t        shm_atime; // last attach time

   time_t        shm_dtime; // last detach time

   time_t        shm_ctime; // last change time of this structure

}

shmget Function

Creates shared memory or accesses existing shared memory.

#include

int shmget(key_t key, size_t size, int oflag);

                     returns: shared memory identifier if OK, -1 on error

Return value would be shared memory identifier and will be used to access shared memory by other three shm_xxx functions.

Size is size of shared memory segment. Oflag means permission for read-write.

shmat  Function

shmat function can access shared memory after it is opened or created.

#include

void *shmat(int shmid, const void *shmaddr, int flag);

              returns: starting address of mapped region if OK, -1 on error

shmid is return value from shmget. The return value of shmat is shared memory starting address.

shmdt Function

Detaches from shared memory when the process finishes.

#include

int shmdt(const void *shmaddr);

                                         returns: 0 if OK, -1 on error

Calling shmdt doesn't mean that it removes shared memory segment.

shmctl Function

Provides various functions related shared memory.

#include

int shmctl(int shmid, int cmd, struct shmid_ds *buff);

                                         returns: 0 if OK, -1 on error

IPC_RMID	Remove the shared memory segment identified by shmid from the system and destroy the shared memory segment
IPC_SET	Set the following three members of the shmid_ds structure for the shared memory segment from the corresponding members in the structure pointed to by the buff argument: shm_perm.uid, shm_perm.gid, and shm_perm.mode. The shm_ctime value is also replaced with the current time.
IPC_STAT	Return to the caller(through the buff argument) the current shmid_ds structure for the specified shared memory segment

* referenced by

Unix Network Programming (Interprocess Communications) by W. Richard Stevens