Skip to main content

System V IPC

Three forms of System V IPC

  • Semaphores

  • Message Queues

  • Shared Memory

System V IPC is different from POSIX API, but both are available in the linux kernel.

GNU C library in kernel version 2.2 includes the interfaces for shared memory and the semaphore according to POSIX.

Access Rights and Numbers

struct kern_ipc_perm

{

key_t key;  //key

uid_t uid; //Owner

gid_t gid;  //Owner

uid_t cuid;  //Creator

gid_t cgid; //Creator

mode_t mode; //Access Mode

unsigned long seq; //counter used to calculate the identifier

};

the user and group id needs 32bit for the Intel 32 bit architecture, so the kernel supported both the IPC_OLD and IPC_64.

Semaphores (System V)

  • Array of semaphores can be setup using the system calls
  • It is always possible to modify a number of semaphores.
  • They can be incremented or decremented in steps greater than 1.

Semaphores are created using the following structure

struct sem_array

{

struct kern_ipc_perm sem_perm; //access permission

time_t sem_otime; //time of the last semaphore operation

time_t sem_ctime; //time of the last change

struct sem *sem_base; //pointer to the first semaphore

struct sem_queue *sem_pending; //operation to be reversed

struct sem_queue **sem_pending_last; //last operation to be carried out

struct sem_undo *undo; //undo operation to be carried out

unsigned long sem_nsems:// Number of semaphores in this array

};

struct sem

{

int semval; //current value of the semaphore

int sempid; //Process ID of the last operation

};

Message Queues

  • Message consists of sequence of bytes and a code.
  • Processes send messages to the queue and can receive message
  • Messages are read in the same order in which they are entered in the message queue.

struct msg_queue

{

struct kern_ipc_perm q_perm; //Access rights

time_t q_stime; //time of last send

time_t q_rtime; //time of last receive

time_t q_ctime; //time of last change

unsigned long q_cbytes; //number of bytes in the queue

unsigned long q_qnum; //number of message in the queue

unsigned long q_qbytes; //capacity of wait queue in bytes

pid_t lspid; //pid of the last sender

pid_t q_lrpid; //pid of the last receiver

};

To send message, the processes use these functions

int sys_msgsnd(int msgid, struct msgbuf *magp, size_t msgsz, int msgflg);

int sys_msgrcv(int msgid, struct msgbuf *magp, size_t msgsz, long msgtyp, int msgflg)

Shared Memory

  • shared memory is the fastest form of Inter process communication
  • exchange data between processes using the machine code commands for reading and writing
  • the main drawback is that the processes need to use additional synchronization mechanism to avoid the race condition
  • Shared segment of memory is identified by a number.
  • The structure shmid_kernel is for the kernel segment and mapped to the user segment in the virtual address space by the processes with the help of attach function, the reverse action will be through the help of detach.

struct shmid_kernel

{

struct kern_ipc_perm shm_perm; //access rights

struct file *shm_file; //file in the shared memory

int id;

unsigned long shm_nattach; //number of attachments

unsigned long shm_segsz; //size of segment

time_t shm_atim; //time of last attach

time_t shm_dtim; //time of last detach

time_t shm_ctim; //time of creation

pid_t shm_cprid; //creator process id

pid_t shm_lprid; //process id of the last operation

};

Information for Semaphore, message queue and shared memory

seminfo

Value

semmni (maximum number of semaphore arrays)

128

semmns (maximum number of semaphores in the system

32000

semmsl (number of semaphores per array)

250

semvmx (maximum value of semaphores

32767

msginfo

Value

msgmni (maximum number of message queue)

16

msgmax (maximum size of a message in bytes)

8192

msgmnb (standarad value for the maximum size of a message queue in bytes)

16384

shminfo

Value

shmmni (maximum no of shared memory segment)

4096

shmmax (maximum size of SHM segment in bytes)

33,554,432

shmmin (mi

nimum size of SHM segment in bytes)

1

shmseg (permitted no of segments/processes)

4096

Comments

Popular posts from this blog

Installing TexLive 2019 in Ubuntu 18.04

Installation of TexLive 2019 in Linux (Ubuntu 18.04 LTS)
TeX (Tech)

Installation of TexLive 2019

Please watch the video for full installation



I used .iso file to download, the Total size is 3.3GB for Linux,

and i used the torrent file to download, it took me just 20 min to download the entire .iso file

Extract the .iso file to a folder and open a terminal

$] sudo ./install-tl
(it goes into a terminal mode, which is faster compared to the GUI Mode)

$] sudo ./install-tl -gui
after the installation, set the PATH, MANPATH and INFOPATH as suggested by LATEX

export PATH=$PATH:/usr/local/texlive/2019/bin/x86_64-linux
export MANPATH=/usr/local/texlive/2019/texmf-dist/doc/man
export INFOPATH=/usr/local/texlive/2019/texmf-dist/doc/info

put these lines in to the /home/pradeepkumar/.bashrc

$] gedit /home/pradeepkumar/.bashrc
We have installed TexLive 2019 and texstudio.

To install texstudio

$] sudo apt install texstudio
The look and feel of TexStudio looks like this image.


texlive, it install everyt…

Implementing a new system call in Kernel version 2.6.32

A system call is used by application or user programs to request service from the operating systems. Since the user programs does not have direct access to the kernel whereas the OS has the direct access. OS can access the hardware through system calls only.The following files has to be modified for implementing a system call/usr/src/linux-2.6.32.5/arch/x86/kernel/syscall_table_32.S/usr/src/linux-2.6.32.5/arch/x86/include/asm/unistd_32.h/usr/src/linux-2.6.32.5/include/linux/syscalls.h/usr/src/linux-2.6.32.5/MakefileNew set of files to be createdCreate a new directory newcall/ inside the path “/usr/src/linux-2.6.32.5/” Create new files Makefile, newcall.c and put them in the /usr/src/linux-2.6.32.5/newcall/ folder Create new user files (in any folder of Linux) to test the system call
testnewcall.c, testnewcall.h (created in /home/pradeepkumar) syscall_table_32.S Find the file /usr/src/linux-2.6.32.5/arch/x86/kernel/syscall_table_32.S and add the following line at the end
"…

Electrical Machine Design (equations)

FactorsDC Machine Transformers Induction Machines Synchronous MachinesOutput EquationPa=CoD2Ln, where Pa=P/h for generators, Pa=P for motorsFor Single Phase
Q=2.22 f Bm Ai Kw Aw d10-3
For Three Phase
Q=3.33 f Bm Ai Kw Aw d 10-3Q=CoD2 L ns
KVA Input Q=
HP * 0.746 / Cos f * hQ=CoD2 L ns
KVA Input Q=
HP * 0.746 / Cos f * h
For Turbo alternators
Q=1.11Bavac KwsVa2 L 10-3/nsOutput CoefficientCo=Bav ac* 10-3where Bav-magnetic loading and ac - electric loadingDNACo=11 Kws Bav ac 10-3Co=11 Kws Bav ac 10-3 Choice of Magnetic LoadingFlux Density in Teeth Frequency of Flux Reversals Size of machineDNAMagnetizing current, Flux Density, Iron lossIron loss, Stability, Voltage Rating, Parallel Operation, Transient ShortCircuit current Choice of Electric LoadingTemperature rise,
speed of machine, Voltage, Armature reaction, CommutationDNAOverload Capacity, Copper losses, Temperature rise, Leakage ReactanceCopper loss, Synchronous reactance, Temperature rise, Stray Load losses,
Voltage rating Flux …