CSCI-3753: Operating SystemsFall 2018

Anh NguyenDepartment of Computer Science University of Colorado Boulder

Announcement

•PA2 is up on Moodle !!!

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Programming Assignment Two

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PA2 – LKM

•How to code a LKM?•module_init()•module_exit()

•How to install the module?•Makefile: obj-m:=xyz.o•make –C /lib/modules/$(uname -r)/build M=$PWD modules

•How to run the module?• sudo insmod xyz.ko

•How to remove the module?• sudo rmmod xyz

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PA2 – Character Device Driver

• A device driver is a program that operates or controls a particular type of device that is attached to a computer.• A driver communicates with

the device through the computer bus or communications subsystem to which the hardware connects. •When a calling program invokes

a routine in the driver, the driver issues commands to the device. • Once the device sends data back to the

driver, the driver may invoke routines in the original calling program.

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PA2 – Character Device Driver

•To work with device drivers, we have to work with the corresponding device files.•These files are stored in /dev folder.•To create device file for a device driver

• sudo mknod –m 777 /dev/simple_character_device c 240 0• mknod was originally used to create the character and

block devices that populate /dev/.• Device files • a storage device (block): cd-roms, hard drives, etc.• a device use for other purpose (character): /dev/zero,

/dev/null, or any other device not used to store info • "b" means block, and "c" means character

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mknod –m <permission> <device_file_location> <type of driver> <major number> <minor number>

Week 4: Inter-Process Communications(IPC) in Linux

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Process

• A software program consist of a sequence of code instructions and data stored on disk.• A program is a passive entity

• A process is a program actively executing from main memory within its own address space

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Code

Data

Program P1

Other…

Process State

•New: The process is being created. •Running: Instructions are being executed. •Waiting: The process is waiting for some event to

occur (such as an I/O completion or reception of a signal). •Ready: The process is waiting to be assigned to a

processor.•Terminated: The process has finished execution.

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Process Cooperation

•Reasons for providing an environment that allows process cooperation: • Information sharing: providing an environment to allow

concurrent access to such information • Computation speedup: break a particular task into

subtasks executing in parallel with the others to run faster•Modularity: dividing the system functions into separate

processes or threads • Convenience: working on many tasks at the same time

•Cooperating processes needs IPC mechanism !!!

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IPC in Linux

•There are two fundamental models of IPC: •Message passing• Communication takes place by means of messages

exchanged between the cooperating processes.• Advantages:• Useful for exchanging small amounts of data,

because no conflicts need be avoided • Easy to implement in a distributed system

• Disadvantages:• Time-consuming as typically implemented using

system calls

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IPC in Linux

•There are two fundamental models of IPC: •Shared memory• A region of memory is shared by cooperating

processes is established.• Processes can then exchange information by reading

and writing data to the shared region.• Advantages:• Fast as system calls are required only to establish

shared-memory• Useful for exchanging large amounts of data

• Disadvantages:• Complicated to implement in a distributed system

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Shared Memory

•One process creates a shared memory segment that other processes (if permitted) can access.

• If succeeds, it returns the shared memory segment ID. • It is also used to get the ID of an existing shared

segment (from a process requesting sharing of some existing memory portion).

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#include <sys/ipc.h>#include <sys/shm.h>int shmget(key_t key, size_t size, int shmflg);

Shared Memory

•Once created, a shared segment can be attached to a process address space.

• If succeeds, it returns a pointer, shmaddr, to the head of the shared segment associated with a valid shmid. •Once attached, a process can read or write to the

segment, as allowed by the permission requested in the attach operation.

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#include <sys/types.h> #include <sys/shm.h>void *shmat(int shmid, const void *shmaddr, int shmflg);

Shared Memory

•To detach the shared memory segment located at the address indicated by shmaddr

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int shmdt(const void *shmaddr);

Shared Memory Example

•Goal: to illustrate the usage of shared memory between two processes in Linux. •The server process creates a shared memory

segment and writes some characters in it. It then waits for the first character in the shared memory to become '*', and then exits. •The client process reads characters from this shared

memory and prints them out on the terminal, writes '*' at the beginning of the shared memory segment, and then exits.

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Week 4 – Checklist

q Discuss PA2q Discuss IPC – Shared memory & exampleq Read more about IPC

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