Operating Systems: Interprocess Communication with System V Semaphores

Operating Systems: Interprocess Communication (IPC)

System V IPC: Semaphores

Lecture 4.1 Florenc Demrozi florenc.demrozi@univr.it University of Verona Department of Computer Science 2021/2022

Operating systems Interprocess communication (IPC) Part 1/3 2021/2022 1 / 45Table of Contents

1. Introduction to System V IPC . Creating and Opening · Data Structures
2. IPCs Commands · ipcs · ipcrm
3. Semaphores . Creating and Opening . Control Operations . Other Operations

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Unix System V Overview

Unix System V (aka "System Five") Unix System V is one of the first commercial versions of the Unix operating system. It was originally developed by AT&T and first released in 1983. Four major versions of System V were released, numbered 1, 2, 3, and 4. System V is sometimes abbreviated to SysV.

Interprocess Communication (IPC) Definition

Interprocess communication (IPC) refers to mechanisms that coordinate activities among cooperating processes. A common example of this need is managing access to a given system resource.

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System V IPC Mechanisms

System V IPCs refers to three different mechanisms for interprocess communication:

• Semaphores let processes to synchronize their actions. A semaphore is a kernel-maintained value, which is appropriately modified by system's processes before performing some critical actions
• Message queues can be used to pass messages among processes.
• Shared memory enables multiple processes to share a their region of memory.

Other IPC Methods

Other IPC · Signals · Pipes · FIFOs

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Creating and Opening IPC Objects

Creating and Opening

Operating systems Interprocess communication (IPC) Part 1/3 2021/2022 sas 6 / 45Creating and opening a System V IPC object

System V IPC Object Creation with get System Calls

Each System V IPC mechanism has an associated get system call (msgget, semget, or shmget ), which is analogous to the open system call. Given an integer key (analogous to a filename), the get system call can either first create a new IPC, and then returns its unique identifier, or returns the identifier of an existing IPC. An IPC identifier is analogous to a file descriptor. It is used in all subsequent system calls to refer to the IPC object.

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Semaphore Creation Example

Example showing how to create a semaphore (overview) / / PERM: rw- id = semget (key, 10 , IPC_CREAT | S_IRUSR | S_IWUSR) ; if (id == - 1) errExit (semget) ; As with all of the get calls, the key is the first argument. It is a value sensible for the application using the IPC object. The returned IPC identifier is a unique code identifying the IPC object in the system. Mapping with the open( ... ) system call: key ->filename id ->file descriptor

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System V IPC Keys and Uniqueness

System V IPC keys are integer values represented using the data type key_t. The IPC get calls translate a key into the corresponding integer IPC identifier. So, how do we provide a unique key that guarantees we do not accidentally obtain the identifier of an existing IPC object used by some other application?

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IPC_PRIVATE Flag for Unique Keys

When creating a new IPC object, the key may be specified as IPC PRIVATE. In this way, we delegate the problem of finding a unique key to the kernel. Example of the usage of IPC PRIVATE: id = semget (IPC_PRIVATE, 10, S_IRUSR | S_IWUSR) ; This technique is especially useful in multiprocess applications where the parent process creates the IPC object prior to performing a fork(), with the result that the child inherits the identifier of the IPC object.

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ftok Function for IPC Keys

The ftok (file to key) function converts a pathname and a proj_id (i.e., project identifier) to a System V IPC key. #include <sys/ipc.h> // Returns integer key on succcess, or -1 on error (check errno) key_t ftok(char *pathname, int proj_id) ; The provided pathname has to refer to an existing, accessible file. The last 8 bits of proj_id are actually used, and they have to be a nonzero value). Typically, pathname refers to one of the files, or directories, created by the application.

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ftok Usage Example

Example shows a typical usage of the function ftok key_t key = ftok("/mydir/myfile", 'a'); if (key == - 1) errExit ("ftok failed") ; int id = semget (key, 10, S_IRUSR | S_IWUSR) ; if (id == - 1) errExit ("semget failed") ; Example: Character "a" . ASCII = 097 · Binary = 01100001

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IPC Data Structures

Data Structures

Operating systems Interprocess communication (IPC) Part 1/3 2021/2022 13 / 45Associated Data Structure - ipc_perm

ipc_perm Structure for IPC Objects

The kernel maintains an associated data structure (msqid_ds, semid_ds, shmid_ds) for each instance of a System V IPC object. As well as data specific to the type of IPC object, each associated data structure includes the substructure ipc perm holding the granted permissions. struct ipc_perm { key_t __ key; /* Key, as supplied to 'get' call */ 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 */ unsigned short mode; /* Permissions */ unsigned short __ seq; /* Sequence number */ }

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ipc_perm Field Details

. The uid and gid fields specify the ownership of the IPC object. . The cuid and cgid fields hold the user and group IDs of the process that created the object. . The mode field holds the permissions mask for the IPC object, which are initialized using the lower 9 bits of the flags specified in the get system call used to create the object. Some important notes about ipc_perm: 1 The cuid and cgid fields are immutable. 2 Only read and write permissions are meaningful for IPC objects. Execute permission is meaningless, and it is ignored.

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Changing Semaphore Owner with semctl

Example shows a typical usage of the semctl to change the owner of a semaphore. struct semid_ds semq; // get the data structure of a semaphore from the kernel if (semctl(semid, 0, IPC_STAT, &semq) == - 1) errExit ("semctl get failed") ; // change the owner of the semaphore semq . sem_perm. uid = newuid; // update the kernel copy of the data structure if (semctl(semid, IPC_SET, &semq) == - 1) errExit("semctl set failed") ; Similarly, the shmctl and msgctl system calls are applied to update the kernel data structure of a shared memory and message queue.

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IPC Commands

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ipcs Command

ipcs

Operating systems Interprocess communication (IPC) Part 1/3 2021/2022 18 / 45The ipcs command

Displaying IPC Object Information with ipcs

Using ipcs, we can obtain information about IPC objects on the system. By default, ipcs displays all objects, as in the following example: user@localhost [~]$ ipcs Message Queues key msqid owner perms used-bytes messages 0x1235 26 student 620 12 20 Shared Memory Segments key shmid owner perms bytes nattch status 0x1234 0 professor 600 8192 2 Semaphore Arrays key semid owner perms nsems 0x1111 102 professor 330 20

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ipcrm Command

ipcrm

Operating systems Interprocess communication (IPC) Part 1/3 2021/2022 20 / 45The ipcrm command

Removing IPC Objects with ipcrm

Using ipcrm, we can remove IPC objects from the system. Remove a message queue: ipcrm -Q 0x1235 ( 0x1235 is the key of a queue ) ipcrm -q 26 ( 26 is the identifier of a queue ) Remove a shared memory segment ipcrm -M 0x1234 ( 0x1234 is the key of a shared memory seg. ) ipcrm -m 0 ( 0 is the identifier of a shared memory seg. ) Remove a semaphore array ipcrm -S 0x1111 ( 0x1111 is the key of a semaphore array ) ipcrm -s 102 ( 102 is the identifier of a semaphore array )

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Semaphores

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Semaphore Creation and Opening

Creating and Opening

Operating systems Interprocess communication (IPC) Part 1/3 2021/2022 23 / 45Creating/Opening a Semaphore Set

semget System Call for Semaphore Sets

The semget system call creates a new semaphore set or obtains the identifier of an existing set. #include <sys/sem.h> // Returns semaphore set identifier on success, or -1 error int semget (key_t key, int nsems, int semflg) ; The key arguments are: an IPC key, nsems specifies the number of semaphores in that set, and must be greater than 0. semflg is a bit mask specifying the permissions (see open( ... ) system call, mode argument) to be places on a new semaphore set or checked against an existing set. In additions, the following flags can be ORed (|) in semflg: . IPC_CREAT: If no semaphore set with the specified key exists, create a new set. . IPC EXCL: in conjunction with IPC_CREAT, it makes semget fail if a semaphore set exists with the specified key.

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Semaphore Set Creation Examples

Example showing how to create a semaphore set having 10 semaphores int semid; ket_t key = // ... (generate a key in some way, i.e. with ftok) // A) delegate the problem of finding a unique key to the kernel semid = semget (IPC_PRIVATE, 10, S_IRUSR | S_IWUSR) ; // B) create a semaphore set with identifier key, if it doesn't already exist semid = semget (key, 10, IPC_CREAT | S_IRUSR | S_IWUSR) ; //C) create a semaphore set with identifier key, but fail if it exists already semid = semget(key, 10, IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR) ;

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