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POSIX shared-memory API

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Several IPC mechanisms are available for POSIX systems, including shared memory and message passing. Here, we explore the POSIX API for shared memory.

POSIX shared memory is organized using memory-mapped files, which associate the region of shared memory with a file. A process must first create a shared-memory object using the shm_open() system call, as follows:

shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666);
Parameters:
name: The first parameter specifies the name of the shared-memory object.
Processes that wish to access this shared memory must refer to the
object by this name.
O_CREAT | O_RDWR : The subsequent parameters specify that the shared-memory
object is to be created if it does not yet exist (O_CREAT) and that the object is
open for reading and writing (O_RDWR).

The last parameter establishes the directory permissions of the
shared-memory object.

A successful call to shm_open() returns an integer file descriptor for the shared-memory object. Once the object is established, the ftruncate() function is used to configure the size of the object in bytes. The call

 ftruncate(shm_fd, 4096);

sets the size of the object to 4, 096 bytes. Finally, the mmap() function establishes a memory-mapped file containing the shared-memory object. It also returns a pointer to the memory-mapped file that is used for accessing the shared-memory object.
 

Programs showing POSIX shared memory API for producer and consumer 

C




//C program for Producer process illustrating POSIX shared-memory API.
 
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
#include <unistd.h>
 
int main()
{
    /* the size (in bytes) of shared memory object */
    const int SIZE = 4096;
 
    /* name of the shared memory object */
    const char* name = "OS";
 
    /* strings written to shared memory */
    const char* message_0 = "Hello";
    const char* message_1 = "World!";
 
    /* shared memory file descriptor */
    int shm_fd;
 
    /* pointer to shared memory object */
    void* ptr;
 
    /* create the shared memory object */
    shm_fd = shm_open(name, O_CREAT | O_RDWR, 0666);
 
    /* configure the size of the shared memory object */
    ftruncate(shm_fd, SIZE);
 
    /* memory map the shared memory object */
    ptr = mmap(0, SIZE, PROT_WRITE, MAP_SHARED, shm_fd, 0);
 
    /* write to the shared memory object */
    sprintf(ptr, "%s", message_0);
 
    ptr += strlen(message_0);
    sprintf(ptr, "%s", message_1);
    ptr += strlen(message_1);
    return 0;
}


C




// C program for Consumer process illustrating
// POSIX shared-memory API.
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <sys/shm.h>
#include <sys/stat.h>
#include <sys/mman.h>
 
int main()
{
    /* the size (in bytes) of shared memory object */
    const int SIZE = 4096;
 
    /* name of the shared memory object */
    const char* name = "OS";
 
    /* shared memory file descriptor */
    int shm_fd;
 
    /* pointer to shared memory object */
    void* ptr;
 
    /* open the shared memory object */
    shm_fd = shm_open(name, O_RDONLY, 0666);
 
    /* memory map the shared memory object */
    ptr = mmap(0, SIZE, PROT_READ, MAP_SHARED, shm_fd, 0);
 
    /* read from the shared memory object */
    printf("%s", (char*)ptr);
 
    /* remove the shared memory object */
    shm_unlink(name);
    return 0;
}


The above programs can be compiled by typing the following commands in terminal opened in working directory –

gcc producer.c -pthread -lrt -o producer
gcc consumer.c -pthread -lrt -o consumer
./consumer & ./producer &

This should output the following –

HelloWorld!

The above programs use the producer–consumer model in implementing shared memory. 

  • The producer establishes a shared memory object and writes to shared memory, and the consumer reads from shared memory.
  • The producer, creates a shared-memory object named OS and writes the famous string “Hello World!” to shared memory.
  • The program memory-maps a shared-memory object of the specified size and allows writing to the object. (Obviously, only writing is necessary for the producer.)
  • The flag MAP SHARED specifies that changes to the shared memory object will be visible to all processes sharing the object. Notice that we write to the shared-memory object by calling the sprintf() function and writing the formatted string to the pointer ptr.
  • After each write, we must increment the pointer by the number of bytes written. The consumer process, reads and outputs the contents of the shared memory.
  • The consumer also invokes the shm_unlink() function, which removes the shared-memory segment after the consumer has accessed it.

References 
Silberschatz’s Operating System concepts 

http://www.cse.cuhk.edu.hk/~ericlo/teaching/os/lab/7-IPC2/sync-pro.html



 



Last Updated : 31 Oct, 2023
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