[TBD]Shared memory

Introduction

Laymen explanation

Technical exaplanation

SystemV based shared memory

Example

Text Box

#include <stdio.h>

#include <sys/shm.h>

#include <sys/stat.h>

int main ()

{

int segment_id;

char* shared_memory;

struct shmid_ds shmbuffer;

int segment_size;

const int shared_segment_size = 0x20;

/* Allocate a shared memory segment. */

segment_id = shmget (IPC_PRIVATE, shared_segment_size,

IPC_CREAT | IPC_EXCL | S_IRUSR | S_IWUSR);

/* Attach the shared memory segment. */

shared_memory = (char*) shmat (segment_id, 0, 0);

printf ("shared memory attached at address %p\n", shared_memory);

/* Determine the segment's size. */

shmctl (segment_id, IPC_STAT, &shmbuffer);

segment_size = shmbuffer.shm_segsz;

printf ("segment size: %d\n", segment_size);

/* Write a string to the shared memory segment. */

sprintf (shared_memory, "Hello, world.");

/* Detach the shared memory segment. */

shmdt (shared_memory);

/* Reattach the shared memory segment, at a different address. */

shared_memory = (char*) shmat (segment_id, (void*) 0x5000000, 0);

printf ("shared memory reattached at address %p\n", shared_memory);

/* Print out the string from shared memory. */

printf ("%s\n", shared_memory);

/* Detach the shared memory segment. */

shmdt (shared_memory);

/* Deallocate the shared memory segment. */

shmctl (segment_id, IPC_RMID, 0);

return 0;

}

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output

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shared memory attached at address 0x7f8f8ee6f000

segment size: 32

shared memory reattached at address 0x5000000

Hello, world.

Use of IPC_CREAT | IPC_EXCL

IPC_CREAT will create the new shared memory segment. IPC_EXCL will ensure that shared memory segment doesn't exist. If exist, shmget will return error.

Useful link: https://stackoverflow.com/questions/11432823/shmget-with-ipc-excl

https://stackoverflow.com/questions/5656530/how-to-use-shared-memory-with-linux-in-c

Reference