Shared Memory in Linux Inter-Process Communication (IPC)

In Linux, shared memory is an efficient method for inter-process communication (IPC). Through shared memory, multiple processes can access the same memory region directly, enabling fast data transfer. This article details the four main functions for shared memory: shmget, shmat, shmdt, and shmctl.

Shared Memory Functions

1. shmget - Create or Get a Shared Memory Segment

The shmget function is used to create or get a shared memory segment.

#include <sys/ipc.h>
#include <sys/shm.h>

int shmget(key_t key, size_t size, int shmflg);

• Parameters:
  • key: A unique identifier for the shared memory segment.
  • size: The size of the shared memory segment in bytes.
  • shmflg: Permission flags and creation control flags, usually a combination of IPC_CREAT, IPC_EXCL, and permission bits.
• Return Value: Returns the shared memory segment identifier (ID) on success, and -1 on failure.

2. shmat - Attach the Shared Memory Segment to the Process’s Address Space

The shmat function attaches the shared memory segment to the address space of the calling process.

#include <sys/ipc.h>
#include <sys/shm.h>

void *shmat(int shmid, const void *shmaddr, int shmflg);

• Parameters:
  • shmid: The shared memory segment ID returned by shmget.
  • shmaddr: Address at which to attach the shared memory segment, typically NULL to let the system choose the address.
  • shmflg: Flags for the attach operation, such as SHM_RDONLY for read-only access.
• Return Value: Returns a pointer to the shared memory segment on success, and (void *) -1 on failure.

3. shmdt - Detach the Shared Memory Segment from the Process’s Address Space

The shmdt function detaches the shared memory segment from the address space of the calling process.

#include <sys/ipc.h>
#include <sys/shm.h>

int shmdt(const void *shmaddr);

• Parameters:
  • shmaddr: Pointer to the shared memory segment, returned by shmat.
• Return Value: Returns 0 on success, and -1 on failure.

4. shmctl - Control Shared Memory Segment

The shmctl function performs various control operations on a shared memory segment, such as deletion or information retrieval.

#include <sys/ipc.h>
#include <sys/shm.h>

int shmctl(int shmid, int cmd, struct shmid_ds *buf);

• Parameters:
  • shmid: The shared memory segment ID.
  • cmd: Control command, such as IPC_RMID (remove the segment), IPC_STAT (get segment status), etc.
  • buf: Pointer to a shmid_ds structure for storing or providing shared memory segment information.
• Return Value: Returns 0 on success, and -1 on failure.

Enabling IPC in the Kernel

Before using shared memory, ensure that IPC support is enabled in the kernel. The following kernel configuration options must be enabled:

CONFIG_SYSVIPC=y
CONFIG_SYSVIPC_SYSCTL=y

Otherwise, you may encounter errors during program execution, such as:

Unable to get shared memory segment(shmget)
shmid = -1, size = 32, size1 = 8192, Error 38

Example Code

Here is a simple example of using shared memory in a program:

#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <string.h>

#define SHM_KEY 1234

int main() {
    int shmid;
    char *shmaddr;
    const int size = 1024;

    // Create the shared memory segment
    shmid = shmget(SHM_KEY, size, IPC_CREAT | 0666);
    if (shmid == -1) {
        perror("shmget failed");
        exit(1);
    }

    // Attach the shared memory segment to the process's address space
    shmaddr = (char *) shmat(shmid, NULL, 0);
    if (shmaddr == (char *) -1) {
        perror("shmat failed");
        exit(1);
    }

    // Write data to the shared memory
    strncpy(shmaddr, "Hello, Shared Memory!", size);

    // Read data from the shared memory
    printf("Data in shared memory: %s\n", shmaddr);

    // Detach the shared memory segment
    if (shmdt(shmaddr) == -1) {
        perror("shmdt failed");
        exit(1);
    }

    // Remove the shared memory segment
    if (shmctl(shmid, IPC_RMID, NULL) == -1) {
        perror("shmctl failed");
        exit(1);
    }

    return 0;
}

Summary

Shared memory is an efficient IPC method, ideal for scenarios requiring rapid data transfer. With the shmget, shmat, shmdt, and shmctl functions, Linux provides comprehensive support for shared memory. Ensure IPC is enabled in the kernel, then use these functions to implement shared memory in your programs easily.