🟦 Inter-Process Communication Using Shared Memory

---

1️⃣ What is Shared Memory?

Shared memory is an IPC mechanism where multiple processes access the same physical memory region.

• Fastest IPC mechanism

• No kernel involvement after setup

• Requires explicit synchronization (semaphores/mutex)

---

2️⃣ Why Shared Memory?

✔ Very high performance
✔ Suitable for large data sharing
❌ Risk of race condition without synchronization

---

3️⃣ System Calls Used (System V Shared Memory)

Function	Purpose
ftok()	Generate IPC key
shmget()	Create shared memory
shmat()	Attach shared memory
shmdt()	Detach shared memory
shmctl()	Control / delete memory

---

4️⃣ Communication Model (Simple Example)

• Parent process writes data

• Child process reads data

• Shared memory is created before fork()

---

5️⃣ Small Example: Parent Writes, Child Reads

🔹 Program Explanation

1. Create shared memory

2. Fork child

3. Parent writes message

4. Child reads message

5. Shared memory deleted

---

6️⃣ C Program: Shared Memory IPC

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

int main() {
    key_t key;
    int shmid;
    char *shared_mem;

    // Generate unique key
    key = ftok("shmfile", 65);

    // Create shared memory segment (1024 bytes)
    shmid = shmget(key, 1024, 0666 | IPC_CREAT);

    // Attach shared memory
    shared_mem = (char*) shmat(shmid, NULL, 0);

    if (fork() == 0) {
        // Child process
        sleep(1);  // ensure parent writes first
        printf("Child reads: %s\n", shared_mem);
        shmdt(shared_mem);
    } else {
        // Parent process
        strcpy(shared_mem, "Hello from parent via shared memory");
        printf("Parent writes message\n");
        wait(NULL);
        shmdt(shared_mem);
        shmctl(shmid, IPC_RMID, NULL);  // delete shared memory
    }

    return 0;
}

---

7️⃣ Sample Output

Parent writes message
Child reads: Hello from parent via shared memory

---

8️⃣ Important Notes 

• Shared memory is not synchronized by default

• Race conditions may occur

• Use semaphores for synchronization

• Must remove shared memory using shmctl()

Explanation of Functions Used in Shared Memory IPC Program

The program uses System V shared memory and a few process control system calls.

---

1️⃣ ftok() — Generate IPC Key

Prototype

key_t ftok(const char *pathname, int proj_id);

Purpose

Generates a unique key used to identify the shared memory segment.

Arguments

Argument	Description
pathname	Existing file name
proj_id	Project identifier (character or number)

Example

key = ftok("shmfile", 65);

📌 Note:
Both processes must use the same key to access shared memory.

---

2️⃣ shmget() — Create or Access Shared Memory

Prototype

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

Purpose

Creates a shared memory segment or accesses an existing one.

Arguments

Argument	Description
key	IPC key
size	Size of shared memory in bytes
shmflg	Permission and flags

Example

shmid = shmget(key, 1024, 0666 | IPC_CREAT);

Flags Explained

• 0666 → Read/write permissions

• IPC_CREAT → Create segment if not present

---

3️⃣ shmat() — Attach Shared Memory

Prototype

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

Purpose

Attaches shared memory to the process’s address space.

Arguments

Argument	Description
shmid	Shared memory ID
shmaddr	Address to attach (NULL = system chooses)
shmflg	Access mode (0 = read/write)

Example

shared_mem = (char*) shmat(shmid, NULL, 0);

📌 Return Value

• Pointer to shared memory

• (void *) -1 on failure

---

4️⃣ fork() — Create Child Process

Prototype

pid_t fork(void);

Purpose

Creates a child process that shares the same memory segment.

Return Values

Return	Meaning
0	Child process
>0	Parent process
<0	Error

Example

if (fork() == 0) { /* child */ }

---

5️⃣ strcpy() — Write to Shared Memory

Prototype

char *strcpy(char *dest, const char *src);

Purpose

Copies message into shared memory.

Example

strcpy(shared_mem, "Hello from parent");

📌 Important:
Shared memory behaves like normal memory.

---

6️⃣ sleep() — Delay Execution

Prototype

unsigned int sleep(unsigned int seconds);

Purpose

Ensures parent writes first before child reads.

Example

sleep(1);

---

7️⃣ shmdt() — Detach Shared Memory

Prototype

int shmdt(const void *shmaddr);

Purpose

Detaches shared memory from process address space.

Arguments

Argument	Description
shmaddr	Address returned by shmat()

Example

shmdt(shared_mem);

---

8️⃣ shmctl() — Control / Delete Shared Memory

Prototype

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

Purpose

Controls shared memory segment.

Arguments

Argument	Description
shmid	Shared memory ID
cmd	Control command
buf	Optional structure

Example

shmctl(shmid, IPC_RMID, NULL);

Commands

Command	Meaning
IPC_RMID	Remove shared memory
IPC_STAT	Get status

---

9️⃣ wait() — Wait for Child Process

Prototype

pid_t wait(int *status);

Purpose

Ensures parent waits for child before deleting memory.

Example

wait(NULL);

---

🔹 Summary Table (Quick Revision)

Function	Purpose
ftok()	Generate key
shmget()	Create shared memory
shmat()	Attach memory
fork()	Create child
strcpy()	Write data
shmdt()	Detach memory
shmctl()	Delete memory
wait()	Synchronize parent