Process Communication

Inter-Process Communication (IPC)

Definition: Inter-Process Communication (IPC) refers to the mechanisms that allow processes to communicate and share data with each other.

Pipes and FIFOs

Pipes:

A pipe is a one-way communication channel that allows the output of one process to be connected to the input of another process.

# Example: Command1's output is passed as input to Command2 command1 | command2

FIFOs (Named Pipes):

A FIFO (First-In-First-Out) is a named pipe that allows unrelated processes to communicate. It is similar to a regular pipe but can be accessed by multiple processes.

# Create a FIFO 
mkfifo my_fifo 
# Example: Command1 writes to FIFO, Command2 reads from FIFO 
command1 > my_fifo & command2 < my_fifo

Signals and Signal Handling

Signals:

Signals are software interrupts sent to a process to communicate an event or request a specific action.

# Example: Send SIGTERM signal to a process with PID <PID> 
kill -15 <PID>

Signal Handling:

Processes can define custom actions to be taken when a specific signal is received.

#include <stdio.h>
#include <signal.h>

void signal_handler(int signum) {
    printf("Received signal %d\n", signum);
}

int main() {
    signal(SIGINT, signal_handler); // Handle SIGINT (Ctrl+C) signal

    while (1) {
        // Code to execute
    }

    return 0;
}

Shared Memory

Shared Memory:

Shared memory allows multiple processes to access and modify the same portion of memory. It is a very efficient method of IPC.

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

int main() {
    key_t key = ftok("shmfile",65);
    int shmid = shmget(key,1024,0666|IPC_CREAT);
    char *str = (char*) shmat(shmid,(void*)0,0);

    printf("Write Data : ");
    gets(str);

    shmdt(str);

    return 0;
}

Message Queues

Message Queues:

Message queues allow processes to communicate by sending and receiving messages.

#include <stdio.h>
#include <sys/ipc.h>
#include <sys/msg.h>

struct msg_buffer {
    long msg_type;
    char msg_text[100];
};

int main() {
    key_t key;
    int msgid;
    struct msg_buffer message;

    // Create unique key
    key = ftok("progfile", 65);

    // Create a message queue
    msgid = msgget(key, 0666 | IPC_CREAT);

    message.msg_type = 1;
    printf("Write Data : ");
    gets(message.msg_text);

    // Send message
    msgsnd(msgid, &message, sizeof(message), 0);

    return 0;
}

Sockets

Sockets:

Sockets provide communication between processes over a network. They can be used for IPC between processes on the same machine or across a network.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

int main() {
    int sock = socket(AF_INET, SOCK_STREAM, 0);

    struct sockaddr_in server;
    server.sin_family = AF_INET;
    server.sin_port = htons(12345);
    server.sin_addr.s_addr = inet_addr("127.0.0.1");

    connect(sock, (struct sockaddr *)&server, sizeof(server));

    char message[100];
    strcpy(message, "Hello, server!");
    send(sock, message, sizeof(message), 0);

    close(sock);

    return 0;
}

These examples provide practical demonstrations of different inter-process communication mechanisms. Each example demonstrates a method for processes to exchange information, either through pipes, FIFOs, signals, shared memory, message queues, or sockets.

BCE-O535 Linux and Shell Programming