System Calls and File Handling in C - C Post 08

System calls are the interface between a user program and the operating system. In C, system calls allow programs to interact with hardware resources, manage processes, and handle files. In this post, we will explore system calls, file handling, and inter-process communication (IPC).

1. Understanding System Calls

A system call is a request made by a program to the operating system for a service such as file I/O, process creation, or memory allocation. Common system calls include:

• fork() - Creates a new process.
• exec() - Replaces the current process image.
• wait() - Makes a process wait for child process termination.
• open(), read(), write(), close() - File handling operations.

1.1 Example: Using fork()

#include <stdio.h>
#include <unistd.h>

int main() {
    pid_t pid = fork();
    if (pid == 0) {
        printf("Child process\n");
    } else {
        printf("Parent process\n");
    }
    return 0;
}

1.2 Example: Using exec()

#include <unistd.h>
int main() {
    char *args[] = {"/bin/ls", "-l", NULL};
    execvp(args[0], args);
    return 0;
}

2. File Handling in C

File handling in C is done using system calls or standard library functions like fopen(), fprintf(), fscanf(), etc.

2.1 Opening and Closing Files

#include <stdio.h>
int main() {
    FILE *file = fopen("example.txt", "w");
    if (file) {
        fprintf(file, "Hello, C!");
        fclose(file);
    }
    return 0;
}

2.2 Reading and Writing Files

#include <stdio.h>
int main() {
    char buffer[100];
    FILE *file = fopen("example.txt", "r");
    if (file) {
        fgets(buffer, sizeof(buffer), file);
        printf("Read from file: %s", buffer);
        fclose(file);
    }
    return 0;
}

3. Inter-Process Communication (IPC)

3.1 Pipes

Pipes enable data transfer between processes.

#include <stdio.h>
#include <unistd.h>
int main() {
    int fd[2];
    pipe(fd);
    write(fd[1], "Hello", 6);
    char buffer[10];
    read(fd[0], buffer, sizeof(buffer));
    printf("Received: %s\n", buffer);
    return 0;
}

3.2 Message Queues

Message queues allow structured message passing.

#include <sys/ipc.h>
#include <sys/msg.h>
#include <stdio.h>
int main() {
    int msgid = msgget(IPC_PRIVATE, 0666 | IPC_CREAT);
    printf("Message Queue ID: %d\n", msgid);
    return 0;
}

System calls and file handling are fundamental to operating system interactions in C. Understanding process management, file operations, and IPC mechanisms enables efficient resource management and communication between processes.

In the next post, we will explore writing and using libraries in C, including static and dynamic libraries.

Happy coding! 🚀