🔧 Linux/Unix: Built-in Functions & System Calls (printf, open, fork) – Explained with Examples

🧲 Introduction – Why Learn Built-in Functions and System Calls?

Linux/Unix isn’t just about shell commands—under the hood, it’s powered by system calls and built-in functions. These low-level operations allow user-space programs to interact with the kernel, making them vital for developers writing C programs, system utilities, or working with custom processes and I/O control.

🎯 In this guide, you’ll learn:

• What system calls are and how they work in Linux/Unix
• Differences between built-in library functions and kernel-level system calls
• Practical use cases of printf(), open(), and fork()
• Hands-on examples in C for each function

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🧠 What Are System Calls?

System calls are interfaces between user applications and the Linux kernel. Whenever a program wants to access hardware, memory, file systems, or processes, it must go through a system call.

🧪 Example Categories:

• File I/O: open(), read(), write(), close()
• Process: fork(), exec(), wait()
• Memory: mmap(), brk()
• Device: ioctl()

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📘 Built-in Functions vs. System Calls

Feature	Built-in Functions (e.g., printf())	System Calls (e.g., fork())
Level	User-space library functions	Kernel-level operations
Source	libc / stdio libraries	OS kernel APIs
Performance	Faster, buffered I/O	Slight overhead due to context switch
Uses	Formatting, standard I/O, computation	File control, process management

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🖨️ printf() – Print Formatted Output (Built-in Function)

✅ Syntax:

int printf(const char *format, ...);

📌 Description:

• Comes from <stdio.h>
• Writes formatted data to stdout
• Does not interact with the kernel directly

🧪 Example:

#include <stdio.h>

int main() {
    printf("Welcome to Linux Programming!\n");
    return 0;
}

💡 Output:

Welcome to Linux Programming!

✅ Use printf() for:

• Displaying variables
• Debugging output
• Formatting strings (%d, %s, %f, etc.)

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📂 open() – Open Files (System Call)

✅ Syntax:

#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>

int open(const char *pathname, int flags);

📌 Description:

• System call to open a file or device
• Returns a file descriptor (int)

Common Flags:

Flag	Purpose
O_RDONLY	Open for reading only
O_WRONLY	Open for writing only
O_RDWR	Open for read/write
O_CREAT	Create file if not exists
O_APPEND	Append to file

🧪 Example:

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

int main() {
    int fd = open("demo.txt", O_CREAT | O_WRONLY, 0644);
    if (fd < 0) {
        perror("open");
        return 1;
    }
    write(fd, "Hello, Linux World!\n", 21);
    close(fd);
    return 0;
}

✅ Use open() for:

• Direct file handling
• Manual control of file descriptors
• System-level file operations

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👥 fork() – Create New Process (System Call)

✅ Syntax:

#include <unistd.h>

pid_t fork(void);

📌 Description:

• Creates a new child process
• Both parent and child continue execution
• Returns:
  • 0 to child
  • Child PID to parent
  • -1 on failure

🧪 Example:

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

int main() {
    pid_t pid = fork();

    if (pid == 0) {
        printf("Child process: PID = %d\n", getpid());
    } else if (pid > 0) {
        printf("Parent process: PID = %d, Child PID = %d\n", getpid(), pid);
    } else {
        perror("fork failed");
    }
    return 0;
}

💡 Output Example:

Parent process: PID = 1234, Child PID = 1235
Child process: PID = 1235

✅ Use fork() for:

• Parallel processing
• Creating child processes in daemons
• Implementing custom shell or multi-process systems

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⚠️ Common Errors and Debug Tips

Error	Cause	Fix
open: Permission denied	File not writable or readable	Check file mode and permissions
fork failed: Resource temporarily unavailable	Max process limit reached	Use ulimit -u to check limits
Output not showing	Buffered I/O issue in printf()	Add fflush(stdout); or use \n

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📌 Summary – Recap & Next Steps

Understanding the difference between built-in functions and system calls helps you interact more deeply with Linux/Unix internals. Whether you’re writing utilities or exploring low-level behavior, functions like printf(), open(), and fork() are fundamental to systems programming.

🔍 Key Takeaways:

• printf() is a high-level library function for formatted output.
• open() is a system call used for file control and returns a file descriptor.
• fork() creates a child process and is essential for multiprocessing tasks.
• System calls provide controlled, secure access to kernel-level operations.
• These functions are foundational for building low-level programs and shells.

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❓ FAQs

❓ Is printf() a system call in Linux?
✅ No. printf() is a standard C library function. It uses write() under the hood for output.

❓ What does fork() do in Linux?
✅ It creates a new process (child) that runs the same code as the parent. Common in multiprocessing.

❓ How do I open a file in Linux using C?
✅ Use open() from <fcntl.h>:

int fd = open("file.txt", O_CREAT | O_WRONLY, 0644);

❓ What is a file descriptor?
✅ An integer returned by system calls like open() that represents an open file or I/O resource.

❓ Can I use fork() in shell scripting?
✅ Not directly. But shell handles process spawning with background jobs (&) and subshells.

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