OS/embedded concepts

Posix Api

Shashi Shekhar's photo
Shashi Shekhar
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2 min read

What are POSIX APIs?

POSIX (Portable Operating System Interface) is a family of standards defined by IEEE to ensure compatibility between operating systems. A POSIX API refers to a set of standardized system calls and library functions that developers can use to write portable applications across POSIX-compliant operating systems, such as Linux, macOS, and some flavors of Unix.

Purpose of POSIX APIs

  • Portability: Write code once and run it on multiple POSIX-compliant systems without modification.

  • Standardization: Provide a consistent and predictable interface for system-level programming.

  • Abstraction: Hide the complexities of underlying hardware and operating system differences.

Key Areas of POSIX APIs

  1. Process Control:

    • Create, manage, and terminate processes.

    • Examples:

      • fork(): Create a new process.

      • exec() family: Replace the current process image with a new one.

      • wait(): Wait for a child process to terminate.

  2. Thread Management:

    • Manage multithreading in applications.

    • Examples:

      • pthread_create(): Create a new thread.

      • pthread_join(): Wait for a thread to terminate.

      • pthread_mutex_lock(): Lock a mutex for thread synchronization.

  3. File and Directory Operations:

    • Perform operations on files and directories.

    • Examples:

      • open(), read(), write(), close(): File I/O operations.

      • stat(): Retrieve metadata about a file.

      • opendir(), readdir(): Work with directories.

  4. Inter-Process Communication (IPC):

    • Facilitate communication between processes.

    • Examples:

      • Pipes: pipe().

      • Shared Memory: shmget(), shmat().

      • Message Queues: msgget(), msgsnd().

      • Semaphores: sem_open(), sem_wait().

  5. Signal Handling:

    • Handle asynchronous events using signals.

    • Examples:

      • signal(): Set a signal handler.

      • kill(): Send a signal to a process.

      • sigaction(): Advanced signal handling.

  6. Network Programming:

    • Enable socket programming for communication over networks.

    • Examples:

      • socket(): Create a socket.

      • bind(), listen(), accept(): Set up a server.

      • connect(): Connect to a server.

  7. Timers and Clocks:

    • Manage time-related functionalities.

    • Examples:

      • time(): Get the current time.

      • nanosleep(): Suspend execution for a specified time.

      • clock_gettime(): Retrieve high-resolution time.

  8. Device I/O:

    • Perform low-level input and output operations with devices.

    • Examples:

      • ioctl(): Device-specific input/output operations.

      • select(), poll(): Monitor multiple file descriptors for readiness.

POSIX Compliance

Operating systems are considered POSIX-compliant if they support the POSIX standard APIs. Examples include:

  • Compliant Systems: Linux, macOS, FreeBSD, Solaris.

  • Non-Compliant Systems: Windows is not POSIX-compliant natively but provides partial support via tools like Windows Subsystem for Linux (WSL) or third-party libraries.

POSIX in Practice

When writing applications, using POSIX APIs ensures that:

  • Code is portable across Unix-like systems.

  • Applications can leverage advanced system-level features like process control, IPC, and multithreading.

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