Operating System Services - Learning Module

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I/O Operations

The Gateway to the Physical World

Computation without input is predetermined; computation without output is invisible. Input/Output (I/O) operations bridge the gap between the abstract world of software and the physical reality of disks, networks, displays, keyboards, and countless other devices. The operating system's I/O services transform the bewildering diversity of hardware into a uniform, manageable interface that applications can use.

Consider the apparent simplicity of reading a file: your program calls read(), and data appears in a buffer. Behind this simplicity, the OS orchestrates device detection, driver loading, buffer management, DMA transfers, interrupt handling, and process scheduling—all invisible to the application. This abstraction is both the OS's greatest convenience and its most complex engineering challenge.

What You Will Learn

By the end of this page, you will understand how operating systems abstract diverse hardware devices into uniform interfaces, the different I/O models available to programs, how device drivers bridge software and hardware, and the critical role of buffering and caching in I/O performance. You'll see how everything from file access to network communication relies on these foundational I/O services.

I/O Abstraction Principles

Operating systems face a fundamental challenge: the diversity of I/O devices is immense. From keyboards to GPUs, from SSDs to network cards, each device has unique characteristics, speeds, protocols, and quirks. Yet applications need a consistent way to interact with all of them.

The abstraction layers:

OS I/O subsystems are built in layers, each hiding complexity from the layer above:

┌─────────────────────────────────────────────────────────────────┐
│                    Application Layer                             │
│    open("file.txt"), read(fd, buf, size), write(socket, ...)   │
├─────────────────────────────────────────────────────────────────┤
│                 Virtual File System (VFS)                        │
│    Unified interface: everything is a file (or file-like)       │
├─────────────────────────────────────────────────────────────────┤
│              File Systems / Network Stacks                       │
│    ext4, NTFS, TCP/IP, etc. — domain-specific logic             │
├─────────────────────────────────────────────────────────────────┤
│                   Block / Character Layer                        │
│    Block devices (disks), character devices (terminals)          │
├─────────────────────────────────────────────────────────────────┤
│                    Device Drivers                                │
│    Translate generic requests to device-specific commands        │
├─────────────────────────────────────────────────────────────────┤
│                    Hardware Devices                              │
│    SSD, HDD, NIC, GPU, keyboard, mouse, USB devices...          │
└─────────────────────────────────────────────────────────────────┘

Everything is a file (Unix philosophy):

Unix systems extend the file abstraction remarkably far:

Regular files: Persistent data storage
Directories: Containers organizing files hierarchically
Device files: Hardware devices (/dev/sda, /dev/tty)
Pipes: Inter-process communication channels
Sockets: Network endpoints
Symbolic links: Pointers to other files
Proc files: Kernel and process information (/proc, /sys)

This uniformity means a single set of calls—open(), read(), write(), close()—works for vastly different I/O types. A program can read from a file, a terminal, or a network socket using nearly identical code.

unified_io_example.c
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#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
 
/**
 * Demonstrates the unified I/O interface
 * The same read() call works for files, devices, pipes, and more
 */
int main() {
    char buffer[1024];
    ssize_t bytes_read;
    
    /* Reading from a regular file */
    int file_fd = open("/etc/hostname", O_RDONLY);
    bytes_read = read(file_fd, buffer, sizeof(buffer));
    printf("From file: %.*s", (int)bytes_read, buffer);
    close(file_fd);
    
    /* Reading from a device (keyboard input) */
    // int tty_fd = open("/dev/tty", O_RDONLY);
    // bytes_read = read(tty_fd, buffer, sizeof(buffer));  // Same read()!
    
    /* Reading from /proc (kernel information) */
    int proc_fd = open("/proc/version", O_RDONLY);
    bytes_read = read(proc_fd, buffer, sizeof(buffer));
    printf("From /proc: %.*s", (int)bytes_read, buffer);
    close(proc_fd);
    
    /* Reading from /dev/urandom (random data device) */
    int random_fd = open("/dev/urandom", O_RDONLY);
    bytes_read = read(random_fd, buffer, 16);
    printf("Random bytes: ");
    for (int i = 0; i < bytes_read; i++)
        printf("%02x ", (unsigned char)buffer[i]);
    printf("\n");
    close(random_fd);
    
    return 0;
}
 
/*
 * Note: open(), read(), write(), close() work uniformly across:
 * - Regular files
 * - Block devices (disks)
 * - Character devices (terminals, serial ports)
 * - Named pipes (FIFOs)
 * - Unix domain sockets
 * - /proc and /sys pseudo-filesystems
 * - Network sockets (with socket() variant)
 */

Device Classification

•Block Devices — Data accessed in fixed-size blocks (sectors). Seek operations supported. Examples: hard drives, SSDs, USB flash drives. Buffering and caching heavily used.
•Character Devices — Data accessed as a stream of bytes. No seeking (usually). Examples: keyboards, mice, serial ports, terminals. Processed byte-by-byte or line-by-line.
•Network Devices — Data transferred as packets over network protocols. Not directly exposed as files; accessed via socket API. Examples: Ethernet NICs, Wi-Fi adapters.
•Pseudo Devices — Virtual devices with no physical hardware. Provide OS services or information. Examples: /dev/null, /dev/zero, /dev/random, /proc, /sys.

Windows I/O Model

Windows uses a similar abstraction through device objects managed by the I/O manager, but files and devices are accessed through different APIs (file operations vs DeviceIoControl). The Handle abstraction provides some uniformity, but Windows doesn't pursue 'everything is a file' as aggressively as Unix.

I/O Models and Strategies

Applications can interact with I/O devices in fundamentally different ways, each with distinct performance characteristics and programming models. Understanding these I/O models is essential for writing efficient programs.

Blocking (Synchronous) I/O:

The simplest model. When a program calls read(), it stops executing until data is available. The OS suspends the process, performs the I/O, and resumes the process when complete.

Process calls read()
        │
        ▼
┌───────────────────┐
│  Process BLOCKS   │   ← Process cannot do any work
│  (waiting for     │
│   I/O to complete)│
└───────────────────┘
        │
        ▼ (I/O completes, data available)
Process resumes with data

Advantages: Simple to program, intuitive flow Disadvantages: Wastes CPU while waiting; thread-per-connection servers don't scale

Non-Blocking I/O:

The process requests I/O, and the call returns immediately—either with data (if available) or an indication that the operation would block. The process must poll repeatedly.

// Set file descriptor to non-blocking mode
int flags = fcntl(fd, F_GETFL, 0);
fcntl(fd, F_SETFL, flags | O_NONBLOCK);

// Now read() returns immediately
while ((bytes = read(fd, buf, size)) == -1 && errno == EAGAIN) {
    // No data yet — do other work, then try again
    do_other_work();
}

Advantages: Process can do other work between I/O attempts Disadvantages: Wastes CPU in polling loop; complex programming model

I/O Multiplexing (select/poll/epoll):

The process monitors multiple I/O sources simultaneously, blocking until any of them is ready. This enables handling many connections with a single thread.

Process monitors fd1, fd2, fd3, fd4
              │
              ▼
┌─────────────────────────────┐
│     BLOCKS in select()       │  ← Waiting on ANY of the fds
│  (watching multiple sources) │
└─────────────────────────────┘
              │
              ▼ (fd2 and fd4 ready)
Process handles ready fds, then select() again

This is the foundation of modern high-performance servers—a single thread can handle thousands of concurrent connections.

io_multiplexing_example.c
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#include <sys/select.h>
#include <sys/epoll.h>
#include <unistd.h>
#include <stdio.h>
 
/**
 * I/O Multiplexing with select() - portable but limited
 */
void select_example(int fd1, int fd2) {
    fd_set read_fds;
    struct timeval timeout;
    
    while (1) {
        FD_ZERO(&read_fds);
        FD_SET(fd1, &read_fds);
        FD_SET(fd2, &read_fds);
        
        timeout.tv_sec = 5;
        timeout.tv_usec = 0;
        
        int max_fd = (fd1 > fd2) ? fd1 : fd2;
        int ready = select(max_fd + 1, &read_fds, NULL, NULL, &timeout);
        
        if (ready > 0) {
            if (FD_ISSET(fd1, &read_fds)) handle_fd1();
            if (FD_ISSET(fd2, &read_fds)) handle_fd2();
        }
    }
}
 
/**
 * I/O Multiplexing with epoll() - Linux-specific, scales better
 * Can handle 100,000+ concurrent connections efficiently
 */
void epoll_example(int listener_fd) {
    int epoll_fd = epoll_create1(0);
    struct epoll_event ev, events[1024];
    
    /* Add listener socket to epoll */
    ev.events = EPOLLIN;
    ev.data.fd = listener_fd;
    epoll_ctl(epoll_fd, EPOLL_CTL_ADD, listener_fd, &ev);
    
    while (1) {
        /* Wait for events on any registered fd */
        int num_ready = epoll_wait(epoll_fd, events, 1024, -1);
        
        for (int i = 0; i < num_ready; i++) {
            if (events[i].data.fd == listener_fd) {
                /* New connection - accept and add to epoll */
                int client_fd = accept(listener_fd, NULL, NULL);
                ev.events = EPOLLIN | EPOLLET;  /* Edge-triggered */
                ev.data.fd = client_fd;
                epoll_ctl(epoll_fd, EPOLL_CTL_ADD, client_fd, &ev);
            } else {
                /* Data from existing client */
                handle_client(events[i].data.fd);
            }
        }
    }
}
 
/*
 * Comparison:
 * - select(): O(n) scanning, limited to ~1024 fds (FD_SETSIZE)
 * - poll(): O(n) scanning, no fd limit
 * - epoll(): O(1) for ready fds, kernel maintains ready list
 * - kqueue(): BSD equivalent of epoll
 * - IOCP: Windows asynchronous I/O completion ports
 */

Asynchronous I/O (AIO):

The process initiates I/O and continues executing immediately. The OS notifies the process when I/O completes—via signal, callback, or completion queue.

Process initiates async read()
        │
        ▼ (returns immediately)
Process does other work
        │
        │ (meanwhile, OS performs I/O in background)
        │
        ▼
Process receives completion notification
Data is now in buffer

True asynchronous I/O is powerful but complex. Linux's io_uring (2019+) finally provides high-performance async I/O, while Windows has had I/O Completion Ports (IOCP) since NT.

I/O Model Comparison
Model	Blocking?	Scalability	Complexity	Use Case
Blocking	Yes	Poor (thread per connection)	Simple	Scripts, simple apps
Non-Blocking	No	Better (requires polling)	Medium	Games, real-time apps
Multiplexing	Yes (on set)	Excellent	Medium-High	Web servers, databases
Asynchronous	No	Excellent	High	High-performance servers

io_uring: The Future of Linux I/O

Linux's io_uring (introduced in kernel 5.1) provides true asynchronous I/O with minimal system call overhead. It uses shared memory ring buffers between user space and kernel, avoiding context switches for I/O submission and completion. High-performance databases and web servers are rapidly adopting io_uring.

Device Drivers

Device drivers are the critical bridge between the OS kernel and physical hardware. They translate generic I/O requests into device-specific commands and handle the peculiarities of each device.

Why drivers exist:

Consider an SSD from Samsung and one from Intel. Both store data, but they have different:

Controller interfaces (NVMe commands, SATA protocols)
Performance characteristics (queue depths, latencies)
Features (TRIM support, encryption, power management)
Failure modes and error handling

A driver encapsulates this device-specific knowledge, presenting a uniform interface to the kernel.

linux_driver_structure.c
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/**
 * Simplified Linux character device driver structure
 * Real drivers are more complex but follow this pattern
 */
 
#include <linux/module.h>
#include <linux/fs.h>
#include <linux/cdev.h>
#include <linux/uaccess.h>
 
#define DEVICE_NAME "mydevice"
 
static int major_number;
static char device_buffer[1024];
static int buffer_size = 0;
 
/* Called when user opens the device file */
static int device_open(struct inode *inode, struct file *file) {
    printk(KERN_INFO "mydevice: opened\n");
    /* Initialize device, acquire resources */
    return 0;
}
 
/* Called when user closes the device file */
static int device_release(struct inode *inode, struct file *file) {
    printk(KERN_INFO "mydevice: closed\n");
    /* Release resources, power down if needed */
    return 0;
}
 
/* Called when user reads from device */
static ssize_t device_read(struct file *file, char __user *buf,
                           size_t count, loff_t *offset) {
    int bytes_to_read = min(count, (size_t)(buffer_size - *offset));
    
    if (bytes_to_read <= 0) return 0;  /* EOF */
    
    /* Copy data from kernel buffer to user space */
    if (copy_to_user(buf, device_buffer + *offset, bytes_to_read)) {
        return -EFAULT;
    }
    
    *offset += bytes_to_read;
    return bytes_to_read;
}
 
/* Called when user writes to device */
static ssize_t device_write(struct file *file, const char __user *buf,
                            size_t count, loff_t *offset) {
    int bytes_to_write = min(count, sizeof(device_buffer) - 1);
    
    /* Copy data from user space to kernel buffer */
    if (copy_from_user(device_buffer, buf, bytes_to_write)) {
        return -EFAULT;
    }
    
    buffer_size = bytes_to_write;
    device_buffer[buffer_size] = '\0';
    
    return bytes_to_write;
}
 
/* File operations structure - maps syscalls to driver functions */
static struct file_operations fops = {
    .owner   = THIS_MODULE,
    .open    = device_open,
    .release = device_release,
    .read    = device_read,
    .write   = device_write,
};
 
/* Module initialization - called when driver loads */
static int __init mydevice_init(void) {
    major_number = register_chrdev(0, DEVICE_NAME, &fops);
    if (major_number < 0) {
        printk(KERN_ALERT "Failed to register device\n");
        return major_number;
    }
    printk(KERN_INFO "mydevice: registered with major number %d\n", major_number);
    return 0;
}
 
/* Module cleanup - called when driver unloads */
static void __exit mydevice_exit(void) {
    unregister_chrdev(major_number, DEVICE_NAME);
    printk(KERN_INFO "mydevice: unregistered\n");
}
 
module_init(mydevice_init);
module_exit(mydevice_exit);
MODULE_LICENSE("GPL");

Driver architecture patterns:

Monolithic drivers: Complete driver code runs in kernel space. High performance but kernel crashes if driver fails. Traditional Linux/Windows model.

Microkernel drivers: Drivers run in user space, communicating with minimal kernel code via message passing. More stable (driver crash doesn't kill kernel) but higher overhead. Used in QNX, MINIX, experimental systems.

User-space drivers (FUSE, UIO, VFIO): Framework allowing drivers in user space for specific device types. File systems (FUSE), network functions (DPDK), virtualization (VFIO).

Driver model comparison:

Traditional (in-kernel):          User-space driver:

┌─────────────────────┐           ┌─────────────────────┐
│   User Application  │           │   User Application  │
└──────────┬──────────┘           └──────────┬──────────┘
           │                                  │
═══════════│══════════════          ═══════════│══════════════
  Kernel   │                          Kernel   │
           ▼                                  ▼
┌─────────────────────┐           ┌─────────────────────┐
│    Device Driver    │           │   Kernel Stub       │
└──────────┬──────────┘           └──────────┬──────────┘
           │                                  │ (IPC)
           ▼                        ═══════════│══════════════
┌─────────────────────┐             User Space │
│    Hardware         │                        ▼
└─────────────────────┘           ┌─────────────────────┐
                                  │  User-Space Driver  │
                                  └──────────┬──────────┘
                                             ▼
                                  ┌─────────────────────┐
                                  │    Hardware         │
                                  └─────────────────────┘

Drivers and System Stability

Device drivers are a leading cause of OS crashes. They run in kernel mode with full hardware access, yet are often written by hardware vendors with varying quality standards. Modern systems implement driver signing, sandboxing, and privilege separation to mitigate risks. Faulty drivers can corrupt memory, hang the system, or create security vulnerabilities.

Buffering and Caching

I/O devices operate at vastly different speeds than CPUs and memory. A modern CPU can execute billions of instructions per second, while an HDD seek takes milliseconds—a difference of millions to one. Buffering and caching are essential strategies to bridge this speed gap.

Buffering temporarily holds data during transfer between components operating at different speeds or with different data-transfer sizes:

Single buffering: One buffer fills while previous data is processed. Simple but can cause blocking.
Double buffering: Two buffers alternate—one fills while the other is processed. Enables continuous operation.
Circular buffering: Ring of buffers for continuous streaming. Used in audio/video, network packets.

Double Buffering Example:

    Time 1:                     Time 2:
    ┌────────────────────┐      ┌────────────────────┐
    │ Buffer A: FILLING  │  →   │ Buffer A: DRAINING │
    │ from device        │      │ to application     │
    ├────────────────────┤      ├────────────────────┤
    │ Buffer B: DRAINING │  →   │ Buffer B: FILLING  │
    │ to application     │      │ from device        │
    └────────────────────┘      └────────────────────┘

Caching stores copies of frequently accessed data in faster storage to reduce repeated I/O:

Page cache (Linux) / System cache (Windows): The OS caches file data in RAM. Repeated reads come from memory instead of disk. Write-back caching delays writes to disk, batching them for efficiency.

$ free -h
              total        used        free      shared  buff/cache   available
Mem:           31Gi       8.2Gi       2.1Gi       1.2Gi        21Gi        21Gi

# 21 GB used for buffers and cache!
# This is memory "available" for applications if needed,
# but currently holding cached file data for fast access

The cache hierarchy:

┌─────────────────────────────────────────────────────────────────┐
│ Application Buffers        │ Speed: Immediate    │ Size: MB     │
├─────────────────────────────────────────────────────────────────┤
│ OS Page Cache              │ Speed: ~ns          │ Size: GB     │
├─────────────────────────────────────────────────────────────────┤
│ Disk Controller Cache      │ Speed: ~μs          │ Size: MB     │
├─────────────────────────────────────────────────────────────────┤
│ SSD/HDD Cache              │ Speed: ~μs-ms       │ Size: MBs    │
├─────────────────────────────────────────────────────────────────┤
│ Persistent Storage         │ Speed: ~ms          │ Size: TB     │
└─────────────────────────────────────────────────────────────────┘

cache_effects_demo.sh
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# Demonstrating the impact of the page cache
 
# Drop all caches (requires root)
$ sudo sh -c 'echo 3 > /proc/sys/vm/drop_caches'
 
# First read - from disk (slow)
$ time cat large_file.bin > /dev/null
real    0m2.341s    # Reading from SSD
user    0m0.012s
sys     0m0.456s
 
# Second read - from page cache (fast!)
$ time cat large_file.bin > /dev/null
real    0m0.089s    # Reading from RAM cache - 26x faster!
user    0m0.008s
sys     0m0.081s
 
# Viewing cache statistics
$ vmstat 1
procs -----------memory---------- ---swap-- -----io---- -system-- ------cpu-----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 1  0      0 234567  12345 789012    0    0   150    50  100  200  5  2 92  1  0
                            ^^^^^^
                            Cache in KB
 
# Sync cached writes to disk
$ sync
 
# Force immediate write-through (bypass cache)
$ dd if=data.bin of=output.bin oflag=direct conv=fdatasync

Buffering Strategies by Use Case

•No buffering (raw I/O) — Direct device access for real-time requirements. Database systems often bypass OS cache (O_DIRECT) to control their own caching.
•Line buffering — Buffer until newline; used for terminals. Commands typed character-by-character but processed line-by-line.
•Block buffering — Standard for files. Data buffered in fixed-size blocks (often 4KB–64KB) for efficient disk I/O.
•Full buffering — Buffer until full or explicit flush. Most file I/O uses this. Need explicit flush for durability guarantees.
•Write-back caching — Writes go to cache immediately, flushed to disk later. Fast but data loss risk on crash.
•Write-through caching — Writes go to cache AND disk. Slower but crash-safe. Database critical paths use this.

Durability vs. Performance Trade-off

Caching improves performance but introduces durability risks. Data in buffers can be lost if the system crashes before flushing to disk. For critical data: • Use fsync() to force data to disk • Open files with O_SYNC or O_DSYNC for synchronous writes • Databases use write-ahead logging (WAL) to ensure consistency

DMA and Interrupt-Driven I/O

Efficient I/O requires minimizing CPU involvement in data transfer. Two key mechanisms enable this: Direct Memory Access (DMA) and interrupt-driven I/O.

Evolution of I/O methods:

1. Programmed I/O (PIO): The CPU manually transfers each byte between device and memory. For each byte:

Read byte from device port
Write byte to memory
Repeat

This approach monopolizes the CPU during transfers—completely unacceptable for modern throughput.

2. Interrupt-Driven I/O: Device sends interrupt when data is ready. CPU then transfers data, but can do other work between interrupts. Better than PIO but still involves CPU in every transfer.

3. Direct Memory Access (DMA): A dedicated DMA controller handles data transfer. CPU initiates the transfer and is interrupted when complete. CPU is free during the entire transfer.

DMA in detail:

The DMA controller is specialized hardware that can access system memory independently of the CPU:

DMA Transfer Process:

┌─────────────────────────────────────────────────────────────────────┐
│ 1. CPU programs DMA controller with:                                │
│    - Source address (device buffer or memory)                       │
│    - Destination address (memory or device buffer)                  │
│    - Transfer size (number of bytes)                                │
│    - Direction (device→memory or memory→device)                     │
│    - Transfer mode (burst, cycle stealing, block)                   │
├─────────────────────────────────────────────────────────────────────┤
│ 2. DMA controller takes control of bus                              │
│    - CPU continues other work (or is briefly paused for bus access) │
├─────────────────────────────────────────────────────────────────────┤
│ 3. DMA controller transfers data directly:                          │
│                                                                      │
│    ┌──────────┐          ┌───────────────┐          ┌────────────┐  │
│    │  Device  │ ──────── │ DMA Controller│ ──────── │   Memory   │  │
│    │  Buffer  │   Data   │               │   Data   │            │  │
│    └──────────┘          └───────────────┘          └────────────┘  │
│                                                                      │
│    CPU is NOT involved in the actual data transfer                   │
├─────────────────────────────────────────────────────────────────────┤
│ 4. DMA controller signals completion via interrupt                   │
│    - CPU processes completion, schedules waiting process             │
└─────────────────────────────────────────────────────────────────────┘

I/O Method Comparison
Method	CPU Usage	Throughput	Use Case
Programmed I/O	100% during transfer	Low	Simple microcontrollers
Interrupt-Driven	Per-byte/packet interrupt	Medium	Low-volume devices
DMA	Setup + completion only	High	Disk, network, video
RDMA	Near-zero	Very High	High-performance computing

Interrupt handling:

When a device needs attention—data ready, transfer complete, error occurred—it triggers a hardware interrupt:

Device signals interrupt via interrupt line to CPU
CPU suspends current execution and saves state
Interrupt controller identifies source, determines priority
CPU looks up handler in Interrupt Vector Table (IVT)
Interrupt handler executes (the driver's ISR - Interrupt Service Routine)
Handler signals completion, CPU resumes previous execution

Performance considerations:

Interrupts have overhead (context save/restore, handler execution). At very high data rates (10 Gbps+ networking), interrupt storms can overwhelm the CPU. Modern strategies:

Interrupt coalescing: Delay interrupt until multiple events accumulate
Polling: At high loads, poll device instead of waiting for interrupts
Adaptive switching: Dynamically switch between interrupt and polling modes

Zero-Copy I/O

Traditional I/O copies data multiple times: device→kernel buffer→user buffer→kernel buffer→device. Zero-copy techniques (sendfile(), splice(), memory-mapped I/O) eliminate intermediate copies. For network servers, this can double throughput by avoiding the CPU touching every byte of data being transferred.

Error Handling in I/O Operations

I/O operations interact with the physical world, where failures are common and unpredictable. Robust error handling distinguishes reliable systems from fragile ones.

Categories of I/O errors:

Transient errors: Temporary conditions that may succeed on retry
- Network timeouts, busy devices, temporary resource exhaustion
- Strategy: Retry with exponential backoff
Permanent errors: Conditions that won't improve
- File not found, permission denied, hardware failure
- Strategy: Report to user/caller, possibly graceful degradation
Partial operations: Operation completed partially
- Short reads/writes (less data than requested)
- Strategy: Loop until complete or handle partial results
Critical errors: System-level failures
- Disk full, out of file descriptors, kernel resource exhaustion
- Strategy: Escalate, log, possibly terminate with cleanup

robust_io.c
C
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#include <errno.h>
#include <unistd.h>
#include <stdio.h>
 
/**
 * Robust read that handles short reads and interrupts
 * ALWAYS use this pattern for real I/O code
 */
ssize_t read_all(int fd, void *buf, size_t count) {
    size_t total_read = 0;
    char *ptr = (char *)buf;
    
    while (total_read < count) {
        ssize_t n = read(fd, ptr + total_read, count - total_read);
        
        if (n > 0) {
            /* Partial read - keep going */
            total_read += n;
        }
        else if (n == 0) {
            /* EOF reached */
            break;
        }
        else {  /* n < 0, error */
            if (errno == EINTR) {
                /* Interrupted by signal - retry */
                continue;
            }
            else if (errno == EAGAIN || errno == EWOULDBLOCK) {
                /* Non-blocking I/O, no data available */
                /* Could wait and retry, or return what we have */
                break;
            }
            else {
                /* Actual error - report it */
                perror("read failed");
                return -1;
            }
        }
    }
    
    return total_read;
}
 
/**
 * Robust write that handles short writes
 */
ssize_t write_all(int fd, const void *buf, size_t count) {
    size_t total_written = 0;
    const char *ptr = (const char *)buf;
    
    while (total_written < count) {
        ssize_t n = write(fd, ptr + total_written, count - total_written);
        
        if (n >= 0) {
            total_written += n;
        }
        else {
            if (errno == EINTR) {
                continue;  /* Retry on interrupt */
            }
            else if (errno == EAGAIN || errno == EWOULDBLOCK) {
                /* Non-blocking - would block try again later */
                usleep(1000);  /* Brief delay before retry */
                continue;
            }
            else {
                /* Real error */
                perror("write failed");
                return -1;
            }
        }
    }
    
    return total_written;
}
 
/* Common errno values for I/O:
 * ENOENT  - File not found
 * EACCES  - Permission denied
 * EEXIST  - File already exists
 * ENOSPC  - No space left on device
 * EMFILE  - Too many open files (process limit)
 * ENFILE  - Too many open files (system limit)
 * EIO     - I/O error (hardware failure)
 * EINTR   - Interrupted by signal
 * EAGAIN  - Try again (non-blocking would block)
 */

I/O Error Handling Best Practices

•Always check return values — Never ignore the result of read(), write(), or any I/O operation. Silent failures corrupt data and mask bugs.
•Handle short reads/writes — A read of 1000 bytes may return 500. Loop until complete or handle partial data correctly.
•Retry transient errors — EINTR (signal interrupt), EAGAIN (would block) are not failures—they're requests to retry.
•Use appropriate timeouts — Don't hang forever on network I/O. Implement timeouts at application or socket level.
•Log and report meaningfully — Include strerror(errno), file paths, operation context. "I/O error" helps no one debug.
•Clean up on failure — If an operation fails midway, release resources, close handles, and leave system in known state.

POSIX Read/Write Semantics

POSIX explicitly allows read() and write() to return fewer bytes than requested—this is not an error. The only guarantee is for pipes/FIFOs under PIPE_BUF (typically 4KB): writes of PIPE_BUF or fewer bytes are atomic. For everything else, always loop until complete.

Summary: I/O Operations

We've explored the sophisticated I/O services that operating systems provide—the essential bridge between software and the physical world. Let's consolidate the key insights:

Key Takeaways

•I/O abstraction unifies diverse devices — The file abstraction (Unix) provides uniform interface for files, devices, sockets, and more through open/read/write/close.
•Multiple I/O models serve different needs — Blocking for simplicity, non-blocking for control, multiplexing for scale, asynchronous for performance.
•Device drivers bridge software and hardware — They encapsulate device-specific complexity, presenting uniform interfaces to the kernel.
•Buffering and caching are essential — They bridge the speed gap between CPU/memory and I/O devices, dramatically improving performance.
•DMA and interrupts minimize CPU overhead — The CPU initiates transfers but dedicated hardware performs data movement.
•Robust error handling is non-negotiable — I/O fails in many ways; production code must handle short reads, interrupts, and transient errors.
•Trade-offs pervade I/O design — Performance vs. durability, simplicity vs. scalability, CPU usage vs. latency.

What's next:

With I/O fundamentals covered, we'll explore File System Manipulation in depth—how the OS organizes persistent data, navigates directory hierarchies, manages permissions, and provides the file abstraction that applications depend on.

Page Complete

You now understand how operating systems handle input/output operations. From device abstraction and I/O models through drivers, buffering, DMA, and error handling—these services enable all interaction between programs and the external world.