Windows Api - Learning Module

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Comparison with POSIX

Two Philosophies of System Programming

The Windows API and the POSIX interface represent two fundamentally different approaches to operating system design and application programming. Understanding these differences is essential for:

Cross-platform developers who need to write code that runs on both Windows and Unix-like systems
System programmers who want to understand the tradeoffs behind each design
Architects evaluating platform choices for new projects
Anyone seeking to deepen their understanding of operating systems

POSIX (Portable Operating System Interface) is a family of standards specified by IEEE for maintaining compatibility between operating systems. It defines the API, shell, and utilities for Unix-like systems. The Windows API, in contrast, evolved specifically for Microsoft's operating system family with different priorities and constraints.

This page provides a comprehensive comparison of these two paradigms, examining their design philosophies, concrete API differences, and practical implications for software development.

What You Will Learn

By the end of this page, you will understand the fundamental design philosophy differences between Windows and POSIX, specific API differences across major functional areas, portability strategies for cross-platform development, and when each approach has advantages.

Design Philosophy Differences

Before examining specific APIs, understanding the underlying design philosophies helps explain why Windows and POSIX differ so fundamentally.

POSIX Philosophy: "Everything is a File"

The Unix philosophy, embodied in POSIX, treats nearly everything as a file:

Regular files, directories
Devices (/dev/sda, /dev/tty)
Inter-process communication (pipes, sockets)
Process information (/proc filesystem)
Kernel parameters (/sys filesystem)

This unified abstraction means:

The same read(), write(), close() calls work on diverse objects
Tools can be composed via pipes
File descriptors are simple integers
Security is based on file permissions (owner, group, world)

Windows Philosophy: "Everything is an Object"

Windows takes an object-oriented approach:

Kernel objects with type-specific behaviors
Handles as opaque references to objects
Rich metadata on every object (security descriptors, names, attributes)
Type-specific APIs for different resource categories

This means:

Objects have methods appropriate to their type
Security is based on discretionary ACLs (access control lists)
The kernel tracks all object references
Strong typing prevents misuse

Fundamental Design Differences
Aspect	POSIX/Unix	Windows
Core abstraction	File descriptor (integer)	Handle (opaque pointer)
Naming philosophy	Hierarchical namespace (/path/to/thing)	Multiple namespaces (\Device\, \Registry\, etc.)
Configuration	Text files	Registry database
Text encoding	UTF-8 (historically ASCII)	UTF-16 (historically codepages)
Line endings	LF (\n)	CRLF (\r\n)
Path separator	Forward slash (/)	Backslash ()
Case sensitivity	Case-sensitive	Case-insensitive (preserving)
Security model	Owner/Group/Other + capabilities	Discretionary ACLs + privileges
Process model	fork() + exec()	CreateProcess()
Threading model	pthreads	Windows threads
Error reporting	errno (integer)	GetLastError() + HRESULT

Neither is "Better"

Each approach has tradeoffs. POSIX's simplicity enables powerful composition but can obscure type information. Windows' rich object model provides stronger typing and security but with more complexity. Production systems on both platforms power critical infrastructure worldwide.

Process Management Comparison

Process creation and management is one of the areas where Windows and POSIX differ most dramatically.

POSIX: fork() + exec()

In POSIX, creating a new process running a different program requires two steps:

posix_process.c
C (POSIX)
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#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
#include <stdlib.h>
 
int main() {
    pid_t pid = fork();  // Create new process - COPIES parent
    
    if (pid < 0) {
        // Fork failed
        perror("fork failed");
        return 1;
    }
    else if (pid == 0) {
        // Child process - this code runs in the COPY
        // Replace process image with new program
        char *const argv[] = {"ls", "-la", NULL};
        char *const envp[] = {NULL};
        
        execve("/bin/ls", argv, envp);  // Replace this process image
        
        // execve only returns on error
        perror("execve failed");
        _exit(1);  // Use _exit in child after fork
    }
    else {
        // Parent process - pid contains child's PID
        int status;
        waitpid(pid, &status, 0);  // Wait for child to finish
        
        if (WIFEXITED(status)) {
            printf("Child exited with status %d\n", WEXITSTATUS(status));
        }
    }
    
    return 0;
}
 
/*
 * fork() characteristics:
 * - Creates COMPLETE copy of parent process
 * - Copy-on-write makes this efficient
 * - Child inherits open file descriptors
 * - Child can modify itself before exec()
 * - Two separate operations: fork (copy) + exec (replace)
 */

Windows: CreateProcess()

Windows combines process creation and program loading into a single call:

windows_process.c
C (Windows)
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#include <windows.h>
#include <stdio.h>
 
int main() {
    STARTUPINFOW si = { sizeof(si) };
    PROCESS_INFORMATION pi;
    
    // Create new process running specified program
    BOOL success = CreateProcessW(
        L"C:\\Windows\\System32\\cmd.exe",  // Executable path
        L"cmd /c dir",           // Command line (mutable!)
        NULL,                    // Process security attributes
        NULL,                    // Thread security attributes
        FALSE,                   // Don't inherit handles
        0,                       // Creation flags
        NULL,                    // Use parent's environment
        NULL,                    // Use parent's current directory
        &si,                     // Startup info
        &pi                      // Receives process information
    );
    
    if (!success) {
        printf("CreateProcess failed: %lu\n", GetLastError());
        return 1;
    }
    
    // Wait for child to finish
    WaitForSingleObject(pi.hProcess, INFINITE);
    
    // Get exit code
    DWORD exitCode;
    GetExitCodeProcess(pi.hProcess, &exitCode);
    printf("Child exited with code %lu\n", exitCode);
    
    // MUST close both handles
    CloseHandle(pi.hProcess);
    CloseHandle(pi.hThread);
    
    return 0;
}
 
/*
 * CreateProcess() characteristics:
 * - Single call creates new process with new program
 * - Parent and child are separate from the start
 * - Explicit handle inheritance control
 * - Returns handles to process AND thread
 * - More parameters but more explicit control
 */

POSIX fork+exec Advantages

•Child can modify environment before exec
•File descriptors inherited by default
•Simple chaining of operations
•fork() alone useful for multiprocessing
•More control between fork and exec

Windows CreateProcess Advantages

•Single atomic operation
•Explicit handle inheritance (security)
•Process and thread handles returned
•More control over initial state
•Clear separation of parent/child

Cross-Platform Strategy

Many cross-platform libraries (like libuv used by Node.js) abstract these differences. They implement spawn/exec semantics that map efficiently to both fork+exec on POSIX and CreateProcess on Windows, hiding the platform differences from application code.

File I/O Comparison

File operations are fundamental to both APIs, but they differ in important ways.

File Operation Equivalents
Operation	POSIX	Windows
Open file	open()	CreateFile()
Read data	read()	ReadFile()
Write data	write()	WriteFile()
Close file	close()	CloseHandle()
Seek	lseek()	SetFilePointer() / SetFilePointerEx()
Get file size	fstat()	GetFileSize() / GetFileSizeEx()
Delete file	unlink()	DeleteFile()
Rename file	rename()	MoveFile() / MoveFileEx()
Create directory	mkdir()	CreateDirectory()
Remove directory	rmdir()	RemoveDirectory()
List directory	opendir()/readdir()	FindFirstFile()/FindNextFile()
File locking	flock() / fcntl()	LockFile() / LockFileEx()
Memory map	mmap()	CreateFileMapping() + MapViewOfFile()

file_io_comparison.c
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// ===== POSIX File I/O =====
#ifdef __unix__
#include <fcntl.h>
#include <unistd.h>
#include <sys/stat.h>
 
void posix_file_example() {
    // Open or create file
    int fd = open("/tmp/test.txt", 
                  O_RDWR | O_CREAT | O_TRUNC,  // Flags
                  0644);  // Mode (permissions)
    
    if (fd < 0) {
        perror("open failed");
        return;
    }
    
    // Write data
    const char *data = "Hello, POSIX!";
    ssize_t written = write(fd, data, strlen(data));
    
    // Seek to beginning
    lseek(fd, 0, SEEK_SET);
    
    // Read data back
    char buffer[256];
    ssize_t bytes_read = read(fd, buffer, sizeof(buffer) - 1);
    buffer[bytes_read] = '\0';
    
    // Close
    close(fd);
}
#endif
 
// ===== Windows File I/O =====
#ifdef _WIN32
#include <windows.h>
 
void windows_file_example() {
    // Open or create file
    HANDLE hFile = CreateFileW(
        L"C:\\Temp\\test.txt",
        GENERIC_READ | GENERIC_WRITE,  // Access
        0,                              // No sharing
        NULL,                           // Default security
        CREATE_ALWAYS,                  // Create/truncate
        FILE_ATTRIBUTE_NORMAL,          // Normal file
        NULL                            // No template
    );
    
    if (hFile == INVALID_HANDLE_VALUE) {
        printf("CreateFile failed: %lu\n", GetLastError());
        return;
    }
    
    // Write data
    const char *data = "Hello, Windows!";
    DWORD written;
    WriteFile(hFile, data, (DWORD)strlen(data), &written, NULL);
    
    // Seek to beginning
    SetFilePointer(hFile, 0, NULL, FILE_BEGIN);
    
    // Read data back
    char buffer[256];
    DWORD bytes_read;
    ReadFile(hFile, buffer, sizeof(buffer) - 1, &bytes_read, NULL);
    buffer[bytes_read] = '\0';
    
    // Close
    CloseHandle(hFile);
}
#endif

Key Differences:

File Descriptors vs Handles: POSIX uses small integers (0, 1, 2, ...) while Windows uses opaque handles
Path Separators: POSIX uses /, Windows uses \ (but also accepts / in many contexts)
Case Sensitivity: Linux filesystems are case-sensitive by default; Windows NTFS is case-insensitive
File Locking: POSIX advisory locks are optional; Windows mandatory locks are enforced by the filesystem
Delete Behavior: POSIX allows deleting open files (unlink removes name, file deleted when closed); Windows typically cannot delete open files without special flags
Share Modes: Windows has explicit share modes (read, write, delete sharing); POSIX has no equivalent concept

Text File Portability

Text files create subtle portability issues: line endings (LF vs CRLF), character encoding (UTF-8 vs UTF-16), and path handling all differ. Use binary mode for data files and explicit encoding specifications. Libraries like ICU for Unicode handling help ensure consistent behavior.

Threading and Synchronization

Both platforms provide comprehensive threading support, but with different APIs and subtle behavioral differences.

Threading API Comparison
Concept	POSIX (pthreads)	Windows
Create thread	pthread_create()	CreateThread() / _beginthreadex()
Wait for thread	pthread_join()	WaitForSingleObject()
Exit thread	pthread_exit()	ExitThread() / return
Thread ID	pthread_t	HANDLE + DWORD thread ID
Mutex	pthread_mutex_t	CRITICAL_SECTION / Mutex object
Condition variable	pthread_cond_t	CONDITION_VARIABLE (Vista+)
Read-write lock	pthread_rwlock_t	SRWLOCK (Vista+)
Semaphore	sem_t	Semaphore object
Thread-local storage	pthread_key_t	__declspec(thread) / TLS API
One-time initialization	pthread_once()	InitOnceExecuteOnce()

threading_comparison.c
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// ===== POSIX Threading =====
#ifdef __unix__
#include <pthread.h>
#include <stdio.h>
 
pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
int shared_counter = 0;
 
void* posix_thread_func(void* arg) {
    int id = *(int*)arg;
    
    for (int i = 0; i < 1000; i++) {
        pthread_mutex_lock(&mutex);
        shared_counter++;
        pthread_mutex_unlock(&mutex);
    }
    
    printf("Thread %d: done\n", id);
    return NULL;
}
 
void posix_threading_example() {
    pthread_t threads[4];
    int thread_ids[4] = {0, 1, 2, 3};
    
    // Create threads
    for (int i = 0; i < 4; i++) {
        pthread_create(&threads[i], NULL, posix_thread_func, &thread_ids[i]);
    }
    
    // Wait for all threads
    for (int i = 0; i < 4; i++) {
        pthread_join(threads[i], NULL);
    }
    
    printf("Final counter: %d\n", shared_counter);
}
#endif
 
// ===== Windows Threading =====
#ifdef _WIN32
#include <windows.h>
#include <stdio.h>
 
CRITICAL_SECTION cs;
volatile LONG shared_counter = 0;
 
DWORD WINAPI windows_thread_func(LPVOID arg) {
    int id = *(int*)arg;
    
    for (int i = 0; i < 1000; i++) {
        EnterCriticalSection(&cs);
        shared_counter++;
        LeaveCriticalSection(&cs);
        
        // Or use interlocked operations:
        // InterlockedIncrement(&shared_counter);
    }
    
    printf("Thread %d: done\n", id);
    return 0;
}
 
void windows_threading_example() {
    HANDLE threads[4];
    int thread_ids[4] = {0, 1, 2, 3};
    
    InitializeCriticalSection(&cs);
    
    // Create threads
    for (int i = 0; i < 4; i++) {
        threads[i] = CreateThread(
            NULL,                   // Default security
            0,                      // Default stack size
            windows_thread_func,    // Thread function
            &thread_ids[i],         // Thread argument
            0,                      // Run immediately
            NULL                    // Don't need thread ID
        );
    }
    
    // Wait for all threads
    WaitForMultipleObjects(4, threads, TRUE, INFINITE);
    
    // Close handles
    for (int i = 0; i < 4; i++) {
        CloseHandle(threads[i]);
    }
    
    DeleteCriticalSection(&cs);
    
    printf("Final counter: %ld\n", shared_counter);
}
#endif

Windows Synchronization Objects

Windows has two categories of synchronization: (1) User-mode primitives like CRITICAL_SECTION and SRWLOCK that never enter kernel mode for uncontended cases (fast), and (2) Kernel objects like Mutex and Semaphore that can be shared across processes but require kernel transitions. Choose based on whether cross-process sharing is needed.

Networking Comparison

Network programming is one area where Windows and POSIX are relatively similar, thanks to Windows' adoption of the Berkeley sockets API. However, important differences remain.

Socket API Comparison
Operation	POSIX	Windows (Winsock)
Header	<sys/socket.h>, <netinet/in.h>	<winsock2.h>, <ws2tcpip.h>
Initialization	None needed	WSAStartup() required
Cleanup	None needed	WSACleanup()
Socket type	int (file descriptor)	SOCKET (unsigned int)
Invalid socket	-1	INVALID_SOCKET
Close socket	close()	closesocket()
Error checking	errno	WSAGetLastError()
Non-blocking	fcntl()	ioctlsocket()
Async I/O	select(), poll(), epoll	select(), WSAAsyncSelect(), IOCP

socket_comparison.c
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// Cross-platform socket abstraction
 
#ifdef _WIN32
    #include <winsock2.h>
    #include <ws2tcpip.h>
    #pragma comment(lib, "ws2_32.lib")
    
    typedef SOCKET socket_t;
    #define SOCKET_ERROR_VALUE INVALID_SOCKET
    #define CLOSE_SOCKET(s) closesocket(s)
    #define GET_SOCKET_ERROR() WSAGetLastError()
    
    int socket_init() {
        WSADATA wsa;
        return WSAStartup(MAKEWORD(2, 2), &wsa);
    }
    void socket_cleanup() {
        WSACleanup();
    }
#else
    #include <sys/socket.h>
    #include <netinet/in.h>
    #include <arpa/inet.h>
    #include <unistd.h>
    #include <errno.h>
    
    typedef int socket_t;
    #define SOCKET_ERROR_VALUE (-1)
    #define CLOSE_SOCKET(s) close(s)
    #define GET_SOCKET_ERROR() errno
    
    int socket_init() { return 0; }  // No-op on POSIX
    void socket_cleanup() {}
#endif
 
// Now the same code works on both platforms
socket_t create_tcp_server(int port) {
    socket_t sock = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (sock == SOCKET_ERROR_VALUE) {
        return SOCKET_ERROR_VALUE;
    }
    
    // Enable address reuse
    int opt = 1;
    setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, (char*)&opt, sizeof(opt));
    
    struct sockaddr_in addr = {0};
    addr.sin_family = AF_INET;
    addr.sin_addr.s_addr = INADDR_ANY;
    addr.sin_port = htons(port);
    
    if (bind(sock, (struct sockaddr*)&addr, sizeof(addr)) < 0) {
        CLOSE_SOCKET(sock);
        return SOCKET_ERROR_VALUE;
    }
    
    if (listen(sock, SOMAXCONN) < 0) {
        CLOSE_SOCKET(sock);
        return SOCKET_ERROR_VALUE;
    }
    
    return sock;
}

Async I/O: The Major Difference

The biggest difference in networking is asynchronous I/O. POSIX offers select(), poll(), and platform-specific mechanisms (epoll on Linux, kqueue on BSD/macOS). Windows offers select() (limited to 64 sockets), but the scalable solution is I/O Completion Ports (IOCP), which uses a different programming model entirely.

Error Handling Comparison

Error handling is fundamentally different between POSIX and Windows, reflecting their different design philosophies.

POSIX Error Handling (errno)

#include <errno.h>
#include <string.h>

int fd = open("/noexist", O_RDONLY);
if (fd < 0) {
    // errno is set automatically
    printf("Error %d: %s\n", 
           errno, 
           strerror(errno));
    
    switch (errno) {
        case ENOENT:
            // File not found
            break;
        case EACCES:
            // Permission denied
            break;
    }
}

errno is thread-local in modern implementations
Standard error codes (EINVAL, ENOENT, etc.)
strerror() for human-readable messages
Some functions return error directly (pthreads)

Windows Error Handling

#include <windows.h>

HANDLE hFile = CreateFile(...);
if (hFile == INVALID_HANDLE_VALUE) {
    DWORD error = GetLastError();
    
    LPWSTR msg = NULL;
    FormatMessageW(
        FORMAT_MESSAGE_ALLOCATE_BUFFER |
        FORMAT_MESSAGE_FROM_SYSTEM,
        NULL, error, 0,
        (LPWSTR)&msg, 0, NULL);
    
    switch (error) {
        case ERROR_FILE_NOT_FOUND:
            break;
        case ERROR_ACCESS_DENIED:
            break;
    }
    LocalFree(msg);
}

GetLastError() is thread-local
Thousands of error codes
FormatMessage() for descriptions
Some APIs use HRESULT (COM)

Common Error Code Equivalents
Meaning	POSIX errno	Windows Error
Invalid argument	EINVAL (22)	ERROR_INVALID_PARAMETER (87)
File not found	ENOENT (2)	ERROR_FILE_NOT_FOUND (2)
Permission denied	EACCES (13)	ERROR_ACCESS_DENIED (5)
File exists	EEXIST (17)	ERROR_FILE_EXISTS (80)
Not enough memory	ENOMEM (12)	ERROR_NOT_ENOUGH_MEMORY (8)
Operation would block	EAGAIN/EWOULDBLOCK (11)	ERROR_IO_PENDING (997)
Broken pipe	EPIPE (32)	ERROR_BROKEN_PIPE (109)
Timed out	ETIMEDOUT (110)	WAIT_TIMEOUT (258)

Portability Strategies

For developers who need code to run on both Windows and POSIX systems, several strategies exist:

Cross-Platform Approaches

•Abstraction Libraries: Use libraries like libuv, Boost.Asio, or APR that provide unified APIs
•Conditional Compilation: Use #ifdef _WIN32 / #else / #endif to provide platform-specific implementations
•POSIX Layer on Windows: Use Cygwin, MSYS2, or Windows Subsystem for Linux (WSL)
•C Standard Library: Stick to standard C (<stdio.h>, <stdlib.h>, etc.) where possible
•Higher-Level Languages: Use Python, Java, Go, or Rust that abstract platform differences

portable_code.c
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// Example: Portable sleep function
 
#ifdef _WIN32
    #include <windows.h>
    #define sleep_ms(ms) Sleep(ms)
#else
    #include <unistd.h>
    #define sleep_ms(ms) usleep((ms) * 1000)
#endif
 
// Example: Portable path handling
#ifdef _WIN32
    #define PATH_SEPARATOR '\\'
    #define PATH_SEPARATOR_STR "\\"
#else
    #define PATH_SEPARATOR '/'
    #define PATH_SEPARATOR_STR "/"
#endif
 
// Example: Portable dynamic library loading
#ifdef _WIN32
    #include <windows.h>
    typedef HMODULE lib_handle;
    #define lib_open(path) LoadLibraryA(path)
    #define lib_symbol(h, name) GetProcAddress(h, name)
    #define lib_close(h) FreeLibrary(h)
#else
    #include <dlfcn.h>
    typedef void* lib_handle;
    #define lib_open(path) dlopen(path, RTLD_LAZY)
    #define lib_symbol(h, name) dlsym(h, name)
    #define lib_close(h) dlclose(h)
#endif
 
// Example: Portable high-resolution timer
#ifdef _WIN32
uint64_t get_time_ns() {
    static LARGE_INTEGER freq = {0};
    if (freq.QuadPart == 0) {
        QueryPerformanceFrequency(&freq);
    }
    LARGE_INTEGER counter;
    QueryPerformanceCounter(&counter);
    return (uint64_t)(counter.QuadPart * 1000000000LL / freq.QuadPart);
}
#else
#include <time.h>
uint64_t get_time_ns() {
    struct timespec ts;
    clock_gettime(CLOCK_MONOTONIC, &ts);
    return (uint64_t)ts.tv_sec * 1000000000ULL + ts.tv_nsec;
}
#endif

Modern Cross-Platform Development

For new projects, consider using Rust or Go, which have excellent cross-platform support built into their standard libraries. For C/C++ projects, CMake with vcpkg or Conan provides portable builds. Libraries like libuv (used by Node.js) provide battle-tested abstractions for async I/O, networking, and process management.

Summary: Windows API vs POSIX

The Windows API and POSIX represent two mature, successful approaches to system programming, each with distinct design philosophies and tradeoffs.

Key Takeaways

•Different philosophies: POSIX sees everything as files; Windows sees everything as objects
•Process creation: POSIX uses fork()+exec(); Windows uses CreateProcess()
•Error handling: POSIX uses errno; Windows uses GetLastError()/HRESULT
•Threading: pthreads vs Windows threads with similar concepts but different APIs
•Networking: Broadly similar (sockets) but async I/O differs significantly
•Text handling: UTF-8/LF vs UTF-16/CRLF affects file portability
•Case sensitivity: POSIX is case-sensitive; Windows is case-insensitive
•Portability strategies: Use abstraction libraries, conditional compilation, or higher-level languages

Module Complete:

With this comparison, we've completed our exploration of the Windows API module. You now understand:

The Win32/Win64 API architecture and conventions
How Windows system calls work internally
The handle-based object model
Registry operations for configuration management
How Windows differs from POSIX systems

This knowledge provides a solid foundation for Windows system programming and helps you make informed decisions about cross-platform development.

Page Complete

You now have a comprehensive understanding of how the Windows API compares to POSIX. This knowledge enables you to write portable code, understand platform-specific behaviors, and make informed choices about which patterns to use in different contexts.