Semaphore

A semaphore is a synchronization object that manages access to a finite number of resources. Windows provides both unnamed and named semaphores for interprocess synchronization.

Overview

Syntax

Remarks

Examples

Key Points

Count represents the number of available resources.
When a thread calls WaitForSingleObject, the count is decremented.
If the count is zero, the thread blocks until a release occurs.
Use ReleaseSemaphore to increase the count and unblock waiting threads.

When to Use

Semaphores are ideal for limiting concurrency, such as controlling the number of threads that can access a pool of database connections.

Function Syntax

#include <windows.h>

HANDLE CreateSemaphore(
    LPSECURITY_ATTRIBUTES lpSemaphoreAttributes,
    LONG                  lInitialCount,
    LONG                  lMaximumCount,
    LPCSTR                lpName
);

BOOL ReleaseSemaphore(
    HANDLE hSemaphore,
    LONG   lReleaseCount,
    LPLONG lpPreviousCount
);

Remarks

Semaphores can be named, allowing multiple processes to share the same synchronization object. The name is case‑insensitive and must be unique within the system.

Parameter	Description
lpSemaphoreAttributes	Optional security descriptor.
lInitialCount	Initial number of resources.
lMaximumCount	Maximum count allowed.
lpName	Optional object name.

Example: Controlling Access to a Resource Pool

#include <windows.h>
#include <stdio.h>

#define POOL_SIZE 3

HANDLE g_hSem;

DWORD WINAPI Worker(LPVOID lpParam) {
    DWORD id = GetCurrentThreadId();
    printf("Thread %lu waiting for resource...\\n", id);
    WaitForSingleObject(g_hSem, INFINITE);
    printf("Thread %lu acquired resource.\\n", id);
    Sleep(2000); // simulate work
    printf("Thread %lu releasing resource.\\n", id);
    ReleaseSemaphore(g_hSem, 1, NULL);
    return 0;
}

int main(void) {
    g_hSem = CreateSemaphore(NULL, POOL_SIZE, POOL_SIZE, NULL);
    if (!g_hSem) {
        printf("CreateSemaphore failed (%lu)\\n", GetLastError());
        return 1;
    }

    HANDLE hThreads[5];
    for (int i = 0; i < 5; ++i) {
        hThreads[i] = CreateThread(NULL, 0, Worker, NULL, 0, NULL);
    }

    WaitForMultipleObjects(5, hThreads, TRUE, INFINITE);
    CloseHandle(g_hSem);
    return 0;
}