Named Mutex

A C program that creates a named mutex and uses GetLastError to detect whether another process already holds it.

Published Mar 31, 1995

The standard single-instance / cross-process lock pattern. CreateMutex with a name never actually fails when the name is already taken - it hands you a handle to the existing mutex and sets GetLastError to ERROR_ALREADY_EXISTS, which is exactly what this snippet checks to tell "I created it" from "someone else already has it." That two-state result is the whole trick behind "only one instance can run" dialogs and simple inter-process critical sections.

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

int main()
{
    LONG LastError;
    HANDLE hMutex;

    hMutex = CreateMutex(0, FALSE, "MyMutex");
    LastError = GetLastError();

    if(hMutex != 0)
    {
        if(LastError == ERROR_SUCCESS)
           printf("%s\n", "Mutex creation succeeded.");
        if(LastError == ERROR_ALREADY_EXISTS)
            printf("%s\n", "Mutex already exists.");
    }
    else printf("%s\n", "Mutex creation failed.");

    ReleaseMutex(hMutex);
    CloseHandle(hMutex);

    return TRUE;
}

Note the call is made non-abandoned (FALSE) so the first caller owns the mutex; in a real single-instance app you would keep the handle open for the life of the process and simply exit when ERROR_ALREADY_EXISTS comes back.