Bare minimum steam client bootstrapper. Can be used as drop-in replacement for steam main executable. (windows only)

uBootstrap.cpp

#include <iostream>
#include <Windows.h>

#define export extern "C" __declspec(dllexport)

typedef void (*SteamUIMainFn) (int, char**);

typedef enum ELauncherType
{
    k_ELauncherTypeDefault, // default
    k_ELauncherTypePw_dota2, // pw_dota2
    k_ELauncherTypeNexon_dota2, // nexon_dota2
    k_ELauncherTypeSteamcmd, // steamcmd
    k_ELauncherTypePw_csgo, // pw_csgo
    k_ELauncherTypeClientui, // clientui
    k_ELauncherTypeSteamhdl, // steamhdl
    k_ELauncherTypeSteamchina, // steamchina
    k_ELauncherTypeSingleapp, // singleapp
} ELauncherType;

std::string g_InstallPath;
std::string g_LogsPath;

export ELauncherType __cdecl GetClientLauncherType()
{
    return k_ELauncherTypeDefault;
}

export const char* __cdecl SteamBootstrapper_GetInstallDir()
{
    return g_InstallPath.c_str();
}

export const char* __cdecl SteamBootstrapper_GetBaseUserDir()
{
    return g_InstallPath.c_str(); // ¯\(°_o)/¯
}

export const char* __cdecl SteamBootstrapper_GetLoggingDir()
{
    return g_LogsPath.c_str();
}

std::string GetExecutablePath()
{
    char szInstallDir[MAX_PATH];
    int pathEnd = GetModuleFileNameA(NULL, szInstallDir, sizeof(szInstallDir));
    if (pathEnd)
    {
        for (int i = pathEnd; i > 0; --i)
        {
            if (szInstallDir[i] == '\\')
            {
                pathEnd = i;
                break;
            }
        }

        return std::string(szInstallDir, pathEnd);
    }

    return std::string("./");
}

int main(int argc, char** argv)
{
    std::cout << "Starting Steam...\n";

    g_InstallPath = GetExecutablePath();
    g_LogsPath = g_InstallPath + "/logs";

    if (!SetCurrentDirectoryA(g_InstallPath.c_str()))
    {
        std::cout << "Could not change current work directory!" << std::endl;
    }

    HMODULE steamUIModule = LoadLibraryW(L"steamui.dll");
    if (!steamUIModule)
    {
        std::cout << "Could not load SteamUI!" << std::endl;
        return -1;
    }

    SteamUIMainFn steamUIMain = (SteamUIMainFn)GetProcAddress(steamUIModule, "SteamDllMain");
    if (!steamUIMain)
    {
        std::cout << "SteamDllMain function not found in steamui.dll!" << std::endl;
        return -1;
    }

    steamUIMain(argc, argv);
}