Quick test of environment variable passing

testenv.cpp

// Quick test of environment variable passing
//   $ cc -nostartfiles -o testenv testenv.cpp
//   $ cl testenv.cpp /link /subsystem:console kernel32.lib
//
// Spawns a copy of itself with a custom-built environment block, and
// the spawned child examines/probes that block.
//
// This is free and unencumbered software released into the public domain.

typedef int I;
typedef char16_t C;
enum H : decltype(sizeof(0));
struct SI {
    I cb = sizeof(SI);
    H _0[3];
    I _1[9];
    H _2[4];
};
struct PI {
    H ph, th;
    I pi, ti;
};

enum { CREATE_UNICODE_ENVIRONMENT = 0x400 };

#define W32(r) extern "C" __declspec(dllimport) r __stdcall
W32(I)    CreateProcessW(C*, C*, H, H, I, I, C*, C*, SI*, PI*);
W32(void) ExitProcess(I);
W32(C *)  GetCommandLineW();
W32(C *)  GetEnvironmentStringsW();
W32(I)    GetEnvironmentVariableW(C const *, C *, I);
W32(I)    GetModuleFileNameW(H, C *, I);
W32(H)    GetStdHandle(I);
W32(I)    WaitForSingleObject(H, I);
W32(I)    WriteConsoleW(H, C const *, I, I *, H);

template<typename T, I N>
constexpr I countof(T const (&)[N]) { return N; };

static I print(C const *s)
{
    H stdout = GetStdHandle(-11);
    I len = 0;
    for (; s[len]; len++) {}
    I dummy;
    WriteConsoleW(stdout, s, len, &dummy, {});
    return len;
}

static void printenv()
{
    C *env = GetEnvironmentStringsW();
    for (C *e = env; *e;) {
        e += print(e) + 1;
        print(u"\n");
    }

    C value[1<<10];
    GetEnvironmentVariableW(u"=x", value, countof(value));
    print(u"\"");
    print(value);
    print(u"\"\n");
}

static I spawn()
{
    C path[1<<10];
    GetModuleFileNameW({}, path, countof(path));

    SI si    = {};
    PI pi    = {};
    C  cmd[] = u"?";
    C  env[] = u"=x=hello world\0";
    I  flags = CREATE_UNICODE_ENVIRONMENT;
    I  ok    = CreateProcessW(path, cmd, {}, {}, 0, flags, env, 0, &si, &pi);
    if (!ok) {
        return 1;
    }

    WaitForSingleObject(pi.ph, -1);
    return 0;
}

extern "C" void __stdcall mainCRTStartup()
{
    I r = 0;
    if (*GetCommandLineW() == '?') {
        printenv();
    } else {
        r = spawn();
    }
    ExitProcess(r);
}