nu/nvram.c: Add helper for NVRAM-class devices

nu/meson.build

@@ -9,5 +9,7 @@ nu_lib = static_library(
     ],
     sources : [
         'guid.c',
+        'nvram.c',
+        'nvram.h',
     ],
 )

nu/nvram.c

@@ -0,0 +1,80 @@
+#include <windows.h>
+
+#include <assert.h>
+#include <stddef.h>
+
+#include "util/dprintf.h"
+
+#include "nu/nvram.h"
+
+HRESULT nvram_open_file(HANDLE *out, wchar_t *path, size_t size)
+{
+    LARGE_INTEGER cur_size;
+    LARGE_INTEGER pos;
+    HANDLE file;
+    HRESULT hr;
+    BOOL ok;
+
+    assert(out != NULL);
+    assert(path != NULL);
+
+    *out = NULL;
+
+    file = CreateFileW(
+            path,
+            GENERIC_READ | GENERIC_WRITE,
+            FILE_SHARE_READ,
+            NULL,
+            OPEN_ALWAYS,
+            FILE_ATTRIBUTE_NORMAL,
+            NULL);
+
+    if (file == INVALID_HANDLE_VALUE) {
+        hr = HRESULT_FROM_WIN32(GetLastError());
+        dprintf("%S: Error opening backing store: %x\n", path, (int) hr);
+
+        goto end;
+    }
+
+    ok = GetFileSizeEx(file, &cur_size);
+
+    if (!ok) {
+        hr = HRESULT_FROM_WIN32(GetLastError());
+        dprintf("%S: GetFileSizeEx failed: %x\n", path, (int) hr);
+
+        goto end;
+    }
+
+    if (cur_size.QuadPart != (uint64_t) size) {
+        pos.QuadPart = (uint64_t) size;
+        ok = SetFilePointerEx(file, pos, NULL, FILE_BEGIN);
+
+        if (!ok) {
+            hr = HRESULT_FROM_WIN32(GetLastError());
+            dprintf("%S: SetFilePointerEx failed: %x\n", path, (int) hr);
+
+            goto end;
+        }
+
+        ok = SetEndOfFile(file);
+
+        if (!ok) {
+            hr = HRESULT_FROM_WIN32(GetLastError());
+            dprintf("%S: SetEndOfFile failed: %x\n", path, (int) hr);
+
+            goto end;
+        }
+    }
+
+    *out = file;
+    file = INVALID_HANDLE_VALUE;
+
+    hr = S_OK;
+
+end:
+    if (file != INVALID_HANDLE_VALUE) {
+        CloseHandle(file);
+    }
+
+    return hr;
+}

nu/nvram.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include <windows.h>
+
+#include <stddef.h>
+
+HRESULT nvram_open_file(HANDLE *out, wchar_t *path, size_t size);