fileMemoryMapCreate

fn fileMemoryMapCreate(
    userdata: ?*anyopaque,
    file: File,
    options: File.MemoryMap.CreateOptions,
) File.MemoryMap.CreateError!File.MemoryMap {
    const t: *Threaded = @ptrCast(@alignCast(userdata));
    const offset = options.offset;
    const len = options.len;

    if (!t.disable_memory_mapping) {
        if (createFileMap(file, options.protection, offset, options.populate, len)) |result| {
            return result;
        } else |err| switch (err) {
            error.Unseekable, error.Canceled, error.AccessDenied => |e| return e,
            error.OperationUnsupported => {},
            else => {
                if (builtin.mode == .debug)
                    std.log.warn("memory mapping failed with {t}, falling back to file operations", .{err});
            },
        }
    }

    const gpa = t.allocator;
    const page_size = std.heap.pageSize();
    const alignment: Alignment = .fromByteUnits(page_size);
    const memory = m: {
        const ptr = gpa.rawAlloc(len, alignment, @returnAddress()) orelse return error.OutOfMemory;
        break :m ptr[0..len];
    };
    errdefer gpa.rawFree(memory, alignment, @returnAddress());

    if (!options.undefined_contents) try mmSyncRead(file, memory, offset);

    return .{
        .file = file,
        .offset = offset,
        .memory = @alignCast(memory),
        .section = null,
    };
}