diff --git a/sys_file_impl.cpp b/sys_file_impl.cpp
new file mode 100644
index 0000000..243d82d
--- /dev/null
+++ b/sys_file_impl.cpp
@@ -0,0 +1,131 @@
+#include "sys_file_impl.hpp"
+
+#include <system_error>
+
+using namespace std::string_literals;
+
+static constexpr size_t rwBlockSize = 8192;
+
+namespace binstore
+{
+
+namespace
+{
+
+std::system_error errnoException(const std::string& message)
+{
+    return std::system_error(errno, std::generic_category(), message);
+}
+
+} // namespace
+
+SysFileImpl::SysFileImpl(const std::string& path, size_t offset,
+                         const internal::Sys* sys) :
+    sys(sys)
+{
+    fd_ = sys->open(path.c_str(), O_RDWR);
+    offset_ = offset;
+
+    if (fd_ < 0)
+    {
+        throw errnoException("Error opening file "s + path);
+    }
+}
+
+SysFileImpl::~SysFileImpl()
+{
+    sys->close(fd_);
+}
+
+void SysFileImpl::lseek(size_t pos) const
+{
+    if (sys->lseek(fd_, offset_ + pos, SEEK_SET) < 0)
+    {
+        throw errnoException("Cannot lseek to pos "s + std::to_string(pos));
+    }
+}
+
+size_t SysFileImpl::readToBuf(size_t pos, size_t count, char* buf) const
+{
+
+    lseek(pos);
+
+    size_t bytesRead = 0;
+
+    do
+    {
+        auto ret = sys->read(fd_, &buf[bytesRead], count - bytesRead);
+        if (ret < 0)
+        {
+            if (errno == EINTR)
+            {
+                continue;
+            }
+
+            throw errnoException("Error reading from file"s);
+        }
+        else if (ret > 0)
+        {
+            bytesRead += ret;
+        }
+        else // ret == 0
+        {
+            break;
+        }
+    } while (bytesRead < count);
+
+    return bytesRead;
+}
+
+std::string SysFileImpl::readAsStr(size_t pos, size_t count) const
+{
+    std::string result;
+
+    /* If count is invalid, return an empty string. */
+    if (count == 0 || count > result.max_size())
+    {
+        return result;
+    }
+
+    result.resize(count);
+    size_t bytesRead = readToBuf(pos, count, result.data());
+    result.resize(bytesRead);
+    return result;
+}
+
+std::string SysFileImpl::readRemainingAsStr(size_t pos) const
+{
+    std::string result;
+    size_t bytesRead, size = 0;
+
+    /* Since we don't know how much to read, read 'rwBlockSize' at a time
+     * until there is nothing to read anymore. */
+    do
+    {
+        result.resize(size + rwBlockSize);
+        bytesRead = readToBuf(pos + size, rwBlockSize, result.data() + size);
+        size += bytesRead;
+    } while (bytesRead == rwBlockSize);
+
+    result.resize(size);
+    return result;
+}
+
+void SysFileImpl::writeStr(const std::string& data, size_t pos)
+{
+    lseek(pos);
+    ssize_t ret;
+    ret = sys->write(fd_, data.data(), data.size());
+    if (ret < 0)
+    {
+        throw errnoException("Error writing to file"s);
+    }
+    if (static_cast<size_t>(ret) != data.size())
+    {
+        throw std::runtime_error(
+            "Tried to send data size "s + std::to_string(data.size()) +
+            " but could only send "s + std::to_string(ret));
+    }
+}
+
+} // namespace binstore