diff --git a/mkvreader.cpp b/mkvreader.cpp
index cb3567f..b4b2459 100644
--- a/mkvreader.cpp
+++ b/mkvreader.cpp
@@ -14,13 +14,20 @@ namespace mkvparser
 {
 
 MkvReader::MkvReader() :
-    m_file(NULL)
-{
+    m_file(NULL),
+    reader_owns_file_(true) {
 }
 
-MkvReader::~MkvReader()
-{
+MkvReader::MkvReader(FILE* fp) :
+    m_file(fp),
+    reader_owns_file_(false) {
+  GetFileSize();
+}
+
+MkvReader::~MkvReader() {
+  if (reader_owns_file_)
     Close();
+  m_file = NULL;
 }
 
 int MkvReader::Open(const char* fileName)
@@ -42,12 +49,17 @@ int MkvReader::Open(const char* fileName)
     if (m_file == NULL)
         return -1;
 #endif
+    return !GetFileSize();
+}
 
+bool MkvReader::GetFileSize() {
+    if (m_file == NULL)
+        return false;
 #ifdef _MSC_VER
     int status = _fseeki64(m_file, 0L, SEEK_END);
 
     if (status)
-        return -1;  //error
+        return false;  //error
 
     m_length = _ftelli64(m_file);
 #else
@@ -56,16 +68,19 @@ int MkvReader::Open(const char* fileName)
 #endif
     assert(m_length >= 0);
 
+    if (m_length < 0)
+        return false;
+
 #ifdef _MSC_VER
     status = _fseeki64(m_file, 0L, SEEK_SET);
 
     if (status)
-        return -1;  //error
+        return false;  //error
 #else
     fseek(m_file, 0L, SEEK_SET);
 #endif
 
-    return 0;
+    return true;
 }
 
 void MkvReader::Close()
diff --git a/mkvreader.hpp b/mkvreader.hpp
index adcc29f..8ebdd99 100644
--- a/mkvreader.hpp
+++ b/mkvreader.hpp
@@ -21,6 +21,7 @@ class MkvReader : public IMkvReader
     MkvReader& operator=(const MkvReader&);
 public:
     MkvReader();
+    MkvReader(FILE* fp);
     virtual ~MkvReader();
 
     int Open(const char*);
@@ -29,8 +30,15 @@ public:
     virtual int Read(long long position, long length, unsigned char* buffer);
     virtual int Length(long long* total, long long* available);
 private:
+
+    // Determines the size of the file. This is called either by the constructor
+    // or by the Open function depending on file ownership. Returns true on
+    // success.
+    bool GetFileSize();
+
     long long m_length;
     FILE* m_file;
+    bool reader_owns_file_;
 };
 
 }  //end namespace mkvparser