FileBase::set_extent(offset, length) constrains seek() and tell()

src/file.h

@@ -59,6 +59,7 @@ public:
     virtual bool isOpen() const { return _fd >= 0; }
     int getFd() const { return _fd; }
     const char *getName() const { return _name; }
+    virtual off_t st_size() const;  // { return _length; }
 
 protected:
     void sopen();
@@ -67,12 +68,15 @@ protected:
     virtual void write(const void *buf, int len);
     virtual void seek(off_t off, int whence);
     virtual off_t tell() const;
+    virtual void set_extent(off_t offset, off_t length);
 
     int _fd;
     int _flags;
     int _shflags;
     int _mode;
     const char *_name;
+    off_t _offset;
+    off_t _length;
 public:
     struct stat st;
 };