Question

I’ve written an Ada program which encrypts files. It reads them block by block to conserve memory on the target machine. Unfortunately Ada’s Directories library reads filesizes in a Long_Integer, limiting the read to almost 2GB files. When trying to read files over 2GB, the program fails at runtime getting a stack overflow error.

The documentation for it here is the origin of my understanding above. How can I read a filesize into a type I define myself? One I can make requiring something like 25 bytes to increase the cap to 100GB.

Answer 1

I just posted GCC bug 55119 on this.

While you’re waiting (!), the code below works on Mac OS X Mountain Lion. On Windows, it’s more complicated; see adainclude/adaint.{c,h}.

The Ada spec:

with Ada.Directories;
package Large_Files is

   function Size (Name : String) return Ada.Directories.File_Size;

end Large_Files;

and body (copied in part from Ada.Directories):

with GNAT.OS_Lib;
with System;
package body Large_Files is

   function Size (Name : String) return Ada.Directories.File_Size
   is
      C_Name : String (1 .. Name'Length + 1);
      function C_Size (Name : System.Address) return Long_Long_Integer;
      pragma Import (C, C_Size, "large_file_length");
   begin
      if not GNAT.OS_Lib.Is_Regular_File (Name) then
         raise Ada.Directories.Name_Error
           with "file """ & Name & """ does not exist";
      else
         C_Name (1 .. Name'Length) := Name;
         C_Name (C_Name'Last) := ASCII.NUL;
         return Ada.Directories.File_Size (C_Size (C_Name'Address));
      end if;
   end Size;

end Large_Files;

and the C interface:

/* large_files_interface.c */

#include <sys/stat.h>

long long large_file_length (const char *name)
{
  struct stat statbuf;
  if (stat(name, &statbuf) != 0) {
    return 0;
  } else {
    return (long long) statbuf.st_size;
  }
}

You might need to use struct stat64 and stat64() on other Unix systems.

Compile the C interface as normal, then add -largs large_files_interface.o to your gnatmake command line.

EDIT: on Mac OS X (and Debian), which are x86_64 machines, sizeof(long) is 8 bytes; so the comment in adaint.c is misleading and Ada.Directories.Size can return up to 2**63-1.