How to read a C generated binary file in Lua

Question

I want to read a 32 bit integer binary file provided by another program. The file contains only integer and no other characters (like spaces or commas). The C code to read this file is as follows:

FILE* pf = fopen("C:/rktemp/filename.dat", "r");
int sz = width*height;
int* vals = new int[sz];
int elread = fread((char*)vals, sizeof(int), sz, pf);
for( int j = 0; j < height; j++ )
{
    for( int k = 0; k < width; k++ )
    {
        int i = j*width+k;
        labels[i] = vals[i];
    }
}
delete [] vals;
fclose(pf);

But I don't know how to read this file into array using Lua.

I've tried to read this file using io.read, but part of the array looks like this:

~~~~~~xxxxxxxxyyyyyyyyyyyyyyzzzzzzzz{{{{{{{{{|||||||||}}}}}}}}}}}~~~~~~~~~xxxxxxxyyyyyyyyyyyyyyzzzzzz{{{{{{{{{{|||||||||}}}}}}}}}}}~~~~~~~~~xxyyyyyyyyyyyyyzzzzz{{{{{{|||}}}yyyyyyyyyyyz{{{yyyyyyyyÞľūơǿȵɶʢ˺̤̼ͽаҩӱľǿجٴȵɶʢܷݸ˺໻⼼ӱľǿ

Also the Matlab code to read this file is like this:

row = image_size(1);
colomn = image_size(2);
fid = fopen(data_path,'r');
A = fread(fid, row * colomn, 'uint32')';
A = A + 1;
B = reshape(A,[colomn, row]);
B = B';
fclose(fid);

I've tried a function to convert bytes to integer, my code is like this:

function bytes_to_int(b1, b2, b3, b4)
      if not b4 then error("need four bytes to convert to int",2) end
      local n = b1 + b2*256 + b3*65536 + b4*16777216
      n = (n > 2147483647) and (n - 4294967296) or n
      return n
end

   local sup_filename = '1.dat'
   fid = io.open(sup_filename, "r")
   st = bytes_to_int(fid:read("*all"):byte(1,4))
   print(st)

   fid:close()

But it still not read this file properly.

Answer 1

You are only calling bytes_to_int once. You need to call it for every int you want to read. e.g.

fid = io.open(sup_filename, "rb")
while true do
  local bytes = fid:read(4)
  if bytes == nil then break end -- EOF
  local st = bytes_to_int(bytes:byte(1,4))
  print(st)
end

fid:close()

Answer 2

Now you can use the new feature of Lua language by calling string.unpack , which has many conversion options for format string. Following options may be useful:

• < sets little endian
• > sets big endian
• = sets native endian
• i[n] a signed int with n bytes (default is native size)
• I[n] an unsigned int with n bytes (default is native size)

The arch of your PC is unknown, so I assume the data to read is unsigned and native-endian.

Since you are reading binary data from the file, you should use io.open(sup_filename, "rb").

The following code may be useful:

local fid = io.open(sup_filename, "rb")
local contents = fid:read("a")
local now
while not now or now < #contents do
  local n, now = string.unpack("=I4", contents, now)
  print(n)
end
fid:close()

see also: Lua 5.4 manual