Question

I am honestly really confused on reading binary files in C.

My data is in a format like:

• int header
• Cell[][] cells, 8×8 matrix
• Each cell is just a short id and a sbyte z.

However, something is clearly messing up and none of the values are accurate.

  int i, j;
  Block block;

  // read 32 bits
  fread( &(block.header), sizeof( int ), 1, mapFile );

  // loop through to fill the 8x8 matrix
  for( i = 0; i < 8; i++ )
  {
    for( j = 0; j < 8; j++ )
    {
      // read 16 bits
      fread( &(block.cells[i][j].tileId), sizeof( short ), 1, mapFile );
      // read 8 bits
      fread( &(block.cells[i][j].z), sizeof( char ), 1, mapFile );

      printf( "[%i][%i]: %x %i", i, j, block.cells[i][j].tileId, block.cells[i][j].z );
    }
  }
  printf( "header: %i", block.header );

Output of above shows a bunch of lines with [n][n]: ffffa800 251.

My C# version works fine though:

            Block block;
            block.header = reader.ReadInt32();
            block.cells = new Cell[8,8];
            for( int i = 0; i < 8; i++ )
            {
                for ( int j = 0; j < 8; j++ )
                {
                    block.cells[i, j].tileId = reader.ReadInt16();
                    block.cells[i, j].z = reader.ReadSByte();
                }
            }

            reader.Close();

Output from that (correctly) shows [n][n]: a8 -5.

Answer 1

You should use more strictly defined types in C. For example, int32_t (stdint.h). It’s possible that your short is 32-bit (or even your int is 16-bit), albeit unlikely.

It’s probably also worthwhile to show us how the Block struct is defined.

For reference:

SByte = int8_t

Int16 = int16_t

Int32 = int32_t