Question

I’m trying to read bytes from binary file but to no success.
I’ve tried many solutions, but I get no get result.
Struct of file:

[offset] [type]          [value]          [description] 
0000     32 bit integer  0x00000803(2051) magic number 
0004     32 bit integer  60000            number of images 
0008     32 bit integer  28               number of rows 
0012     32 bit integer  28               number of columns 
0016     unsigned byte   ??               pixel 
0017     unsigned byte   ??               pixel 
........ 
xxxx     unsigned byte   ??               pixel

How I tried (doesn’t work):

auto myfile = fopen("t10k-images.idx3-ubyte", "r");
char buf[30];
auto x = fread(buf, 1, sizeof(int), myfile);

Answer 1

Knowing the endianness of your file layout whence reading multi-byte numerics is important. Assuming big-endian is always the written format, and assuming the value is indeed a 32bit unsigned value:

uint32_t magic = 0;
unsigned char[4] bytes;
if (1 == fread(bytes, sizeof(bytes), 1, f))
{
   magic = (uint32_t)((bytes[0] << 24) | 
                      (bytes[1] << 16) | 
                      (bytes[2] << 8) | 
                      bytes[3]);
}

Note: this will work regardless of whether the reader (your program) is little endian or big-endian. I’m sure I missed at least one cast in there, but hopefully you get the point. The only safe, and portable way of reading multi-byte numerics is to (a) know the endianness they were written with, and (b) read-and-assemble them byte by byte.