C: Difference between U64 and uint64_t

Question

I am trying to learn C after working with Java for a couple of years. I've found some code that I wanted to reproduce which looked something like this:

U64 attack_table[...]; // ~840 KiB 

struct SMagic {
   U64* ptr;  // pointer to attack_table for each particular square
   U64 mask;  // to mask relevant squares of both lines (no outer squares)
   U64 magic; // magic 64-bit factor
   int shift; // shift right
};

SMagic mBishopTbl[64];
SMagic mRookTbl[64];

U64 bishopAttacks(U64 occ, enumSquare sq) {
   U64* aptr = mBishopTbl[sq].ptr;
   occ      &= mBishopTbl[sq].mask;
   occ      *= mBishopTbl[sq].magic;
   occ     >>= mBishopTbl[sq].shift;
   return aptr[occ]; 
}

U64 rookAttacks(U64 occ, enumSquare sq) {
   U64* aptr = mRookTbl[sq].ptr;
   occ      &= mRookTbl[sq].mask;
   occ      *= mRookTbl[sq].magic;
   occ     >>= mRookTbl[sq].shift;
   return aptr[occ];
}

The code is not that important but I already failed at using the same datatype: U64 , I only found uint64_t . Now I would like to know where the difference in U64 , uint64_t and long is.

I am very happy if someone could briefly explain this one to me, including the advantage of each of them.

Greetings, Finn

Answer 1

TL;DR - for a 64-bit exact width unsigned integer, #include <stdint.h> and use uint64_t .

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Presumably, U64 is a custom typedef for 64-bit wide unsigned integer.

If you're using at least a C99 compliant compiler, it would have <stdint.h> with a typedef for 64-bit wide unsigned integer with no padding bits : uint64_t . However, it might be that the code targets a compiler that doesn't have the standard uint64_t defined. In that case, there might be some configuration header where a type is chosen for U64 . Perhaps you can grep the files for typedef.*U64;

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long on the other hand, is a signed type . Due to various undefined and implementation-defined aspects of signed math, you wouldn't want to use a signed type for bit-twiddling at all. Another complication is that unlike in Java, the C long doesn't have a standardized width; instead long is allowed to be only 32 bits wide - and it is so on most 32-bit platforms, and even on 64-bit Windows. If you ever need exact width types, you wouldn't use int or long . Only long long and unsigned long long are guaranteed to be at least 64 bits wide.