What's the difference between long long and long

Going by the standard, all that's guaranteed is:

• int must be at least 16 bits
• long must be at least 32 bits
• long long must be at least 64 bits

On major 32-bit platforms:

• int is 32 bits
• long is 32 bits as well
• long long is 64 bits

On major 64-bit platforms:

• int is 32 bits
• long is either 32 or 64 bits
• long long is 64 bits as well

If you need a specific integer size for a particular application, rather than trusting the compiler to pick the size you want, #include <stdint.h> (or <cstdint>) so you can use these types:

• int8_t and uint8_t
• int16_t and uint16_t
• int32_t and uint32_t
• int64_t and uint64_t

You may also be interested in #include <stddef.h> (or <cstddef>):

• size_t
• ptrdiff_t

long long does not exist in C++98/C++03, but does exist in C99 and c++0x.

long is guaranteed at least 32 bits.

long long is guaranteed at least 64 bits.

To elaborate on @ildjarn's comment:

And they both don't work with 12 digit numbers (600851475143), am I forgetting something?

The compiler looks at the literal value 600851475143 without considering the variable that you're assigning it to/initializing it with. You've written it as an int typed literal, and it won't fit in an int.

Use 600851475143LL to get a long long typed literal.