11.1.6 Integer Variations

The integer types of C have standard names, but what they mean varies depending on the kind of platform in use: which kind of computer, which operating system, and which compiler. It may even depend on the compiler options used.

Plain char may be signed or unsigned; this depends on the platform, too. Even for GNU C, there is no general rule.

In theory, all of the integer types’ sizes can vary. char is always considered one “byte” for C, but it is not necessarily an 8-bit byte; on some platforms it may be more than 8 bits. ISO C specifies only that none of these types is narrower than the ones above it in the list in Basic Integers, and that short has at least 16 bits.

It is possible that in the future GNU C will support platforms where int is 64 bits long. In practice, however, on today’s real computers, there is little variation; you can rely on the table given previously (see Basic Integers).

To be completely sure of the size of an integer type, use the types int16_t, int32_t and int64_t. Their corresponding unsigned types add ‘u’ at the front: uint16_t, uint32_t and uint64_t. To define all these types, include the header file stdint.h.

The GNU C Compiler can compile for some embedded controllers that use two bytes for int. On some, int is just one “byte,” and so is short int—but that “byte” may contain 16 bits or even 32 bits. These processors can’t support an ordinary operating system (they may have their own specialized operating systems), and most C programs do not try to support them.