Все наверное указано в limits.h
/*
* ANSI required limits
*/
#define CHAR_BIT 8 /* number of bits in a byte */
#ifdef __CHAR_SIGNED__
#define CHAR_MIN (-128) /* minimum value of a char */
#define CHAR_MAX 127 /* maximum value of a char */
#else
#define CHAR_MIN 0 /* minimum value of a char */
#define CHAR_MAX 255 /* maximum value of a char */
#endif
#define SCHAR_MIN (-128) /* minimum value of a signed char */
#define SCHAR_MAX 127 /* maximum value of a signed char */
#define UCHAR_MAX 255U /* maximum value of an unsigned char */