/* Number of bits in a `char'.    */
#  define CHAR_BIT    8

/* Minimum and maximum values a `signed char' can hold.  */
#  define SCHAR_MIN    (-128)
#  define SCHAR_MAX    127

/* Maximum value an `unsigned char' can hold.  (Minimum is 0.)  */
#  define UCHAR_MAX    255

/* Minimum and maximum values a `char' can hold.  */
#  ifdef __CHAR_UNSIGNED__
#   define CHAR_MIN    0
#   define CHAR_MAX    UCHAR_MAX
#  else
#   define CHAR_MIN    SCHAR_MIN
#   define CHAR_MAX    SCHAR_MAX
#  endif

/* The character type that char matches (i.e., signed or unsigned)
 */
#if CHAR_MIN < 0
typedef signed char underlying_char_type;
#else
typedef unsigned char underlying_char_type;
#endif

#include <stdio.h>

int main(void) 
{
    char a = -1;
    signed char b = -1;
    unsigned char c = -1;

    printf("a=%d, b=%d, c=%d\n", a, b, c);
    return 0;
}

# ./test_char.elf 
a=-1, b=-1, c=255 // 从这里可以看出 默认的 char 类型就是 signed char 类型