对字符的理解差不多都在这里了：

#include<stdio.h>
#include<ctype.h>       //刚才测试的没调这个库也可以实现
void main(){
        printf("单引号：\' 双引号：\" 反斜杠\\ \n");
        printf("字符也是整数：A=%d,a=%d\n",'A','a');
        printf("a-A=%d\n",'a'-'A');

        /*常用函数*/
/*
        int isalpha(int ch);  // 若ch是字母（'A'-'Z','a'-'z'）返回非0值，否则返回0。
        int isalnum(int ch);  // 若ch是字母（'A'-'Z','a'-'z'）或数字（'0'-'9'），返回非0值,否则返回0。
        int isdigit(int ch);  // 若ch是数字（'0'-'9'）返回非0值，否则返回0。
        int islower(int ch);  // 若ch是小写字母（'a'-'z'）返回非0值，否则返回0。
        int isupper(int ch);  // 若ch是大写字母（'A'-'Z'）返回非0值，否则返回0。
        int tolower(int ch);  // 若ch是大写字母（'A'-'Z'）返回相应的小写字母（'a'-'z'）。
        int toupper(int ch);  // 若ch是小写字母（'a'-'z'）返回相应的大写字母（'A'-'Z'）

*/

        char Up='F';
        char Low='g';

        printf("Up=%c   Low=%c\n\n",Up,Low);
        printf("isalpha:Up %d   Low %d\n",isalpha(Up),isalpha(Low));
        printf("isalnum:Up %d   Low %d\n",isalnum(Up),isalnum(Low));
        printf("isdigit:Up %d   Low %d\n",isdigit(Up),isdigit(Low));
        printf("islower:Up %d   Low %d\n",islower(Up),islower(Low));
        printf("isupper:Up %d   Low %d\n",isupper(Up),isupper(Low));
        printf("tolower:Up %c\n",tolower(Up));
        printf("toupper:Low %c\n",toupper(Low));
}

对于调用的函数，是几个常用的
对于字符操作基本的函数，man 手册的解释：

SYNOPSIS
#include <ctype.h>

   int isalnum(int c);
   int isalpha(int c);
   int isascii(int c);
   int isblank(int c);
   int iscntrl(int c);
   int isdigit(int c);
   int isgraph(int c);
   int islower(int c);
   int isprint(int c);
   int ispunct(int c);
   int isspace(int c);
   int isupper(int c);
   int isxdigit(int c);

Feature Test Macro Requirements for glibc (see
feature_test_macros(7)):

   isascii(): _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE
   isblank(): _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE; or cc -std=c99

DESCRIPTION
These functions check whether c, which must have the value of an unsigned char or EOF, falls into a certain character
class according to the current locale.

   isalnum()
          checks for an alphanumeric character; it is equivalent to (isalpha(c) || isdigit(c)).

   isalpha()
          checks  for  an  alphabetic character; in the standard "C" locale, it is equivalent to (isupper(c) || islower(c)).
          In some locales, there may be additional characters for which isalpha() is true—letters which  are  neither  upper
          case nor lower case.

   isascii()
          checks whether c is a 7-bit unsigned char value that fits into the ASCII character set.

   isblank()
          checks for a blank character; that is, a space or a tab.

   iscntrl()
          checks for a control character.

   isdigit()
          checks for a digit (0 through 9).

   isgraph()
          checks for any printable character except space.

   islower()
          checks for a lower-case character.

   isprint()
          In some locales, there may be additional characters for which isalpha() is true—letters which  are  neither  upper
          case nor lower case.

   isascii()
          checks whether c is a 7-bit unsigned char value that fits into the ASCII character set.

   isblank()
          checks for a blank character; that is, a space or a tab.

   iscntrl()
          checks for a control character.

   isdigit()
          checks for a digit (0 through 9).

   isgraph()
          checks for any printable character except space.

   islower()
          checks for a lower-case character.

   isprint()
          checks for any printable character including space.

   ispunct()
          checks for any printable character which is not a space or an alphanumeric character.

   isspace()
          checks  for  white-space  characters.  In the "C" and "POSIX" locales, these are: space, form-feed ('\f'), newline
          ('\n'), carriage return ('\r'), horizontal tab ('\t'), and vertical tab ('\v').

   isupper()
          checks for an uppercase letter.

   isxdigit()
          checks for a hexadecimal digits, that is, one of
          0 1 2 3 4 5 6 7 8 9 a b c d e f A B C D E F.

这里
int isalpha(int ch);
int isalnum(int ch);
int isdigit(int ch);
返回值不太满意，如果判断为是会返回随机一个数(强迫症无法接受)；
所以这里将其改写为bool型加入自建库

/*字符函数*/
/*isalpha判断是否为大小写字母'a'-'z'或'A'-'Z';改为bool型*/
bool Isalpha(int ch){
        if(ch>'A'&&ch<'z')      return TRUE;    //A=65,z=122
        else    return FALSE;
}

/*isdigit判断是否为数字'0'-'9';改为bool型*/
bool Isdigit(int ch){
        if(ch>'0'&&ch<'9')      return TRUE;
        else    return FALSE;   
}

/*islower判断是否为小写字母'a'-'z';改为bool型*/
bool Islower(int ch){
        if(ch>'a'&&ch<'z')      return TRUE;
        else    return FALSE;
}


/*isupper判断是否为大写字母'A'-'Z';改为bool型*/
bool Isupper(int ch){
        if(ch>'A'&&ch<'Z')      return TRUE;
        else    return FALSE;
}

注意这里bool型，C语言对bool是没有返回值的（其实是int型)
所以这里要对bool进行宏定义，不然会报错

#define bool int
#define true 1
#define false 0

int islower(int ch);
int isupper(int ch);
返回值为int型但由于字符=整数，所以没必要修改
可以理解其工作原理是对ch进行了加减
测试函数：

#include<stdio.h>
#include"_public.h"
void main(){
        /*测试自建的字符函数*/
        printf("输入字符/数字：");
        char temp;
        scanf("%c",&temp);
        printf("temp int %d;char %c\n",temp,temp);

        if(Isdigit(temp)) printf("是数字\n");
        if(Isalpha(temp)){
                printf("是字符\n");
                if(Islower(temp)){
                        printf("是小写字符\n");
                        printf("转大写：%c\n",toupper(temp));
                }
                else if(Isupper(temp)){
                        printf("是大写字符\n");
                        printf("转小写：%c\n",tolower(temp));   //这里判断没有必要
                }
        }
}