C语言编程 将大写字母A、B变成小写字母

#include<stdio.h>

char low_to_upper(char ch);

void main()
{
char ch_low, ch_upper;
printf("please input some chars :\n");
while((ch_upper=getchar())!='\n')
{
ch_low= low_to_upper(ch_upper);
putchar(ch_low);
}
}

char low_to_upper(char ch)
{
return(ch+32);
}

1. 库函数 tolower
2. if('A' <= c && 'Z' >= c) c -= ('a'-'A')