文件内容大写改小写

#include<bits/stdc++.h>
#define INPUT_FILE "a.txt"
#define OUTPUT_FILE "b.txt"
using namespace std;
int main()
{
    char c;
    FILE *fin,*fout;
    fin=fopen(INPUT_FILE,"r");
    if(!fin)
    {
        perror(INPUT_FILE);
        exit(1);
    }
    fout=fopen(OUTPUT_FILE,"w");
    if(!fout)
    {
        perror(OUTPUT_FILE);
        exit(2);
    }
    while((c=fgetc(fin))!=EOF)
    {
        if(c>='A'&&c<='Z')
            c=c+32;
        fputc(c,fout);
    }
    return 0;
}