std::ungetc

#include <cstdio>
#include <iostream>
                                                                                                                
using std::ungetc,
    std::getc,
    std::cerr;

int main(){
    FILE* f = fopen("ztext.txt", "a+");
    char c = 'r';
    c = getc(f);
    cerr << c << "\n";
    if(ungetc(c, f) == c){
        cerr << "ungetc success\n";
    }
    c = getc(f);
    cerr  c << "\n";
    fclose(f);
    return 0;
}