std::fflush

#include <cstdio>
#include <iostream>
                                                                                                                
using std::FILE,
    std::fflush,
    std::setbuf,
    std::fopen,
    std::fclose,
    std::fwrite,
    std::cerr;

int main(){
    char buffer[BUFSIZ]{"I woke up this morning and shot an elephant in my pajamas; how he got in my pajamas I’ll never know. \0"};
    FILE* f = fopen("ztext.txt", "w");

    setbuf(f, buffer);

    size_t s = fwrite(buffer, sizeof(char), 25, f);
    
    int r = fflush(f);
    if(r == 0){
        cerr << "flush successful" << "\n";
    }

    fclose(f);

    f = fopen("ztext.txt", "w");
    r = setvbuf(f, buffer, _IOFBF, BUFSIZ);
    if(r == 0){
        cerr << "setvbuf successful" << "\n";
    }
    fclose(f);
    return 0;
}