std::setvbuf

int main() {
    char buf[BUFSIZ];
    std::setbuf(stdin, buf);
} // lifetime of buf ends, undefined behavior

#include <iostream>
#include <cstdio>
#include <stdlib.h>
#include <sys/stat.h>
 
int main()
{
    std::FILE* fp = std::fopen("test.txt", "r");
    if(!fp) {
       std::perror("fopen"); return 1;
    }
 
    struct stat stats;
    if(fstat(fileno(fp), &stats) == -1) { // POSIX only
        std::perror("fstat"); return 1;
    }
 
    std::cout << "BUFSIZ is " << BUFSIZ << ", but optimal block size is "
              << stats.st_blksize << '\n';
    if(std::setvbuf(fp, NULL, _IOFBF, stats.st_blksize) != 0) {
       perror("setvbuf failed"); // POSIX version sets errno
       return 1;
    }
 
    int ch;
    while((ch=std::fgetc(fp)) != EOF); // read entire file: use truss/strace to
                                       // observe the read(2) syscalls used
    std::fclose(fp);
}

BUFSIZ is 8192, but optimal block size is 65536

 ? cppreference.com