test.c

#include<stdio.h>

int main() {
    puts("to terminal 1");
    fflush(stdout);

    int fd = dup(fileno(stdout));
    freopen("out-file.txt", "w", stdout);

    puts("to file");
    fflush(stdout);
    dup2(fd, fileno(stdout));
    puts("to terminal 2");

    return 0;
}