Let's learn about open(2) (called by fopen(3))

test.c

#include <stdio.h>
#include <unistd.h>

#define TEST_TXT "test.txt"

void phase1(void) {
    FILE * fp = fopen(TEST_TXT, "w");

    fprintf(fp, "hoge\n");
    fclose(fp);

    sleep(10);  // During this, try `$ cat test.txt`
}

void phase2(void) {
    FILE * fp = fopen(TEST_TXT, "w");

    fprintf(fp, "fuga\n");

    sleep(10);  // During this, try `$ cat test.txt`

    fclose(fp);
}

int main(void) {
    phase1();
    phase2();

    return 0;
}

$ cat test.txt twice as instructed.
You should find that

1. you get hoge on the first attempt, and
2. you get nothing (neither hoge nor fuga) on the second attempt.

In this code fopen(3) is called with w option. This calls syscall open(2) with O_TRUNC set. On Linux, open(2) calls do_truncate, as a result of calling do_sys_open, do_filp_open, path_openat, do_last, and handle_truncate. Since do_truncate instantly updates inode upon call, the target file becomes empty immediately. Changes made by fprintf(3) in this code would not be flushed until fclose(3) called.