write_test.c

#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>

int main() {
    int fd;
    char buf[2];
    fd = open("filename.txt", 2);
    read(fd, buf, 2);
    buf[0] = 'a';
    buf[1] = 'b';
    lseek(fd, 0, 0);
    write(fd, buf, 2);
    close(fd);
}