How do concurrent writes by parent and child processes behave with a shared file descriptor?

fork_fd_test.c

/* Write a program that opens a file (with the open() system call) and then
 * calls fork() to create a new process. Can both the child and parent access
 * the file descriptor returned by open()? What happens when they are writing to
 * the file concurrently, i.e., at the same time? */

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


int main(int argc, char *argv[])
{
    int fd = open("./test.txt", O_CREAT|O_WRONLY|O_TRUNC, S_IRWXU);

    if (fd < 0) {
        fprintf(stderr, "Error opening file for writing. Exiting...\n");
        exit(1);
    }
    int rc = fork();

    if (rc < 0) {
        fprintf(stderr, "Error forking. Exiting...\n");
        exit(1);
    } else if (rc == 0) {
        printf("I'm a child. Writing childish things to file.\n");
        for(int i = 0; i < 1000; i++)
        {
            int result = write(fd, "WAAAAAAAAAAAAAAHHHHHHH :(\n", 26);
            fprintf(stdout, "Result = %d\n", result);
        }
    } else {
        printf("I'm a parent. Writing parental things to file.\n");
        for(int i = 0; i < 1000; i++)
        {
            int result = write(fd, ".....why............\n", 21);
            fprintf(stdout, "Result = %d\n", result);
        }
    }
    return 0;
}