read() System call and Null-terminated string - c

Question

If I have the following array :
char message_buffer [100] ="Hello World ";
If I then use an ordinary pipe to send this buffer from the parent process to a child and then at the child , I have used the following code :
#include <unistd.h>
#include <stdio.h>
#include <sys/wait.h>
#include <stdlib.h>
int main()
{
char message_buffer[100] = "Hello World ";
char read_buffer[100];
int return_value;
int fd[2];
return_value = pipe(fd);
if (return_value < 0)
{
printf("Error creating the pipe");
exit(1);
}
int rc = fork();
if (rc < 0)
{
printf("Error forking a child");
exit(1);
}
if (rc > 0)
{
printf("I am the parent , I will write the message now into the pipe. \n");
close(fd[0]);
write(fd[1], message_buffer, 100);
close(fd[1]);
wait(NULL);
}
else
{
printf("I am the child , I will read the message from the pipe. \n");
close(fd[1]);
read(fd[0], read_buffer, 100);
close(fd[0]);
printf("Message recieved: %s", read_buffer);
}
In this situation, the print statement will work without printing any garbage value. However, assume we have a file copying program that copies the content of a file to another file, where the source file will have the content "hello world" and then we read this message into a buffer and send it through a pipe to a child:
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
void main()
{
char buffer[100];
char childbuff[100];
int fd[2], des, bytes, target;
pipe(fd);
if (fork())
{
/* parent process closes the downstream */
close(fd[0]);
/* reads the file */
des = open("test.txt", O_RDONLY);
bytes = read(des, buffer, sizeof(buffer));
/* puts data in pipe */
write(fd[1], buffer, bytes);
}
else
{
/* Child process closes the upstream */
close(fd[1]);
/* reads from the pipe */
read(fd[0], childbuff, sizeof(childbuff));
close(fd[0]);
/* output the received string */
printf("\nReceived string is -- %s", childbuff);
target = open("copy.txt", O_CREAT, 00777);
write(target, childbuff, (strlen(childbuff) - 1));
}
}
In this case, I get a garbage value along the message received when I run the print statement. I know that the read() doesn't Null terminate the buffer. But my question is why the print statement has worked fine in the first case (in which we have only transferred an array), while it didn't work fine in the second case (in which we have transferred a content of a file)?