Question

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?

Answer 1

CODE:

#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <stdio.h>
#include <string.h>

int main(void)
{
int fd;

fd = open("output",O_CREAT|O_TRUNC|O_WRONLY, 0666);

if(!fork()) {
/* child process */
  
char msg[1024];
sprintf(msg,"This is child with fd=%d\n",fd);
write(fd, msg , strlen(msg));
_exit(0);
} else {
/* parent process */
// int status;

// wait(&status);
char msg[1024];
sprintf(msg,"This is Parent with fd=%d\n",fd);
  
write(fd, msg , strlen(msg));
}
return 0;
}

1) When a fork system call is called. a new child process is created which has a copy of all variables of the parent process which called it.

So, When a child is forked then it inherits parent's file descriptors. So a copy of file descriptors of parent is made in the child process.

So, both the values of file descriptors are same in both the processes.

2) Both processes can access the same file descriptor concurrently.

After parent calls forks, parents process executes as is unless we mention wait() which makes parent to wait until the child process completes its execution. Otherwise both the processes run simultaneously.

while writing into a file, the offset address of file in file descriptor(or in File Table )is incremented automatically by write() function cal.

The writing can only be done by one processes at a time.

Now since the both processes are running simultaneosly and they share same fd and file offset, both processes tries to write at the same time one after the other.

So the output  contains data from both child and parent intermixingly.

To be more clear. run the below code which writes multiple lines.

CODE: -

#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>
#include <stdio.h>
#include <string.h>

int main(void)
{
int fd;

fd = open("output",O_CREAT|O_TRUNC|O_WRONLY, 0666);
  

if(!fork()) {
/* child process */
  
char msg[1024];
sprintf(msg,"This is child with fd=%d\n",fd);
for (int i=0;i<100;i++)
write(fd, msg , strlen(msg));
_exit(0);
} else {
/* parent process */
// int status;

// wait(&status);
  
char msg[1024];

sprintf(msg,"This is Parent with fd=%d\n",fd);
for (int i=0;i<100;i++)
write(fd, msg , strlen(msg));
}
return 0;
}

Output file :

This is Parent with fd=3
This is child with fd=3
This is Parent with fd=3
This is child with fd=3

***Thank you . For further clarifications, please comment***